Size-Dependent Chemical Speciation in Cementitious Quarry Waste

Abstract

Quarry waste, especially adjacent to cementitious rock formations, presents an underutilized mineral resource given that geo-processes generate gradients in composition across vast scales. We inferred, and demonstrate, that mechanical breakdown depends on chemical composition; hence, chemical speciation can be correlated to particle size. Using samples derived from Thika, Kenya, we evaluate composition and morphology and their potential as modified cementitious materials. Raw samples were mechanically sized using various filters (75–1180 μm) and both bulk and surface composition mapped. Results indicate that size-fractionated samples have a narrow particle size distribution dominated by intermediate fractions. We confirm this distribution using tomographic imaging of packed beds and in-situ sizing across the packed bed. An inverse correlation between SiO2 and CaO contents across the particle sizes indicates pronounced size-dependent compositional heterogeneity, although all fractions exhibit comparable crystalline phases. The overall chemical composition broadly aligns with that of Ordinary Portland Cement (OPC), albeit at lower CaO concentrations. Morphological analysis indicates substantial variability arising from repeated fracture events during comminution. Calcium distribution shows marked size dependence, with negligible surface concentrations at 1180 and 106 μm, and elevated atomic percentages at 75 μm (4.18 at. %) and 150 μm (3.94 at. %). The 106 μm fraction demonstrates the most uniform and densely packed microstructure among the samples examined. Calcination induces partial amorphization of clay constituents and modification of particle surface characteristics. Collectively, these findings indicate that felicitous choice of rock form and sizing conditions, coupled with thermal activation can elevate quarry waste into a precursor cementitious material, especially when appropriately modified. In the current sample, the addition of lime followed by calcination brings the composition to that of pozzolanic materials appropriate for clinker formation.