## 3 Plant-Based Biopolymers
### 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, Macrocytsis, 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 [PERSON] 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 \(\alpha\)-L-guluronic acid (G) and 1,4-linked \(\beta\)-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  
Figure 9: Chemical structure of alginate [from PubChem Database]. 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.