Valorization of Cashew Industry Waste into Anacardate-Based Green Foaming Agents for Foam Concrete Applications

Traditional foaming agents and foam concrete rely on synthetic chemicals and ordinary portland cement (OPC) with a high carbon footprint. This paper presents the formulation of novel anacardate-based green foaming agents derived from cashew industry waste to address the environmental concerns associated with synthetic surfactants. Unsaturated and saturated anacardic acids were extracted and modified using sodium and potassium-based alkalis to synthesize anionic surfactants. The salts of unsaturated anacardic acid-ene mixture demonstrated superior foaming capabilities over their saturated counterparts, which exhibited unfavorable gelling behavior in an aqueous solution. This is beneficial considering the economy and sustainability; as an intermediate, extracting unsaturated anacardic acid is straightforward, less energy-intensive, and more cost-efficient than extracting saturated anacardic acid. Disodium anacardate (unsaturated) yielded foam with favorable density and stability but required further stabilization for use in foam concrete. Thus, three polymer admixtures, namely, xanthan gum, guar gum, and sodium carboxymethyl cellulose (Na-CMC), were investigated. Xanthan gum showed maximum viscosity enhancement of foaming solution and foam stabilization, followed by Na-CMC and guar gum. The compatibility of the stabilized disodium anacardate (unsaturated) foaming agents with both OPC and geopolymer binders was evaluated in terms of reaction kinetics, fresh-state stability, compressive strength, and water absorption. Isothermal calorimetry results confirmed that the anacardate-based foaming agent did not retard binder reactions, irrespective of the admixture used. Further, geopolymer foam concrete exhibited higher compressive strength and lower water absorption than OPC-based mixes at comparable densities, indicating a better synergy. Thus, anacardate-based foams offer a viable, sustainable alternative to synthetic surfactants in foam concrete.