Preparation and characterization of unfired lightweight bricks using dealkalized calcium silicate residue from low-calcium sintering red mud

In order to achieve the full utilization of red mud, the unfired lightweight bricks were prepared by compaction molding technology using dealkalized calcium silicate residue, cement and sand, and the effects of raw material formula and molding pressure on the physical properties and phase transition of unfired bricks were investigated by X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) methods. The dealkalized calcium silicate residue presented as irregular honeycomb particles is mainly composed of calcium silicate hydrate (C-S-H) and some CaTiO3, CaCO3 and SiO2. Increasing the content of calcium silicate residue increases the compressive strength of unfired bricks and reduces bulk density, but excessive calcium silicate residue reduces the compressive strength and softening coefficient, and increases water absorption. When the consumption of calcium silicate residue reaches 50%-70%, the compressive strength, bulk density, softening coefficient and water absorption of the unfired bricks are in the range of 15.10-24.16 MPa, 1.3-1.5 g/cm(3), 0.77-0.91 and 28.41%-43.52%, respectively. During the curing process, the calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) gels produced by hydration reactions flocculate the raw material particles together to strengthen the strength of the unfired bricks. The leaching results of heavy metals and Na ions show that the unfired lightweight bricks are harmless to the environment.