Microstructure Evolution of Cementitious Matrices Utilising Graphite Mine Tailing as Supplementary Cementitious Materials

The tailing used in this study, is a by-product of graphite mining and is generated from the floatation process of the finer ore concentrate. It is predominantly silicate and was provided in dry state which comprised a few lumps up to a size of 20 cm. The mineralogy of the mine tailing was studied, based on which, a treatment process of calcination at 850 ℃ for 2 h was adopted. This study examines the microstructure of hydrated cementitious pastes with 10%, 20%, and 40% treated mine tailing as ordinary Portland cement (OPC) replacement and with w/c ratios of 0.3, 0.4, and 0.5. Microstructure was examined after 7, 14, 28, 56, and 90 days of curing. Calcium hydroxide (C-H), calcium silicate hydrate (C-S-H), ettringite, biotite, and quartz were found in cementitious paste with mine tailing samples. Though the hydration products C-H, ettringite, and C-S-H were present in both OPC and OPC-mine tailing mixes, the C-S-H morphology in OPC-mine tailing mixes was different. Higher silica content in mine tailing created a constrained space, resulting in formation of a crumpled foil C-S-H. In OPC-mine tailings samples, C-H nucleation was also observed in later ages such as 56 days indicating secondary hydration. This also results in more compact matrices at the later ages. These findings together with the findings of FTIR and XRD studies give an insight into the hydration of cementitious pastes containing mine tailing.