Suitability of Excavated London Clay from Tunnelling Operations as a Supplementary Cementitious Material and Expanded Clay Aggregate

London Clay is a marine geological formation that is relatively abundant in the London Basin area in the United Kingdom. It primarily originates from reworked Jurassic shale, greensand and chalk and lateritic Eocene soils with a mineralogy consisting of kaolinite, illite, chlorite and smectites. As part of a major tunnelling project in the United Kingdom, significant volumes of London Clay will be excavated in the wider London area. The excavated material would normally be treated as waste, sent to landfill or incorporated into landscaping. Instead, a method is developed to turn the excavated London Clay spoil into a construction resource such as supplementary cementitious material (SCM) and/or lightweight aggregate. This paper reports the developments within the Re-Purposed Excavated Arisings Loop (REAL) research project aiming to transform London Clay spoil into such construction resources. It encompasses investigations on the plastic, microstructural and chemical properties of raw and calcined London Clays, as well as hardened concrete and mortar properties produced with those clays. The material characterisation results demonstrated that London Clay, in its calcined form, can be a suitable SCM for low/medium strength concrete. The REAL project aims at developing robust concrete mixes with compressive strengths of up to 40 MPa, and Portland cement replacement levels of up to 70%. Additionally, the possibility of producing expanded clay lightweight aggregate from excavated London Clay is also explored. Preliminary lab test results indicate material suitability for certain construction applications—an important step towards a circular economy approach for large construction projects.