Effect of construction demolition waste as fine aggregate and NaOH molarity on strength and fracture parameters of slag based geopolymer mortars

Abstract To achieve sustainable development, it is imperative that we find ways to manage sand resources wisely. One approach to mitigate the impact of sand extraction is the recycling of construction demolition waste (CDW) and its reuse in construction activities. This research paper primarily delves into the investigation of the strength and fracture properties of slag-based geopolymer mortar (SGPM) while varying the molarities of sodium hydroxide and altering the proportions of recycled fine aggregate (RFA) sourced from CDW. Two distinct binder-to-fine aggregate mix ratios, namely 1:1 and 1:3, were taken into consideration. The study employed sodium hydroxide (NaOH) and sodium silicate solutions as alkali-activators. Various NaOH concentrations of 2 M, 4 M, 6 M, 8 M, and 10 M were utilized, and RFA was employed to replace natural fine aggregate at levels of 10%, 20%, 30%, and 40%. The workability of SGPM was assessed in terms of flowability, while the compressive and flexural strengths were measured following a 28-day curing period. Moreover, fracture parameters, including load–deflection behavior, peak load, deflection at peak load, and fracture energy, were investigated through a three-point bending (TPB) test conducted on notched beams. The test results revealed that the flowability, expressed as a percentage of flow, decreased as the molarity of NaOH and the percentage of RFA increased. Conversely, the compressive and flexural strength values exhibited an upward trend as the NaOH concentration increased from 2 to 10 M. A substantial improvement in both compressive and flexural strengths was observed at a 10 M NaOH solution, with increases of 20.73% and 10.63% in the 1:1 mix ratio and 18.94% and 10.46% in the 1:3 mix ratio. However, these values decreased with an increase in the percentage of RFA, ranging from 10 to 40%. Notably, specimens containing 8 M and 10 M NaOH concentrations demonstrated a more brittle behavior compared to specimens with other NaOH concentrations. Moreover, a decline in fracture energies was evident as the molarity of NaOH and the percentage of RFA increased. These findings highlight effect of NaOH concentration, RFA content on mechanical and fracture properties of SGPM, providing valuable insights for sustainable construction materials.