PERFORMANCE OF ALKALI- ACTIVATED HYBRID CEMENT MORTAR INCORPORATING FLY ASH AND CONCRETE WASTE POWDER

Abstract

This paper explores the performance of alkali-activated hybrid cement mortars, which use fly ash (FA) and recycled concrete waste powder (CWP) as partial substitutes for ordinary Portland cement (OPC). Mortars were mixed with the ratio of water to binder of 0.45 and binder to sand of 1:2.75, and sodium silicate (Ms = 1.5) was used as an alkaline activator. Flowability, compressive and flexural strength, porosity, water absorption, and microstructure (SEM) were tested. This led to a finding that alkali activation had a major impact on mechanical and durability properties. But contrary to the anticipated constant increase in strength, mix F5P5 (50 FA + 50 OPC) demonstrated a decline in compressive strength during 7 to 28 days, which suggests slower strength development or even a decrease, although it had a lower porosity and water absorption than the reference mortar that had not been activated. Incorporation of CWP resulted in reduced strength and higher porosity and absorption at 7 and 28 days. This is explained by the fact that CWP is less reactive, is high in CaCO₃, and is prone to forming micro-voids and particles that are not reactive in the matrix. SEM analyses confirmed that activated mixes formed denser gels, while CWP-containing mixes exhibited discontinuous gel phases and weak interfacial zones, which negatively affected long-term durability.