Abstract
In this research, waste gypsum (CaSO4·2H2O), a by-product material from industrial factory, was upgraded and then used as raw material for building materials. The by-product gypsum possessed a high acidic value of 3-point pH scale and moisture content of 40 %. The two properties had an impact on setting reaction and hardening of gypsum. Therefore, the studies of gypsum phase transformation to calcium sulfate hemihydrate (CaSO4·0.5H2O) were focused on washing process and amount of calcium carbonate (CaCO3) added at 0, 1, 3 and 5 % wt. After washing, waste gypsum and washed water were reduced from high acidic value to neutralization (pH = 7) as a result of CaCO3. Next, the neutralized gypsum was heated to the optimal temperature at 160 °C for 2 hours and transformed to hemihydrate gypsum phase observed by XRD. Finally, the relationship of amount of CaCO3-mechanical property such as bending strength will be investigated.
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