Utilisation of GGBS and Fly Ash as Supplementary Cementitious Materials in Concrete: A Review of Properties and Sustainability

The production of Portland cement is one of the largest industrial sources of carbon dioxide, and partial replacement of cement clinker by supplementary cementitious materials (SCMs) is the most practical way of reducing the carbon footprint of concrete.Ground granulated blast furnace slag (GGBS) and fly ash are the two SCMs that are available in the largest quantities and are already accepted in standards and practice.This review summarises published research on the use of GGBS and fly ash in concrete.The origin, chemical composition and reaction mechanisms of both materials are first described.Their influence on the fresh properties, compressive and tensile strength, and durability of concrete is then discussed, followed by the role of curing, the use of prediction models, and the environmental and economic benefits.The literature shows that both materials improve workability and long-term strength, refine the pore structure and increase resistance to chloride and sulphate attack, while early-age strength is usually reduced and carbonation resistance may be lower if curing is inadequate.GGBS is latent hydraulic and gains strength faster, whereas fly ash reacts more slowly through the pozzolanic reaction and gives better workability and lower heat of hydration.Adequate moist curing is essential for both.The review concludes that GGBS and fly ash can routinely replace 30-50% and 20-35% of cement respectively in structural concrete, and identifies the need for performance-based mix design, data-driven strength prediction and field durability data as directions for future work.