Reduce the cost and embodied carbon of ultrahigh performance concrete using waste clay

Recent research has recognized limestone calcined clay cement (LC3) a promising sustainable cementitious material. While LC3 could be used in a wide range of applications, it is important to explore its feasibility in developing (ultra-)high performance concrete. In this paper we attempt to produce ultrahigh performance LC3 concrete (UHP-LC3). With 20%, 30%, and 40% OPC replaced by coal gangue waste calcined clay and limestone (LC2), the mechanical, hydration, and microstructural properties were extensively studied. The mechanical properties were investigated by compressive and flexural tensile tests. The hydration behavior of UHP-LC3 was evaluated by thermogravimetric analysis (TGA), quantitative X-ray powder diffraction (QXRD) analysis, 29Si nuclear magnetic resonance (NMR) spectroscopy, and scanning electron microscope (SEM). Finally, the microstructure of UHP-LC3 was studied by mercury intrusion porosimetry (MIP). Results show that the substitution of LC2 improve the early strength by quickly pozzolanic reaction to form denser hydration products as well as the synergistic interaction between silica fume and limestone/calcined clay in the system. However, the long-term strength would drop as LC3 caused insufficient hydration reaction due to the high water-demand of calcined clay that reduced the amount of free water. Also, there are abundant unreacted calcined clay particles in the hydrated system. Meanwhile, LC3 substitution of OPC seems to improve the polymerization of C-A-S-H gel and densify the pore structure, thus display excellent carbonation resistance. This paper explores new options to prepare more sustainable and cost-effective UHPC.