Abstract

The low-temperature preparation of belite (β-C2S) from a mixture of lime and white sand (Ca/Si=2) in presence of 0.5-5 M NaOH solution was investigated by hydrothermal treatment in a stainless steel capsule at 135 ̊C for 3 hours followed by calcination at 1000 ˚C for 3 hours. All materials were analyzed by FTIR, SEM-EDX, and XRD with semi-quantitative phase analysis calculation. The addition of NaOH changed the composition of the hydrothermal and calcination products. Different percent of β-C2S and other calcium and sodium silicate phases were produced with increasing the concentration of NaOH. In presence of 0.5M NaOH, 32.6% of β-C2S formed with the formation of calcium silicate with mole ratio CaO/SiO2=1.5 (rankinite, 30.9%) and sodium-calcium silicate (combeite, 8.6%). Whereas, in the presence of 5M NaOH, the hydrothermal reaction between lime and silica was effectively deactivated i.e. 32.9% of β-C2S formed. The formation of sodium silicate (31%), rankinite (15.4%) and sodium-calcium silicate (16.7%) phases were encouraged. The optimum condition for preparation of low-temperature β-C2S rich cement from lime and sand under these conditions can be achieved by the addition of 2M NaOH were, 77.7% of β-C2S formed and 12.4% combeite.

Keywords: White sand; Lime; Hydrothermal treatment; Calcination