Gravel wash fines with mixed mineralogy as sources of supplementary cementitious material in cement mortars

Given the scale of concrete production worldwide there is a need to reduce the significant carbon footprint of concrete by replacing ordinary Portland cement (OPC) binder with supplementary cementitious materials (SCMs). For several decades, waste by-products such as fly ash have had widespread use as SCMs, however with the replacement of coal burning power stations being replaced by renewable energy systems, emphasis now is on alternative SCMs such as locally available calcined clay resources. Calcined clay minerals have demonstrated potential as SCMs, but generally high purity (e.g., >40 % kaolinite) materials have been reported to be most effective at cement replacement. Three local gravel wash fines (GWF) products, a waste accumulated during the washing of coarse aggregates at quarry plants, having high clay content and distinct mineralogy, were studied as potential SCMs. The GWF materials contained ≈ 40 % clay content comprising a mix of kaolinite, smectite and illite, and were calcined at 400–800°C prior to assessment as prospective SCMs using thermogravimetric analysis, Fourier transform infrared spectroscopy, X-ray diffraction, electron microscopy and mechanical testing. The calcination temperature to induce effective pozzolanic behaviour depended on the mineralogy of the GWF: 700°C for a smectite-rich GWF, 600°C for a GWF co-dominated by kaolin-smectite and as high as 800°C for a GWF in which illite and kaolinite were co-dominant. Replacement levels in mortar mixes as high as 40 wt% of cement could be achieved that attained strengths comparable to, or greater than control mortar. Analytical results on the phase development in mixes up to 40 wt% replacement of OPC with calcined gravel wash fines (CGF) revealed pozzolan reactivity as evidenced by consumption of portlandite and the formation of calcium silicate hydrates. Cement replacement with CGF reduced the embodied carbon per unit compressive strength of mortar by as much as 40 % compared to OPC. As such locally available sources of GWF can be useful for low embodied carbon alternatives to OPC. • Gravel wash fines (GWF) were thermally activated into pozzolanic materials, with calcination temperature varying with mineralogy. • A modified mortar mix design used reactive GWF clay to replace OPC, while inert GWF fraction replaced fine aggregate. • Mortars with calcined GWF (CGF) showed higher strength than control at 40 % OPC replacement and similar strength at 50 %. • The embodied carbon per unit compressive strength of CGF-based mortars was reduced by 35–40 % compared to OPC-based mixes.