Repurposing Basaltic Laterite Waste as a Potential Supplementary Cementitious Material or Fine Aggregate in Portland-Limestone Cement–Based Mortars

Abstract This study investigates the feasibility of using basaltic laterite (BL), generated as a waste from Northern Ireland’s aggregate industry, as a supplementary cementitious material (SCM) and/or a fine-grained aggregate in portland cement (PC)-based construction mortars. BL was pulverized and calcined (BL_C) at 700°C, before being blended with Limestone-PC at 10%–50% replacement by mass. The calcination process resulted in the dehydroxylation of secondary mineral phases within the BL, but not of the aluminosilicate clay weathering products. The engineering performance of mortars was assessed based on compressive strength ( f c ) and ultrasonic pulse velocity (UPV) measurements after 7 and 28 days of curing, with insights gained into changing microstructure, geochemistry and mineralogy. Using 10%–21% BL_C content had relatively little effect on 28-day f c or UPV values; whereby strengths ranged between 33 and 35 MPa. However, increasing BL_C content beyond 21% caused significant reductions in mechanical strength, whereby C─S─H formation was inhibited and the BL_C content served more as a microfiller agent. Sigmoidal trends were observed between f c , BL_C content, and corresponding CaO, SiO 2 , Al 2 O 3 , and Fe 2 O 3 compositions. A new exponential empirical correlation has been proposed for predicting f c based on UPV measurements, which has superior performance over previously published correlations. In summary, BL shows potential as an SCM at low replacement levels, particularly 10% which maintains strength and reduces cement content. However, using higher replacement levels ( > 30 % ) reduced the strength and density of mortars, accompanied by an increase in porosity. This highlights that BL pretreatment requires optimization through thermal and mechanochemical activation methods. Furthermore, the slightly coarser particle size distribution of the BL_C considered in this study meant that the material provided more microfiller effects, rather than purely functioning as an SCM.