Enhanced compressive strength and durability of low-molarity alkali-activated concrete with recycled aggregates: An experimental and response surface methodology approach

Abstract

The construction industry faces increasing challenges related to environmental impact, including high carbon emissions from Portland cement production and the management of construction and demolition waste. This study addresses these issues by developing sustainable alkali-activated concrete (AAC) using low-molarity (2M) activators, incorporating fly ash and ground granulated blast furnace slag (GGBS) as binders, and exploring the use of recycled coarse aggregates (RCA) as a replacement for natural aggregates (CA). The objective was to evaluate the influence of varying sodium silicate-to-sodium hydroxide (SS:SH) ratios on the compressive strength properties, durability, and sustainability of AAC. Nine mixes were designed with SS:SH ratios ranging from 1.5 to 3.5, incorporating both CA and RCA in varying proportions. Compressive strength, water absorption, and sulphate resistance were assessed, with specific emphasis on the role of SS:SH ratio in mitigating the weaknesses of RCA. Response surface methodology (RSM) with central composite design (CCD) was also employed to analyse the combined effects of the SS:SH ratio (1.5 to 3.5) and RCA content (0% to 100%). Experimental results revealed that higher SS:SH ratios significantly improved performance. M3 (SS:SH = 3.5, 100% CA) achieved the highest compressive strength of 34 MPa, superior impact resistance, and excellent sulphate resistance with a residual strength retention of 92 percent. RCA mixes, while exhibiting higher porosity and reduced initial performance, showed substantial improvements with optimized SS:SH ratios. M9 (SS:SH = 3.5, 100% RCA) demonstrated balanced properties, achieving a compressive strength of 28 MPa and retaining 87 percent of its strength under sulphate exposure. The optimal mix identified through RSM CCD consists of an SS:SH ratio of 3.32 and 48.6% RCA. This mix achieved the best balance between sustainability and performance, demonstrating compressive strengths above 30.09 MPa, residual sulphate strength retention of 85.51%, and superior impact resistance, water absorption, and durability. The study highlights the critical role of SS:SH ratio in enhancing matrix densification, mitigating the limitations of RCA, and ensuring the durability of low-molarity systems. These findings promote the application of alkali-activated concrete in sustainable construction, enabling the integration of RCA in structural elements without compromising performance or durability.