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, pumping, 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[7; 8]. Based on their physical effects on cement-based materials, VMAs can be classified into five classes (by [PERSON]), as shown in Table 2[7]. Moreover, based on their chemical origin and composition, VMAs can be classified into three main groups, presented in Table 3, as proposed by [PERSON] [6; 9]. \begin{table}
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**Mechanisms of Action** & **Definition** \\ \hline 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. \\ 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. \\ 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. \\ Within the system, a surge in osmotic pressure occurs, leading to flocculation, as nonadsorbed polymers are drawn away from a “volume exclusion shell” enveloping 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. \\ 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. \\ \hline \end{tabular}
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Table 1: Mechanisms of action of VMAs. 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.