A Novel, Eco-Friendly, and Cost-Effective Calcium Hydroxide Treatment for Sustainable Sisal Fiber-Reinforced Geopolymers

This paper explores a new, sustainable, and economical approach to increase the compatibility between natural fibers and geopolymeric matrices by using a calcium hydroxide Ca(OH)2 solution. Untreated and treated sisal fibers have been used as reinforcement in geopolymers, replacing 2 wt% of the river sand aggregate. To assess the effect of treatment duration on the properties of the resulting geopolymer composites, sisal fibers have been specifically soaked in an aqueous solution (2 g/L) of calcium hydroxide for 24, 48, and 72 h. The tensile properties and the thermal resistance of sisal fibers were investigated, together with their chemical composition and morphological features. The workability of fresh geopolymer materials was indirectly evaluated in this study by determining the initial and final setting times via needle penetration testing. Binary optical photographs were processed with image analysis software to quantify the porosity content of geopolymers. Quasi-static flexural, compression, and Brazilian (indirect tensile) tests were conducted on all geopolymer composites and the neat geopolymeric matrix. Experimental results demonstrated that, although slightly reducing fiber tensile properties, the proposed alkaline treatment improved the mechanical response of the resulting composites due to enhanced fiber-matrix compatibility, particularly toughness and post-peak behavior, regardless of loading configuration. Notably, the most favorable outcomes were achieved by treating sisal fiber in calcium hydroxide solution for 24 h.