A Comprehensive Review of Plant-Based Biopolymers as Viscosity-Modifying Admixtures in Cement-Based Materials

Abstract

As the construction industry is facing the challenge of meeting the ever-increasing demand for environmentally friendly and durable concrete, the role of viscosity-modifying admixtures (VMAs) has become increasingly essential to improve the rheological properties, stability, and mechanical properties of concrete. Additionally, natural polymers are ever evolving, offering multiple opportunities for innovative applications and sustainable solutions. This comprehensive review delves into the historical context and classifications of VMAs, accentuating their impact in enhancing the rheological properties, stability, and mechanical properties of concrete. Emphasis is placed on the environmental impact of synthetic VMAs, promoting the exploration of sustainable alternatives derived from plant-based biopolymers. Indeed, biopolymers, such as cellulose, starch, alginate, pectin, and carrageenan are considered in this paper, focusing on understanding their efficacy in improving concrete properties while enhancing the environmental sustainability within the concrete.

1. Introduction

Concrete is the most widely used material in the construction industry [1]. Due to abundant resources and rapid global population growth, concrete’s intensive use and production are expected to more than double the current annual consumption of 15 billion tons by 2050 [2]. However, the escalating concrete production leads to significant environmental concerns, mitigating the alarming potential for global warming. In fact, ordinary Portland cement is responsible for about 6% of global CO₂ emissions and the depletion of over 50 billion tons of aggregates annually [2]. It is estimated that the production of 1 m³ of concrete requires 2775 MJ of energy. Adopting appropriate building solutions will reduce not only energy consumption but also pollutant emissions. Meeting this high production demand, these environmental concerns and enhancing concrete’s performance in various construction conditions are key objectives for both industry and research. For example, the introduction of high-performance self-consolidating concrete (SCC) has revolutionized the casting process, speed up the construction activities, and enhanced concrete durability [3]. SCC, recognized for exceptional fluidity, adaptability, and low internal flow resistance, facilitates flow and moderate viscosity, ensuring homogeneous component suspension. SCC is used to cast confined spaces and crowded structural elements designed for seismic regions, where placement and consolidation access is restricted. Furthermore, SCC is proportioned with a higher powder content compared to traditional concrete and integrates a high-range water-reducer (HRWR) to enhance fluidity without increasing the water–cement ratio (W/C) [4].

Flowable concrete, owing to its high fluidity and low yield stress, poses a significant risk of bleeding and segregation, potentially compromising its stability in the fresh state and affecting its hardened properties. Contrary to Newtonian fluids, where the shear stress, τ, is proportional to the shear rate, γ̇ (Equation (1)), cement suspensions are typically described by the Bingham model (Equation (2)) or the Herschel–Bulkley model (Equation (3)). In these cases, the materials require a minimum shear stress, referred to as yield stress, to initiate flow. Additionally, these materials can exhibit shear-thinning behavior, characterized by a decrease in apparent viscosity with shear rate [5].

τ = μ₀ γ̇

(1)

τ = τ₀ + μ_p γ̇

(2)

τ = τ₀ + k γ̇ ⁿ

(3)

Here, τ is the shear stress (Pa), γ̇ is the shear rate (s⁻¹), μ₀ is the material viscosity (Pa s), τ₀ is the yield stress (Pa), μ_p is the plastic viscosity (Pa s), k is the consistency, and n is the power index representing the deviation from the Newtonian behavior.

To mitigate these issues, viscosity-modifying admixtures (VMAs) are incorporated to minimize segregation and enhance concrete’s stability during transport, placement, and consolidation [6]. VMAs, also known as viscosity-enhancing admixtures (VEAs) or anti-washout admixtures, primarily consist of water-soluble polysaccharides. When properly used in cement-based materials, VMAs enhance rheology, stability, cohesion, and water retention capacity of concrete [4]. Initially adopted in Germany during the mid-1970s, followed by Japan in the 1980s, and eventually integrated into North American practices by the late 1980s, VMAs are available in both powder-based and liquid-based forms [6].

VMAs have diverse applications in various types of concrete, fulfilling specific purposes such as in tremie concrete used in constructing curtain walls and deep foundation walls, the underwater repairing of marine and hydraulic structures, and shotcrete for repairing deteriorated structures [6]. Additionally, in applications where exceptional sedimentation and bleeding resistances are essential for effective corrosion protection for stressed tendons, VMAs are used to fill post-tensioning ducts [4]. VMAs are thus used to enhance the stability of the fluid system by increasing the viscosity and promoting the homogeneous dispersion of solid particles while also retaining water until hardening.

Moreover, VMAs play a crucial role in enhancing the thixotropy of cement-based materials. Thixotropy refers to the property of a material where its viscosity decreases with an increase in shear rate, then returns to its initial value once the stress is removed, making it invaluable in the construction industry. High thixotropy significantly affects various aspects of concrete, including transport, pum**, and implementation. As concrete is poured into formwork, its reduced viscosity at higher shear rates allows for the smooth casting and filling of intricate formworks, ensuring proper consolidation and reducing the likelihood of air voids or void spaces [4,6].

The mechanism of action of VMAs is influenced by many factors, including the nature and concentration of the VMAs used, their compatibility with cement particles, their molecular weight, and their conformation (chain length and shape). To date, six modes of action have been identified, as shown in the following

Table 1. Mechanisms of action of VMAs.

Mechanisms of Action	Definition
Bridging flocculation	This mechanism involves the adsorption of high-molecular-weight polymer chains to the periphery of water molecules and onto cement particles. This dual interaction physically binds the cement particles together, leading to the immobilization of a portion of the mixing water and an expansion in the overall mixture volume. Consequently, this process increases the viscosity of both the mixing water and the cement-based system while also improving the yield stress of cement suspensions.
Polymer–polymer Association	In this mechanism, adjacent polymer chains develop attractive forces, impeding the movement of water and leading to the formation of a gel-like structure and an increase in viscosity. Associative polymers feature segments distributed along their chains with a natural tendency to interact, fostering both intramolecular and intermolecular associations among polymer chains. This intricate interplay creates a three-dimensional network within the solution, further amplifying its viscosity.
Entanglement	At high concentrations, VMAs polymer chains have the capacity to entangle, thereby increasing the apparent viscosity within both the interstitial solution and cement suspension. This intertwining phenomenon occurs particularly at low shear rates and high polymer concentrations, where the chains interlace and become entangled, consequently increasing the apparent viscosity.
Depletion flocculation	Within the system, a surge in osmotic pressure occurs, leading to flocculation, as nonadsorbed polymers are drawn away from a “volume exclusion shell” envelo** larger particles. It is important to note that this mechanism does not modify the plastic viscosity of suspensions. However, it results in a notable elevation in the yield stress, stemming from the variation in polymer concentration between the bulk solution and the depleted zone encircling larger particles.
Solvation and swelling	The VMAs polymer chains undergo swelling to enhance their interactions with the solvent. It is important to mention that the dimensions of the polymer are contingent upon its molecular properties, the surrounding environment, and the shear rate applied.
Alignment under shear	Experiencing shear forces can induce transformative changes in the polymer structure, leading to the alignment, stretching, and/or disentanglement of polymer chains in the direction of the flow.

[7,8].

Based on their physical effects on cement-based materials, VMAs can be classified into five classes (by Mailvaganam), as shown in

Table 2. Classification of VMAs—Mailvaganam.

Classes	Definition
Class A	Includes cellulose ethers and polyethylene oxides, and it holds water-soluble synthetic and natural organic polymers. These VMAs play an important role in increasing the viscosity of the mixing water.
Class B	Contains water-soluble flocculants, such as styrene copolymers with carboxyl groups, natural gums, and synthetic polyelectrolytes, which readily adsorb onto cement grains. This adsorption process involves the attachment of these polymers to the surface of cement particles. Importantly, it fosters interparticle attraction among cement grains, a phenomenon that contributes significantly to an increase in the overall viscosity of the cement-based system.
Class C	Encompasses a variety of organic materials with the primary objective of enhancing interparticle attraction and introducing superfine particles into the cement paste. Notable among these materials are acrylic emulsions and aqueous clay dispersions.
Class D	Consists of water-swellable inorganic materials with a high surface area, including bentonites, silica fume, and milled asbestos. Their primary function lies in increasing the water-retaining capacity of the paste.
Class E	Comprises an array of inorganic materials characterized by their high surface area, which serves to enhance the fine dosage within the paste and subsequently improve thixotropy. Noteworthy among these materials are fly ash, kaolin, hydrated lime, and various rock dusts, among others.

[7].

Moreover, based on their chemical origin and composition, VMAs can be classified into three main groups, presented in

Table 3. Classification of VMAs—Kawai.

Classes	Definition
Synthetic polymers	These are artificial compounds derived through chemical synthesis. These include vinyl-based polymers, like polyvinyl alcohol, and ethylene-based polymers such as polyethylene oxide, polyacrylamide, and polyacrylate.
Semi-synthetic polymers	These compounds have partially synthetic origins and include cellulose-ether (CE) derivatives like hydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose (HEMC), hydroxyethyl cellulose (HEC), and carboxymethyl cellulose (CMC). Additionally, semi-synthetic polymers encompass decomposed starch and its derivatives, along with electrolytes such as propylene glycol alginate and sodium alginate.
Natural polymers	Constitute a broad spectrum of substances primarily derived from natural sources. Locust bean gum, starches, guar gum, alginates, agar, rhamsan gum, gellan gum, arabic gum, Welan gum, xanthan gum, and various plant proteins are part of this polymer type. Moreover, natural polymers can be obtained from agricultural resources, such as polysaccharides, cellulose, starch, alginate, pectin, and carrageenan, or obtained through fermentation from microorganisms, like polyhydroxyalkanoates, including polyhydroxybutyrate. Natural polymers can also be produced through conventional synthetic biotechnology; examples include polylactides (PLAs), polybutadiene succinate, biopolyethylene, polytrimethylene terephthalate, and poly-p-phenylene [10].

, as proposed by Kawai [6,9].

Indeed, studies have demonstrated that synthetic VMAs or those obtained from microorganisms, such as Welan gum, Diutan gum, and xanthan gum, improve the rheological properties and stability of cement-based materials [11,12], However, their negative environmental impact is significant. Synthetic VMAs are manufactured through industrial chemical synthesis, involving petroleum-derived components that emit toxic species, resulting in notable environmental impacts and increased construction costs [10]. Furthermore, when combined with high-range water-reducer agents, unpredictable and complex interactions may occur, leading to a loss of fluidity, delayed setting, and reduced mechanical resistance [11,12].

Addressing these challenges requires a decrease in the cost of producing these admixtures, particularly in develo** countries where highly industrialized VMAs are challenging to produce domestically, leading to dependence on expensive imports and transportation, thus raising the production cost of specialized concrete [13]. To mitigate these issues, exploring local biological sources of polymers, such as plants, algae, animals, or microorganisms, has become increasingly important. By leveraging these resources, low-cost and eco-friendly natural admixtures can be developed. Nowadays, several plant-based biopolymers have been used in concrete as VMAs to enhance both the fresh and hardened properties of concrete, offering economic and environmentally sustainable alternatives.

This review represents the pioneering effort in highlighting the use of biopolymers as viscosity-modifying admixtures in cement-based materials. While previous reviews, such as [14,15], have discussed natural polymers used in concrete, this paper goes further by providing a comprehensive bibliometric analysis of VMAs in construction materials. It serves as a guideline for both researchers and practitioners, offering a detailed synthesis of the latest advancements in utilizing plant-based biopolymers to enhance the rheology and stability of cement-based materials. In addition to providing an in-depth exploration of the characteristics, origin, and chemical composition of selected biopolymers, this review underscores their profound impact on the rheological properties of cement-based materials. By elucidating the intricate interactions between these biopolymers and the cementitious system matrix, it opens new avenues for optimizing both fresh and hardened properties, thereby improving the durability, workability, and overall performance of concrete structures. Furthermore, the incorporation of prominent plant-based biopolymers such as cellulose, starch, alginate, pectin, and carrageenan highlights the diversity and adaptability of natural materials that can be used to tackle pressing challenges in modern construction. By focusing on these key components, the paper not only enhances the understanding of their individual contributions but also establishes a foundation for future research endeavors aimed at harnessing their full potential.

2. Bibliometric Analysis

To analyze the scientific literature on VMAs in construction materials, a bibliometric analysis was conducted using the Web of Science (WoS) database by Clarivate Analytics. The primary research questions for the bibliometric analysis were as follows:

(1)

How has research on the use of VMAs in construction materials evolved over time?

(2)

Who are the prominent researchers, institutions, and countries actively engaged in VMA research?

(3)

In which scientific journals is research on VMAs/construction materials typically published?

(4)

What are the primary search keywords associated with the use of VMAs in construction materials?

The data for the bibliometric analysis were obtained from the Web of Sciences (WoS) database in August 2023 using the following search keywords: “VMA” OR “viscosity modifying admixtures” OR “viscosity modifying agent” AND “concrete” AND “cement”. A total of 2356 results were retrieved (

Table 4. Web of Science search query.

Search Query		Results
Search date	23 August 2023
Search topic	“VMA” OR “viscosity modifying admixtures” or “viscosity modifying agent” and “concrete” and “cement”	2356
Publication years	2000–2023	2312
Document type	Include only articles	1181
Language screening	Include only English	952
Research aera	Include only research aeras related to construction materials	604
Data extraction and analysis		604

). The analysis focused only on English articles related to construction materials and published between 2000 and 2023.

Figure 1 illustrates the evolving trends in VMA and construction materials research, reflecting a significant increase in publications in 2017. This upward trend suggests a growing focus on these topics, with researchers likely to continue exploring them in the coming years. Notably, 61.1% (369 out of 604) of articles were published between 2017 and 2023. Among this percentage, 23.7% (143 out of 604) were published in the journal Construction and Building Materials.

In Figure 2, the top 10 journals contributing to VMA and construction materials research are highlighted. Among these, 54.6% (330 out of 604) of articles were published in the following journals: (1) Construction and Building Materials (23.7%, Elsevier, Impact Factor: 7.4); (2) Journal of Materials in Civil Engineering (5.8%, ASCE, IF: 3.2); (3) Cement and Concrete Research (5.6%, Elsevier, IF: 11.4); (4) Cement and Concrete Composites (4.5%, Elsevier, IF: 10.5); (5) ACI Materials Journal (4.0%, ACI, IF: 1.83); (6) Materials (3.3%, MDPI, IF: 3.4); (7) Transportation Research Record (1.82%, Sage Journals, IF: 1.7), (8) Materials and Structure (1.5%, Springer, IF: 3.8); (9) Applied Sciences (1.2%, MDPI, IF: 2.7), and (10) Advances in Materials and Civil Engineering (1.2%, ASTM, IF: 1.4).

Research in the field of VMAs/construction materials has received contributions from 69 countries, as illustrated in Figure 3, showcasing the global distribution of publications counts. China emerges as the most productive country, contributing 163 articles (27.0%), followed by the USA (115 articles, 19.0%), Canada (54 articles, 9.0%), India (42 articles, 7.0%), France (41 articles, 6.8%), Poland (31 articles, 5.1%), Spain (24 articles, 4.0%), Turkey (24 articles, 4.0%), Germany (21 articles, 3.5%), and Iran (17 articles, 2.8%). These 10 countries collectively account for 88.1% of publications in the VMA and construction materials field. This high level of contribution from certain countries may be attributed to factors such as their large populations, active commercial activity, a high demand for construction materials, and a significant number of scientific research institutions. The University of Sherbrooke in Canada stands out as the most productive with 22 publications (3.6%) related to the use of VMAs in construction materials (Figure 4). The top 10 most productive research institutions span seven different countries: Canada (University of Sherbrooke and Toronto Metropolitan University), France (Centre National de la Recherche Scientifique CNRS), China (Southeast University, Tongji University, and Wuhan University of Technology), India (Indian Institute of Technology), Poland (Silesian University of Technology), Ireland (Queen’s University Belfast), and the USA (Missouri University of Science Technology).

Moreover, based on our data, Khayat K.H. is the most productive author, with 18 publications (2.98%) (Figure 5). His most cited publication (166 citations) about the use of VMA in constructions materials was published in 2017 and presented important results regarding the dosage of VMA in self-consolidating concrete to enhance steel fiber distribution and flexural performance [16]. In general, Khayat’s studies focus on the mechanism of action of VMAs in cement-based materials, the ability of VMAs to ensure the required rheological properties, the effect of VMAs on the concrete properties in the hardened state, and the compatibility between VMAs and HRWR admixtures. The main VMAs tested in Khayat’s publications are Welan gum, Diutan gum, and cellulose ether. After Khayat K.H., Hossein K.M.A. and Lachemi M., who belong to the same research team at the University of Toronto, are ranked second among the ten most productive authors, with 14 publications between 2000 and 2023 in this field. Their most cited publication (187 citations) was published in 2004 and presented the performance of four new polysaccharide-based VMAs in order to identify and produce a new type of low-cost VMA [13]. Hossein K.M.A. and Lachemi M. focused, in their studies, on the performance of VMAs in binder manufactured with fly ash, slag cement and metakaolin; the development of new low cost VMAs; and the bond behavior of SCC made with different VMAs. The main VMAs tested in Hossein K.M.A. and Lachemi M.’s publications are Welan gum and new polysaccharide-based VMAs. Finally, Yahia A. holds the fourth position in this field with nine studies published between 2000 and 2023. His most cited publication (59 citations) was published in 2009 and was co-authored by Kamal K.H. and Erdem T.K. The authors tested, in their study, a low (1.3 L/m³) and medium (2.9 L/m³) VMA incorporation in SCC, and they observed a strong correlation between the yield stress, plastic viscosity, and thixotropy of SCC and the corresponding Concrete-Equivalent Mortar mixtures [17]. In general, Yahia’s studies focus on the interaction between VMAs and HRWRs, the use of VMAs in the 3D printing of cement-based materials, the effect of VMAs on the air-void system in SCC, and the use of kappa-carrageenan as a new low-cost VMA in cement-based materials. The main VMAs tested in Yahia studies are Welan gum and carrageenan.

However, keyword analysis can unveil the diverse research topics explored in the field of VMAs and construction materials, providing valuable insights into areas that may require further investigation. Figure 6 depicts three major topics studied between 2000 and 2023 in the VMA and construction materials field, namely cement paste, self-consolidating concrete, and asphalt mixtures. Within the realm of concrete research, studies predominantly focus on examining the impact of VMAs on the fresh state (rheological properties) of concrete using cement paste. In the field of construction materials, VMAs find primary applications in SCC.

Moreover, there is a noticeable absence of keywords related to the impact of VMAs on the hardened properties of concrete. While some durability indicators such as permeability and air content are present, further investigations are needed in the field of VMAs and construction materials to analyze the effects of this addition on the durability of concrete structures. Notably, no study, to the best of our knowledge, has been published to date about the environmental and economic assessment of VMA applications in concrete. Additionally, no keywords related to this topic, such as integrated life cycle assessment or life cycle cost assessment, have been detected. Therefore, in addition to the considerable attention given to the rheological adaptation of construction materials using VMAs, further research is essential to ensure the economic and environmental sustainability of this additional component.

Bibliometric analysis, a quantitative methodology frequently applied in literature reviews, is utilized to investigate worldwide research trends within a specific field. Its aim is to assess both the current status and potential future directions of research within that field. To achieve this objective, this article first examines the annual publication count to gain insights into the overall evolution of VMA research in construction materials. Subsequently, it evaluates the distribution of publications across different countries to understand the growth of research interests in this area. The third stage involves studying prominent authors and their research topics to discern the prevailing research direction. Additionally, the paper evaluates keywords and suggests potential future research topics within the realm of VMAs and construction materials.

The yearly distribution of published studies demonstrates a significant growth in research from 2017. This could be attributed to the increased demand for SCC, particularly used in the production of precast concrete materials. Moreover, the geographic distribution of publication counts by country provides valuable insights into the worldwide dispersion of research efforts in the VMA and construction materials field. The analysis indicates that advanced economies such as China, the USA, and Canada have generated a large number of articles. This underscores the influential role that developed nations play in both identifying VMAs and influencing the evolution of SCC mix designs incorporating VMAs. However, the analysis of prominent authors, their research topics, and the keywords associated with this field provides valuable insight into the areas already explored and those that might necessitate further investigation. The data indicate that most studies in the VMA/construction materials fields have focused on two primary areas: identifying new VMAs and assessing their impacts on the fresh state of concrete (rheological behavior). Additional investigations are necessary to identify the influence of these VMAs on the durability of concrete and to guarantee the economic and environmental sustainability of integrating these additional components into concrete mix designs.

3. Plant-Based Biopolymers

The exploration and integration of plant-based biopolymers as viscosity-modifying admixtures in cement-based materials represent a significant advancement in sustainable construction practices. Biopolymers such as cellulose, starch, alginate, and pectin have been widely used for this purpose. A notable addition to this lineup is carrageenan, extracted from red seaweeds, which has been used as a viscosity-modifying admixture for over four years and earned a patent in 2020 [18]. The incorporation of these biopolymers reflects the growing momentum towards sustainable and environmentally friendly construction practices. Ongoing research and industry initiatives are aimed at exploring new applications, demonstrating a collective commitment to innovation and the optimization of both performance and environmental impact in construction materials.

3.1. Cellulose-Based VMAs

Cellulose, an omnipresent biopolymer, holds the distinction of being the Earth’s most abundant natural substance, with an impressive annual production of approximately 2 × 10¹¹ tons [19,20]. It serves as the fundamental structural foundation of cell walls for a multitude of life forms, spanning the realms of plants, seaweeds, protists, and fungi [20,21]. Often combined with hemicellulose from hardwoods and cotton, cellulose exhibits diversity, imparting distinct characteristics. Moreover, hemicelluloses (xylan and glucomannan) have demonstrated the potential to create bioinspired nanocomposites with promising functional properties, particularly in enhancing barrier properties. Additionally, their strong affinity to cellulose within native plant cell walls is widely recognized [22]. At its core, cellulose manifests as a macromolecule, presenting an elongated, unbranched chain of variable length [20]. Its structure unfolds as a linear β-(1→4)-linked glucan structure, marked by a remarkable ability to foster cooperative networks of intra- and intermolecular hydrogen bonds, creating a robust, water-resistant framework of fibrils and fibers (Figure 7) [23]. However, to reveal its pure form, cellulose, entwined with the complex cell walls of plants, must undergo liberation from its entangled companions, notably lignin and hemicellulose, through the alchemical process of pul** [8].

Natural cellulose typically exhibits a degree of polymerization (DP) above 1 × 10⁴, corresponding to a substantial molecular weight of roughly 2 × 10⁶ g·mol⁻¹. Moreover, cellulose’s versatility depends on three critical factors: molecular mass, degree of substitution (DS), and the chemical composition of the substitution group. As molecular mass increases, solubility decreases, while shear-thinning behavior intensifies [8]. Furthermore, cellulose has found extensive applications in various industrial sectors (food, cosmetics, pharmacy, latex paints, and ceramics) [24]. Particularly in the field of construction materials, the chemical modification of cellulose is recognized as a crucial step to unlock its full potential and achieve desired outcomes in cementitious materials [20,23]. Indeed, in cement-based materials, cellulose serves as a VMA, primarily in the form of ether derivatives, with an annual usage tallying around 1 × 10⁵ tons in dry mortars [25]. These versatile derivatives play a multitude of roles in construction, including thickening, binding, and water retention [23].

When incorporated into cementitious materials, cellulose ethers increase viscosity and water retention capacity, reduce the yield stress, and improve shear-thinning behavior [24]. Additionally, it has been reported that the effectiveness of cellulose-based VMAs in modifying the properties of cementitious materials depends on several factors, such as the type of cellulose ether used, as well as its concentration, molecular substitution, molecular weight, and chemical composition [24,26]. Additionally, Roussel et al. highlight the influence of other intrinsic parameters of cementitious materials, such as pH and ion levels in the cement paste [27,28].

Furthermore, other studies have shown that the behavior of cellulose incorporated in cementitious materials varies depending on the nature of the superplasticizer with which it is combined. Saric-Coric et al. investigated the impact of a cellulose-based VMA on the properties of cement pastes containing two different types of superplasticizers: polynaphthalene sulfonate (PNS) and polymelamine sulfonate (PMS). It was found that while the combination of cellulose VMA with PNS did not significantly affect rheological parameters, the presence of cellulose with PMS led to a notable increase in both viscosity and yield stress. Other differences were observed also between the two cement pastes, particularly in terms of bleeding, leaching, setting time, and compressive strength development. For example, the cement paste made with PMS and cellulose VMA showed a higher mechanical strength than the cement paste prepared with PNS and cellulose [29]. On the other hand, Khayat et al. showed that some incompatibilities may appear and a pronounced reduction in fluidity may result when the HPMC is combined with a PNS HRWR. Thus, it has been acknowledged that PMS is more compatible with cellulose-based VMAs [11,30].

In addition, cellulose has been used in other forms, including cellulose nanofibers (CFFs) and cellulose filaments (CFs), which constitute a new class of nanocellulose materials. It was agreed that CFs (95% cellulose and 5% hemicellulose) improve the rheological properties particularly the yield stress, the plastic viscosity, and the stability of SCC [31,32]. Hisseine et al. claim that the rheological and the stability improvements are due to the hydrophilicity, the high aspect ratio, and the flexibility of CFs [32]. Furthermore, CFs increase the heat of hydration without causing a delay of the hydration. CFs also lead to performances comparable to Welan gum at a dosage of 0.05% in terms of yield stress and plastic viscosity. Moreover, it was added, in terms of CF dosages, that they should be less than 0.12% to ensure adequate dispersion and stability [31]. As for CFFs, which were used as admixtures for self-leveling mortars, they reduced the spreading diameter and increased the flow time. Identically to CFs, they increase the yield stress and the plastic viscosity. Finally, it was mentioned that using 0.03% of CFFs, the performances of concrete were favorable for projected concrete applications [33].

On the other hand, in a recent study, the impact of paper pulp as a VMA in cement-based materials was assessed [34]. Paper pulp, primarily composed of cellulose, differs from conventional VMAs biopolymers due to its insolubility in water, attributed to the dimensions of the fibers in terms of diameter and length. Two variants of paper pulp were examined: low-energy and high-energy, depending on the mechanical grinding process. The findings indicated that these variants significantly influenced the rheological properties of the mixtures, serving as versatile VMAs comparable to conventional admixtures. Low-energy variants increased dynamic yield stress and plastic viscosity, outperforming MasterMatrix (MM) at equivalent dosages, while high-energy variants exhibited similarities to Diutan gum, offering even more substantial enhancements. This study highlighted the environmentally friendly and effective role of paper pulp as a VMA, modifying the rheology of cement mixtures through a combination of mechanisms, including the flocculation and water affinity of pulp fibers.

3.2. Starch-Based VMAs

Starch, a predominant dietary component, stands out for its diverse range and the versatility of its derivatives across various applications [35]. Serving as the principal storage polysaccharides in plant-derived foods, starches play an important role in global food production [36]. Approximately 48 × 10⁶ tons are produced annually, with 70% of this output used in the worldwide food sector [36]. Indeed, it serves diverse functions including gelling, thickening, moisture retention, stabilizing, and texturizing in various food products. For instance, in cereal-based foods, starch significantly impacts texture by efficiently absorbing water and filling the dough matrix. Furthermore, derived from various plants (corn, cassava, sweet potato, wheat, and potato), starches exhibit varied characteristics such as in size, shape, and molecular organization, influenced by factors like plant species, genetics, and environmental conditions [37].

The extraction of starch from roots and tubers involves a systematic process, starting with grating the raw material to rupture plant cells and release the starch, followed by passing the fibrous material through sieves of varying mesh sizes and concentrating the resulting slurry through processes like decantation or centrifugation [38]. Starch, a polysaccharide [8], consists of two D-glucose polymers: amylose and amylopectin (Figure 8). Amylose is characterized by its predominantly unbranched α [1 → 4] glucan linkage while amylopectin features chains of α [1 → 4] linked glucose units organized in a heavily branched configuration through α [1 → 6] branching connections [39]. Furthermore, starch may contain minor constituents such as proteins and lipids [37]. Except for waxy starches with 100% amylopectin, the proportions of amylose and amylopectin vary depending on the starch source. Typically, starch consists of approximately 20–30% amylose and 70–80% amylopectin [8].

On the other hand, starch-based polymers, widely used in industries, such as in food and construction, can undergo crucial alterations through chemical, physical, or enzymatic methods from their native state [20], aiming to enhance positive traits and mitigate negative ones [40]. In addition, their modification can result in the development of innovative polymers with a range of functional and value-added properties to meet the requirements of various industries [40]. Moreover, both amylose and amylopectin can undergo these modifications, affecting granule arrangement and polymer structures. The precise controlling of modification conditions is thus essential for tailoring the properties of these modified starches, contributing to the development of innovative products across diverse industries [41].

Indeed, modified starches are also widely exploited in various construction applications, such as improving the adhesion of plaster to walls, minimizing settlement, providing initial strength, improving consistency, and optimizing rheological properties. For example, hydroxypropyl starch (HPS), a modified starch, has been used to optimize the material properties of construction products such as concrete and dry mortar [35]. It has been reported that the HPS with a degree of substitution (DS) of less than 0.5 and a dosage of 0.05% causes a rapid increase in viscosity, promoting the adhesion of plaster to walls. Moreover, when the HPS is combined with CE, it prevents plaster drips after spraying and reduces trowel stickiness. It is also used to reinforce tile adhesives and prevent water loss in porous formations [34]. In addition, starch derivatives improve the thickness and smoothness of cement-based formulations. Indeed, although their rheological characteristics under shear stress are undesirable, mortars with starch derivatives are more viscous at rest. On the other hand, the starch derivatives explored in another study, one assessing the properties of self-leveling mortars incorporating several VMAs, exhibit an intermediate behavior between a natural polysaccharide and microsilica [42]. It was found that while maintaining a constant water/binder ratio (W/B), VMAs, including starch, reduce mortar flow and increase flow time, simultaneously leading to an increase in yield stress and plastic viscosity. However, it is essential to mention that the composition of cement mortars is influenced by the type of starch modification and chain length. For example, in the study, hydrolyzed starch, with a short polymer chain, reduced the yield stress and the plastic viscosity, resulting in larger flow diameters. Meanwhile, unhydrolyzed potato starch thickened the cement mixture and increased the yield stress [43].

In addition, starch and modified starch have been compared with conventional VMAs, not only for comparative purposes but also to assess their affinity with different superplasticizers. For example, it has been found that starch has a low affinity with polycarboxylate (PCE)-based HRWRs, unlike Welan gum [44]. Another example can be found in the study by Khayat et al. where modified starch was combined with a polycarboxylate ether-based HRWR [9]. Modified starch alone led to a reduction in the yield stress, slightly affecting plastic viscosity. However, when combined with the HRWR, viscosity, segregation resistance, and toughness increased further. It should also be noted that the demand for the HRWR proved to be limited, demonstrating its effectiveness.

On the other hand, starch has proven its potential in improving shear-thinning behavior, particularly at high dosages. It has been shown to have a significant effect on improving apparent viscosity and yield stress, as well as on reducing the spreading diameter [44]. In the context of research into alternatives to conventional VMAs, notably Welan gum, starch has proven its potential for increasing segregation resistance [45]. Furthermore, starch ether has also been shown to be effective in eliminating bleeding, unlike Welan gum, which only reduces it. It is important to mention, however, that starch ethers have been used at high dosages due to inherent distinctions in its structure [46].

Furthermore, it has been shown that the behavior of starch in concrete can vary according to its origin. For example, corn starch improves workability and density at a rate of 1% due to its gelatinous properties. Nevertheless, when a certain dosage of starch is reached, the density and the compressive strength decrease while compressive strength remains higher than that of ordinary concrete [47]. In contrast to corn starch, cassava starch reduces workability and increases the setting time. The versatility of cassava starch lies in its modification of viscosity in parallel with setting delay without compromising the compressive strength of the concrete [48].

3.3. Alginate-Based VMAs

Alginate, an abundant polysaccharide in natural sources [49], is found in the cell wall in marine brown algae such as Ascophyllum, Hydroclathrus, Laminaria, Lessonia, Macrocystis, Sargassum, Durvillaea, etc. [49,50]. Growing throughout the year in the littoral and sublittoral regions [51], these marine brown seaweeds use alginate in their intercellular matrix to provide flexibility, robust structural support, and protection against powerful seawater waves [50]. Although 30,000 metric tons of alginate are industrially manufactured annually, this amount accounts for less than 10% of the total bio-synthesized alginate from macroalgae thanks to the feasibility of macroalgae cultivation and technically viable fermentation-based production [52]. Discovered in kelp by Stanford in 1881, alginate’s hydrogel-forming ability has led to its recognition as a stabilizer, thickener, gelling agent, and emulsifier. Alginate is moreover biocompatible, non-toxic, biodegradable, and easily processed [53]. These properties make alginate indeed valuable in diverse industrial applications including food, medicine, pharmaceuticals, and textiles [49].

Alginates consist of blocks of repeated 1,4 α-L-guluronic acid (G) and 1,4-linked β-D-mannuronic acid (M) residues, comprising sequences of repeated G residues, repeated M residues, and alternating G and M residues (Figure 9) [20,50]. Additionally, alginates contain several naturally occurring elements that are biologically active such as peptides, carbohydrates, lipids, pigments, etc. [54]. Moreover, the viscosity of alginate solutions is influenced by pH and molecular weight. For example, viscosity peaks at pH 3–3.5 due to protonation of carboxylate groups, forming hydrogen bonds. In addition, higher molecular weight not only increases the viscosity, which can be problematic for incorporating proteins or cells, but also improves gel properties [55].

On the other hand, alginate has been used as a viscosity-enhancing admixture in fluid concrete in numerous studies. Various sources of alginate have been employed for this purpose, including pure alginate, brown seaweed, sodium alginate (SA), and water-soluble alginate polymers (WSPs). Alginate has indeed demonstrated its potential in improving the overall performance of SCC [56]. For example, it has been shown to enhance workability, reflected by both reduced spreading diameter as well as water retention [51]. Additionally, it has been demonstrated to increase slump and slightly increase fresh density [57]. Nevertheless, the incorporation of alginate in fluid concrete has consequently led to a reduction in properties in the hardened state. Alginate reduces not only compressive strength [51,57] but also tensile and flexural strength [57]. This has been attributed to the weakening of the bond between the aggregates and the cementitious paste due to alginate’s smooth surface [57]. Moreover, because of the significant increase in water retention, the demand for water also increases, making concrete increasingly porous and reducing its compressive strength [51]. However, it was demonstrated that the tensile strength upon casting is reduced in a separate study. It was further noted that this reduction is mitigated following the addition of nano- and microsilica alongside alginate [56].

Furthermore, SA is a common form of alginate used as a VMA in fluid concrete. It effectively reduces the segregation of SCC [58]. Indeed, its performance surpasses that of brown seaweed itself (10% to 40% of alginate), primarily due to its purity, although its cost remains relatively high. The impact of SA, particularly in terms of viscosity, is more pronounced compared to that of seaweed-based alginate [59]. In another investigation study, SA was evaluated in combination with a HRWR, specifically PCE. Viscosity decreased slightly at low doses of SA but increased significantly with higher dosages. Furthermore, SA delayed the setting and influenced PCE adsorption, becoming competitive at higher dosages. The gel formation between SA and calcium ions Ca²⁺ enhances both plastic viscosity and yield stress. Additionally, the abundant presence of carboxyl and hydroxyl groups in the molecular structure of SA facilitates bonding with Ca²⁺ ions, making the adsorption of SA to cement particles more effective than that of PCE [60].

Indeed, it is noteworthy to mention that alginate serves not only as a VMA for fluid concretes but also as a concrete durability enhancer. Initially, it demonstrated its potential as a self-healing agent for concrete cracks [61,62]. Additionally, alginate has been found to reduce ion diffusion, attributed to the development of small alginate spheres within the concrete matrix [51]. Furthermore, WSPs have been used as internal hardening agents to reinforce concrete. These characteristics make alginate a unique agent as it exhibits gelling properties due to the presence of calcium ions. Lower calcium content increases viscosity while higher concentrations lead to gel formation through the cross-linking of the alginate chain [63].

3.4. Pectin-Based VMAs

Pectin is among the most complex polysaccharides [64], being known for its unique chemical and functional characteristics [65]. It comprises approximately one-third of the dry substance in the cell walls of higher plants [66], particularly around actively dividing cells, soft plant tissues, cell corners, and middle lamella [64], and is responsible for intercellular adhesion and contributing to the overall plant structure and firmness [67]. Also found at intercellular junctions in cells with secondary walls (xylem and woody fiber cells), pectin is not limited to angiosperms but is also found in gymnosperms, pteridophytes, bryophytes, and Chara (charophycean algae closely related to land plants) [64]. Additionally, commercially pectin is primarily derived from citrus peel (20–30% pectin) and apple pomace (10–15%), which have similar functional properties. Moreover, alternative sources of pectin include sugarbeet waste, sunflower heads, and mango waste [68].

Indeed, pectin, a high-molecular-weight polysaccharide, is known for its biocompatibility, lack of toxicity, and anionic properties [69]. It is characterized by a high content of galacturonic acid (GalA) (Figure 10) [70], which forms the core of three structurally well-characterized polysaccharide motifs: homogalacturonan (HGA), rhamnogalacturonan I (RG I), and rhamnogalacturonan II (RG II) [69]. On the other hand, pectin serves various industries as a hydrating agent, regulating water movement and stabilizing formulations [67]. In the food industry, pectin is widely used in products like jam and jellies for its gelling, thickening, and stabilizing properties [69], which arise from interactions like bonding with calcium ions, hydrogen bonding, or hydrophobic interactions [67]. However, the properties of pectin can vary significantly depending on its source, molecular size, extraction conditions, location, and degree of esterification [66,67].

On the other hand, in the realm of construction, pectin has emerged as a biopolymer that enhances the properties of fresh cement-based materials. Indeed, it has been shown to improve both the viscosity of cement suspensions [71,72] as well as their water retention capacity [71,72,73]. However, its viscosity enhancement depends on the concentration of pectin used and that of Ca²⁺ ions. For example, when pectin has a reduced number of free galacturonic acid groups, the interactions with calcium ions become more robust, leading to an increase in the viscosity as a result of pectin chains being bound by Ca²⁺ ions. However, when the amount of Ca²⁺ ions increases, viscosity is reduced, and gel precipitation occurs [71]. These gelling properties occurring as a result of the interaction of pectin and calcium ions were also highlighted in a study [74].

In addition, pectin improves concrete plasticity, reduces water demand [72], and enhances workability [72,75]. Similarly, it was concluded that the improvement in concrete viscosity and plasticity is mainly due to the binding of calcium ions by pectin, thus promoting intermolecular associations via calcium bridges [72]. In another study [74], pectin-rich cactus extract was shown to increase flexibility and water absorption. This improvement has been proven to occur due to interactions between the polysaccharides or proteins in the extract and the calcium hydroxide generated during cement hydration.

Pectin has also demonstrated its potential for improving the hardened properties and durability of concrete [71,72]. For example, the interactions that occur between the polysaccharide’s cactus extract and calcium hydroxide lead to the formation of complexes increasing the water resistance of concrete [74]. Furthermore, in separate studies [73,75], not only was concrete porosity enhanced but so was resistance to sulfate attack [75]. Additionally, the compressive strength, splitting tensile strength, and deflection of concrete are improved [75].

Regarding the effect of pectin on the hydration kinetics of cementitious suspensions, two trends were observed. Firstly, pectin has been found to accelerate setting [71,74] by forming three-dimensional gels with Ca²⁺, a result that is contrary to that obtained with organic admixtures, which retard the setting [71]. On the other hand, conflicting results have been reported in other studies, where pectin was shown to delay the setting [31,73,75,76,77]. This is because pectin captures and creates complex compounds in the presence of Ca²⁺ ions, making them unavailable for hydration, thus slowing the development of concrete strength [76,77]. Despite the delayed setting, long-term mechanical properties are improved [75]. This variation in the hydration kinetics of cementitious suspensions, following the addition of pectin, could be attributed to the disparity of pectin sources and their respective degrees of purity.

3.5. Carrageenan-Based VMAs

Carrageenan, a high-molecular-weight hydrophilic polysaccharide known for its linear structure [78], is naturally found in various species of red seaweeds belonging to the Rhodophyceae class [79], including Solieriaceae, Rhabdoniaceae, Hypneaceae, Phyllophoraceae, Gigartinaceae, Furcellariaceae, and Rhodophyllidaceae [80,81]. In fact, the cell walls of marine algae consist of a two-phase system, including a crystalline phase (skeleton) and a more amorphous phase (matrix) [82]. Moreover, examples like Chondrus crispus and Kappaphycus sp. are notable for their high carrageenan contents, reaching up to 71% and 88%, respectively. Moreover, carrageenan finds extensive applications in the food industry (bakery fillings, ice cream, pet food) [83], making it the third most important polysaccharide in the food industry [84]. Additionally, since the 1830s, carrageenan has also been employed in various medical applications, such as in pharmaceutical lotions and medicinal creams, as an anti-coagulant in blood products, and for treating gastrointestinal issues like diarrhea, constipation, and dysentery, due to its stabilizing properties [83].

Moreover, carrageenans are a type of sulfated galactans characterized by their structural composition consisting of alternating disaccharide repeating units. These repeating units consist of three-linked β-D-galactopyranose (G-units) and either four-linked α-D-galactopyranose (D-units) or four-linked 3,6-anhydro-α-D-galactopyranose (DA-units) [79,85,86]. There are several categories of carrageenan, based on the presence of 3,6-anhydrogalactose on the four-linked residue and the position and quantity of sulfate groups [85], with the most well-known being kappa (k)-, iota (i)-, (Figure 11), and lambda (λ)-carrageenans. Other categories include mu (μ)-, nu (ν)-, and theta (θ)-carrageenans, with mu- and nu-carrageenans being the biological precursors of kappa- and iota-carrageenan and theta the successor of lambda [80]. Indeed, the difference in their molecular structures contributes to variations in their functionalities and applications. For example, kappa- and iota-carrageenan, both containing the 3,6-anhydro unit, exhibit gel-forming properties. In contrast, lambda-carrageenan, composed solely of galactose groups, functions primarily as a thickening agent [85].

Indeed, as the applications of carrageenan expand across various industrial domains, they are proving to be valuable components in concrete formulations, providing unique characteristics. On one hand, they enhance the fresh properties of cement-based materials, particularly rheological and viscoelastic properties, build-up kinetics, and stability [78,87]. In addition, kappa-carrageenan, when used as a VMA in cement-based materials, leads to a significant improvement of the shear-thinning behavior, accompanied by an increase in plastic viscosity at both low and high shear rates. In addition, an elastic network is formed, and stiffness and stability are improved by absorbing water and enhancing dispersion [4,88]. Furthermore, the combination of kappa- and iota-carrageenan incorporated in cementitious suspensions has resulted in an increase in the elasticity of kappa-carrageenan gels under shear. As a result, thixotropic gels suitable for specialized applications (3D printing, the casting of high structures by reducing lateral pressure on the formwork) are formed. Consequently, the combination of the two polymers affected the thixotropic behavior, pseudoplasticity, plastic viscosity, bond strengths and build-up kinetics in one study [88].

Concerning the hydration kinetics of cement pastes incorporating kappa-carrageenan, a notable delay in setting occurs due to reduced ion concentrations. Moreover, this delay increases with higher dosages [4,88]. However, the combination of kappa- and iota-carrageenan mitigates this delay, thereby reducing the overall setting time [88]. The delay reduced by kappa-carrageenan at high dosages correlates with a reduction in compressive strength, although significant improvements are observed at lower dosages [4]. Furthermore, in other studies, carrageenan has demonstrated significant improvements in the mechanical properties of concrete. It exhibits a more pronounced effect than xanthan gum in enhancing compressive strength. Similarly, a direct correlation between carrageenan content and compressive strength has been found, with notable improvements at a low concentration (0.1%) [78,87].

In addition, a comprehensive study explored the use of biobased superabsorbent polymers (SAPs) developed from kappa-carrageenan and polyacrylic acid in high-performance concrete to counteract cracking due to autogenous shrinkage. This study demonstrated that SAPs incorporated into low W/C ratio pastes effectively mitigate shrinkage, thereby enhancing concrete performance by reducing early autogenous shrinkage and enhancing freeze-thaw resistance [79].

Nevertheless, it is important to note that the influence of carrageenan biopolymers on cementitious suspensions varies not only according to the type and dosages used but also based on the presence or absence of HRWRs. For instance, when combined with a HRWR, kappa-carrageenan reduces shear stress and plastic viscosity. While stability is significantly improved, build-up kinetics and compressive strength development may be affected [4]. On the other hand, the combination of kappa-iota-carrageenan with a HRWR leads to an increase in the degree of pseudoplasticity at high dosages. However, the yield stress and the plastic viscosity are attenuated. In addition, setting time is prolonged, resulting in a decrease in compressive strength at high dosages due to delayed hydration reactions and the formation of weaker gels [88].

Furthermore, the alkaline extraction process for commercial kappa-carrageenan is costly. Then, a study investigated the use of the red alga Kappaphycus alvarezii as an alternative to kappa-carrageenan [89]. Various parameters were optimized to maximize the efficiency of native (κ)-carrageenan from K. alvarezii without alkaline extraction. The main distinction between the two lies in the chemical structure, notably the absence of 3,6-anhydrogalactose bridges in the native (κ)-carrageenan of K. alvarezii. It was found that the studied seaweed powder improves the viscosity and stiffness of cement-based materials. Furthermore, at the dosage of 0.5%, both demonstrate comparable performances, especially regarding rheological parameters. Nonetheless, the rigidity and build-up kinetics are not as enhanced with the K. alvarezii as they are with kappa-carrageenan. It is essential to note that the seaweed improved the rheological properties without disturbing the hydration kinetics of the cement, unlike (κ)-carrageenan, which significantly delays the setting time. This research effectively highlights the potential of K. alvarezii as an environmentally friendly and cost-effective alternative to conventional VMAs, offering sustainable solutions in the construction sector.

3.6. Synthesizing Table

A synthesizing table (

Table 5. Synthesis of the advantages and drawbacks of the plant-based biopolymers.

Biopolymers	Advantages	Drawbacks
Cellulose	Cellulose ethers (CEs): - Thickening agents. - Binding agents. - Enhance the water retention capacity. - Increase the shear-thinning behavior and the viscosity. - Attenuate the forced bleeding and the leaching. - When combined with PMS, significantly increase the viscosity and the yield stress. Cellulose filaments (CFs) improve the stability of SCC. Cellulose nanofibers (CFFs) raise the flow time and reduce the spreading diameter. Both CF and CFF improve the viscosity and the yield stress.	Cellulose ethers: - Decrease the yield stress. - Reduce the compressive strength. - When combined with PNS, do not improve the viscosity along with the yield stress.
Starch	Starch: - Improves the viscosity and yield stress. - Increases the flow time and reduces the mortar flow. Modified starch: - Improves the adhesion of plaster to walls. - Provides the initial strength. - Enhances the consistency. Corn starch improves the workability and density at 1%	Modified starch decreases the yield stress and slightly affects the viscosity. Starch derivatives lead to undesirable rheological characteristics under shear. Cassava starch decreases the workability and amplifies the setting time
Alginate	Pure alginate improves the workability and the water retention capacity of SCC. Sodium alginate reduces the segregation of SCC and enhances the viscosity at high dosages. The gel formed between sodium alginate and calcium ions leads to an increase in both the yield stress and the viscosity. Alginate is used to enhance the durability of concrete: - Self-healing agent for concrete cracks. - Ion diffusion reducer. - Internal hardening agent.	Alginate worsens the hardened properties of concrete, particularly the compressive, tensile, and flexural strength. At low dosages of sodium alginate, the viscosity decreases.
Pectin	Pectin enhances the following fresh properties of concrete: - Plasticity. - Workability. - Flexibility. - Water absorption. Pectin improves the hardened properties of concrete: - Compressive strength. - Splitting tensile strength. - Deflection of concrete. Pectin boosts the durability of concrete: - Water resistance. - Porosity. - Resistances to sulfate attacks.	The delay of hydration of concrete that can be caused by pectin leads to the deceleration of the development of concrete strength.
Carrageenan	(k)-carrageenan: - Improves the shear-thinning behavior and the plastic viscosity at high and low shear rates. - Enhances the stability, the water’s absorption, and the dispersion. (k)-(i)-carrageenan: - Enhances both the elasticity as well as the thixotropy of (k)-carrageenan gels under shear. - Reduces the setting times. (k)-carrageenan-based superabsorbent polymers enhance the durability of concrete: - The shrinkage is reduced. - The freeze-thaw resistance is improved. K. alvarezii: - Offers comparable performances with (k)-carrageenan in terms of yield stress and plastic viscosity. - Causes no hydration delay.	(k)-carrageenan leads to a delay of hydration, which is significant at high dosages. This delay causes a decrease in the compressive strength at high dosages. The combination between carrageenan and the HRWR leads to a decrease in both yield stress and plastic viscosity. The build-up kinetics and the compressive strength are also affected. K. alvarezii does not enhance the rigidity and the build-up kinetics of the cement-based materials.

) has been created to summarize the benefits and limitations of plant-based biopolymers as viscosity-modifying admixtures in concrete. This table serves as a database for ongoing research in this area, highlighting the distinct advantages of each biopolymer in enhancing rheological properties. However, it also notes potential drawbacks or side effects. The table aids in selecting the right viscosity-modifying admixture for specific applications and desired performance criteria.

4. Concluding Remarks

This paper has extensively explored the realm of viscosity-modifying admixtures within the context of cement-based materials, highlighting their impact in enhancing concrete’s performance. With the construction industry facing increasing demands for concrete, driven by global population growth and infrastructure development, optimizing concrete properties in both fresh and hardened states becomes imperative to ensure superior durability and resilience of construction projects. VMAs, characterized as water-soluble polysaccharides, have emerged as indispensable agents in this pursuit, showcasing their efficacy in improving the rheological properties, water retention capacity, stability, and mechanical properties of concrete.

This comprehensive review has unveiled the history and classifications of VMAs, emphasizing their critical role in modifying rheology and addressing common issues such as bleeding and segregation, particularly in specialized concrete applications. Furthermore, the paper has explored environmental concerns linked to synthetic VMAs and advocates for the exploration of natural sources. Plant-based biopolymers like cellulose, starch, alginate, pectin, and carrageenan have been discussed, shedding light on their effectiveness in enhancing concrete properties. In summary, this review serves as a valuable resource for understanding past research endeavors and offers insightful recommendation for future investigation aimed at fostering more efficient, eco-friendly, and cost-effective construction practices. It highlights the importance of leveraging natural biopolymers as alternatives to synthetic VMAs, thereby contributing to sustainable development in the construction industry. This review has yielded the following conclusions:

• The incorporation of cellulose ethers into cementitious materials enhances viscosity, water retention capacity, and shear-thinning behavior. However, the effectiveness of this enhancement depends on various factors, including the type of the cellulose ether, its concentration and molecular properties, the intrinsic parameters of the cement paste, and the type of the HRWR.
• PMS exhibits superior compatibility with cellulose-based VMAs in comparison to PNS.
• The behavior of starch in concrete can vary according to its origin, the length of its molecular chains, whether it is modified or not, and the modification’s type.
• The behavior of starch varies depending on the HRWR it is combined with. It exhibits indeed a lower affinity with polycarboxylate (PCE)-based superplasticizers compared to Welan gum.
• Starch exhibits promise in enhancing shear-thinning behavior, apparent viscosity, and yield stress and reducing spreading diameter, presenting itself as a viable alternative to conventional VMAs such as Welan gum.
• Alginate enhances workability by reducing spreading diameter and improving water retention in concrete. However, its inclusion reduces hardened-state properties due to weakened aggregate–cement paste bonds and increased water retention, leading to higher water demand and increased porosity.
• Alginate serves also as a concrete durability enhancer, particularly as a self-healing agent for cracks, and has the ability to reduce ion diffusion.
• Sodium alginate serves as an effective viscosity-modifying admixture in fluid concrete, surpassing raw brown seaweed alginate in performance due to its higher purity despite a higher cost.
• Pectin enhances the viscosity of cement suspensions and their water retention capacity, in addition to concrete’s plasticity and workability.
• Pectin’s improvements are due to calcium ion bindings, and promoting intermolecular associations through calcium bridges, but it depends, however, on the concentration of pectin and Ca²⁺.
• The influence of carrageenan biopolymers on cementitious suspensions varies not only according to the type and dosages used but also according to the presence or absence of HRWRs.
• Incorporating kappa-carrageenan in cement pastes results in a notable delay in setting, projected onto the reduction of the compressive strength due to reduced ion concentrations, which is further prolonged with increasing dosage, but this delay is mitigated by the addition of both kappa- and iota-carrageenan, leading to a reduction in setting time.
• K. alvarezii enhances the viscosity and stiffness of cement-based materials, with comparable performance to kappa-carrageenan at a dosage of 0.5%, although kappa-carrageenan exhibits a superior enhancement of rigidity and build-up kinetics. Unlike kappa-carrageenan, the seaweed causes no delays of the setting time.

On the other hand, future research could concentrate on refining extraction methods, evaluating environmental impacts, and optimizing the synthesis and production processes of plant-based biopolymers. This focus would enhance their scalability and sustainability. Exploring new sources like agricultural waste or marine resources could broaden options and address supply chain challenges. Investigating the complex interactions between these biopolymers and cementitious matrix ions could optimize their performance in concrete formulations, leading to more efficient, eco-friendly, and cost-effective construction practices.