Sustainable use of recycled sand in alkali-activated cement and Portland cement mortars: A comparative study with natural sand

The recycling and application of recycled aggregate from building demolition would broad the accesses to fine aggregate for construction industry and relieve the pressure of natural sand (NS) shortage. This work investigates the workability, compressive strength and alkali-silica reaction (ASR) of Portland cement (PC) and alkali-activated cement (AAC) mortars containing NS and recycled sand (RS). Compared to NS, the higher water absorption characteristic of RS leads to the decrease flowability of fresh mortars. The compressive strength of AAC mortar containing RS is comparable to that of AAC containing NS, while the compressive strength of PC mortar containing RS presents obvious decrease. Pore structure analysis shows that the use of RS has negative impact on the densification of microstructure. This is more obvious in the PC system, which is one of the reasons for the decrease of strengths. It can be found that, with the use of RS, the lower alkalinity and less available calcium in pore solution are beneficial for the mitigation of ASR in PC system. While the soluble aluminum species contributes to the rapid formation of gel layer covered on the RS surface in AAC system, and thereby inhibiting the further dissolution of silica from NS and controlling of ASR expansion. Meanwhile, the application of RS can achieve considerable reduction in total energy consumption, carbon emission and manufacture cost. Such study would offer insights into the application of recycled fine aggregate in construction field.