Using diatomite as a partial replacement of cement for improving the performance of recycled aggregate concrete (RAC)-Effects and mechanism

Abstract

This work proposes that diatomite can be used to improve the performance of recycled aggregate concrete (RAC). A calcined diatomite sample with an amorphous silica content over 90% was added to the RAC mixture as a partial replacement of the ordinary Portland cement (OPC) at varying rates from 0 to 20 wt%. Test results show that both the strength and impermeability of RAC can be obviously enhanced with the greatest enhancement observed at a replacement level of 15 wt% for which the 28-day compressive/splitting tensile strengths were improved by 29.2% and 33.9%, respectively, and the chloride diffusion coefficient was reduced by 86.27%. The microscopic measurements confirmed the nucleation effects and the high pozzolanic reactivity of diatomite which both contributed to the more complete hydration of cement in RAC. It was also found that the proportion of large capillary pores in the new mortar was reduced by using diatomite, meanwhile the average porosity of the new interfacial transition zone (ITZ) became smaller. This work suggests that the calcined diatomite used for improving RAC performance should be no larger than 15 wt% of cement, as the scarcity of CH at a higher replacement ratio would hamper the pozzolanic reactions, meanwhile the diluted cement clinkers would have a negative effect on the pore structure of RAC.