Roles of recycled fine aggregate and carbonated recycled fine aggregate in alkali-activated slag and glass powder mortar

Alkali-activated materials have attracted a lot of research interest due to the advantage of less CO₂ emission than ordinary Portland cement (OPC) systems. The use of recycled aggregate or carbonated recycled aggregate in new concrete is considered a good way to solve the problem of construction and demolition waste. This study aims to investigate the influences of recycled fine aggregate (RFA) and carbonated RFA (CRFA) on the properties of alkali-activated slag and glass powder mortar (AASGM). The macro properties (compressive strength, workability, setting time, and shrinkage) and microstructural properties of AASGMs prepared with RFA/CRFA were evaluated. The hydration evolutions of AASGMs and the reactions between RFA/CRFA and sodium silicate were investigated to explore the reaction mechanisms of AASGM with RFA/CRFA. The results showed that with the increase in RFA content from 0 % to 100 %, the flow value of AASGM decreased from 240 mm to 145 mm, the initial setting time was shortened by 44 %, the 7 days autogenous shrinkage decreased by 82 %, the 25 days drying shrinkage decreased by 31 %, and the 28 days compressive strength firstly increased (RFA content ≤ 50 %) and then decreased. On the contrary, with the increase in CRFA content from 0 % to 100 %, the flow value of AASGM increased from 240 mm to 270 mm, the final setting time increased by 37 %, and the compressive strength decreased by 97 %. The roles of RFA and CRFA in AASGM were different from their roles in OPC-based materials. That was because the calcium hydroxide in RFA could react with sodium silicate (SS) and increase the concentration of alkali, which accelerated the polymerization reaction of AASGM. However, the calcium carbonate and silica gel in CRFA reacted with SS and reduced the concentration of alkali, and thus the polymerization reaction of AASGM was decelerated.