The use of low-carbon supplementary cementitious materials (SCM), such as calcined clays, to replace cement clinker has been recognized by the Cement Industry to achieve reductions in greenhouse gas emissions. This paper investigates eight low-grade clays, with <20% kaolinite, obtained from different geological formations, that have been calcined to produce potential SCMs. The clays were characterised before and after calcining at 750, 800, 850 and 900 °C, and the mineralogical changes and amorphous phase contents determined. The pozzolanic activity and the strength activity index of the different calcined clays were evaluated. The results show that calcined clays from the Oxford and Ampthill geological formations in the UK can produce SCMs with pozzolanic activity higher than conventional SCMs such as PFA. These clays were rich in illite and smectite and produced ∼60% amorphous phase when calcined at 850 °C. Mortars produced using calcined clays had higher compressive strengths than mortars containing pulverised fuel ash and achieved ∼90% of the compressive strength of 100% Portland cement mortar samples at 28 days. The research demonstrates that low-grade clay resources can be calcined to produce SCMs and that these can be used to form cementitious materials with reduced total associated CO2 emissions.