Effect of silicate modulus and alkali content on the microstructure and macroscopic properties of alkali-activated recycled powder mortar

Abstract

The use of recycled powder (RP) as a green raw material to prepare alkali-activated materials provides a promising approach for the sustainable utilization of construction waste. However, limited attention has been paid in the literature to the impact of silicate modulus and alkali content of alkaline activators on the performance of alkali-activated recycled powder mortar (AAM). Therefore, this study investigates the influence of different silicate modulus and alkali content on the microstructure, mechanical properties, water absorption and porosity of AAM. The results revealed that the RP exhibits irregular shapes, comprising hydrated products and inert materials like quartz and calcite, and can participate in polymerization reactions under the influence of alkaline activators. Suitable silicate modulus and alkali content can promote further dissolution of RP to form C-A-S-H/N-A-S-H gels, which benefits the development of the alkali-activated recycled powder paste microstructure. When the silicate modulus is constant, the mechanical performance of AAM initially improves with increasing alkali content, but eventually decreases beyond a certain threshold, especially at an alkali content of 8%. The 28-d compressive strength of M1.8N8 decreased by 4.92% compared to M1.8N6. Meanwhile, water absorption and porosity decline with increasing alkali content. When alkali content is constant, increasing the silicate modulus is beneficial for the development of the microstructure of the alkali-activated recycled powder paste, and improves the AAM mechanical performance. The 28-d flexural strength of N6M1.8 increased by 17.63% compared to N6M1.2. The addition of supplementary cementitious materials (SCMs) can reduce the porosity and water absorption of AAM and significantly improve its mechanical properties, with the best performance enhancement achieved by metakaolin (MK). Therefore, RP can be used as a raw material to prepare environmentally friendly alkali-activated materials with excellent macroscopic performance.