Recycling of Concrete Demolition Waste Powder as a Sustainable Material in Portland Cement Pastes Modified with Nano-silica

Abstract

In recent years, researchers dedicated themselves to explore the possibility of introducingconcrete waste powder (CWP) into Portland cement as a sustainable material for Portland cement to reduce as possible the environmental pollution with this concrete demolition wastes. To optimize the effect of CWP on cement-based materials, this paper usesnano-silica (NS) to improve the hydration and mechanical properties of cement-based materials with CWP. Results indicated that after adding NS into CWP cement pastes, the setting time of cement pastes was significantly reduced, while the early rate of hydration and hydration heat increased. Besides, the mechanical strengths of the cement pastes increased as CWP replaced at the expense of the cement only up till 15 weights %, and then decreased. The addition of nanosilica (NS) can compensate the mechanical strength loss caused by CWP as supplementary cementing materials. The higher ratios of CWP than 15 weight % increased the pore volume or porosity. In contrast, NS in CWP blended cement significantly decreased the porosity, and increased the proportion of harmless pores. Hence, NS reduced the porosity, which in turn improved and enhanced the bulk density and mechanical properties. The optimum amount of NS is 2.5 weight % which resulted in the best results. The heat of hydration of the different cement batches in the two groups adversely affected with the incorporation of CWP, but little improved with NS. The obtained results were confirmed by the ultrasonic pulse velocity (USPV) test, where the cement batch PP15 (Group I) and PS2.5 (Group II) achieved the highest conformance.