Low carbon dioxide binders with cement-equivalent strength: a spectroscopic hydration study of limestone–calcined clay cement (LC3)

Low-grade calcined clays exhibit phase evolution in limestone–calcined clay cement (LC³) systems that differs from conventional metakaolin. In this study, LC³ binders with different clinker contents (50–90%) at a constant water/binder ratio of 0.485 were developed to investigate the effects of clinker reduction in two phases. In phase I, mortar samples were characterised using X-ray diffraction and Fourier transform infrared spectroscopy to monitor the hydration products. Mortar specimens were also tested for compressive strength up to 90 days. Although the early-age strength was reduced owing to the absence of performance-enhancing additives, significant later-age strength gains were observed. These gains correlated with the increased formation of hemicarbonate and monocarbonate phases and enhanced development of calcium silicate hydrate gel. In phase II, concrete mixes were produced using the optimal binder (based on compressive strength) from phase I. These mixes were evaluated for compressive strength, embodied carbon dioxide emissions (CDE) and cost. CDE and cost analyses were conducted using constituent-based factors applied to actual mix proportions. The ACI strength prediction model accurately estimated the LC³ concrete strengths within 10% of experimental results. The concrete made with the optimal LC³ achieved comparable later-age strength to concrete made with ordinary Portland cement, at 11% lower cost and with a 26% reduction in CDE, demonstrating the viability of sustainable, low carbon dioxide cement using low-reactivity clays.