Suitability of excavated London clay as a supplementary cementitious material: mineralogy and reactivity

This study evaluated the potential of producing supplementary cementitious materials (SCMs) using London Clay excavated from construction activities of the High Speed 2 rail project. A trade-off between enhancing reactivity versus decomposition of impurities (e.g., pyrite, carbonates) present in different London Clay samples was considered in selecting the calcination temperature. The additional reactivity obtained by calcining at 800 °C is deemed to be worth the cost of the small additional process emissions from decomposition of carbonate minerals. Blended cement formulations were developed with the produced SCMs, with replacement levels of 50 and 70 wt%. The optimal gypsum dosage was found to be 1 wt%, which resulted in improved reaction kinetics at early ages. Mortars produced with these binders developed ~50 MPa compressive strength after 90 days of curing even with 70 wt% replacement, which is sufficient for potential production of low to medium strength concretes. These findings demonstrate the excellent potential of London Clays for SCM production and present a systematic approach for characterisation, processing and utilization of excavated mixed clays obtained from infrastructure projects.