This project examines the leaching behaviour of elements from cementitious materials incorporating supplementary cementitious materials (SCMs), focusing on their environmental and functional impacts. Historically, pulverised fly ash (PFA) has been used for LLW encapsulation. Produced as a fine by-product of coal-fired power stations, PFA serves as a supplementary cementitious material (SCM). However, with the UK’s Net Zero legislation requiring the closure of coal-fired power plants, a suitable alternative SCM is needed to maintain or improve the grout’s mechanical and chemical properties.