Soils contaminated with hydrocarbons, per- and polyfluoroalkyl substances (PFAS) and other organic contaminants are emerging problems for countries around the world, with global estimate of 7000 Mt requiring treatment, of which, 100,000 sites are in the EU alone. Destruction of these contaminants via heat-treatment also valorises the waste soil by activating latent pozzolanic properties, through calcination. To divert these wastes away from landfill, this study developed a modified mix design employing valorised heat-treated soil (HTS) as partial cement replacement in mortars. A previous study reported need to re-calcinate HTS to induce pozzolanic behaviour. However, after adjusting the mix design to account for reactive fines, as-received HTS returned strength activity index test (SAI) of 93% that of the compressive strength of a control Portland cement mortar at 7 days. Consequently, higher replacement levels with HTS were achieved. At 30 wt% HTS, the compressive strength was 72% that of OPC, while it was only half the strength of OPC at 40 wt% replacement. The pozzolanic activity of HTS mortar mixes at different replacement ratios was assessed by FTIR analysis indicating differences in portlandite, calcium silicate hydrate, and carbonate formation compared to reference mortars. The results herein refute the need for recalcination of HTS used as a pozzolan as considered in a previous study, thereby eliminating additional embodied carbon. Utilisation of valorised HTS in cementitious systems may simultaneously divert otherwise waste materials away from landfill and enable the localised production of low embodied carbon cement and concrete materials.