Improving the water resistance of gypsum-based building materials with slag activated by calcium oxide

The water resistance of building gypsum products was poor, which led to certain limitations in their application. In this paper, calcium oxide and slag were added to building gypsum to study the effect of calcium oxide alkali activation of slag on the water resistance of building gypsum. Low-field nuclear magnetic resonance technology, X-ray diffraction technology, and other modern testing techniques were employed to investigate the hydration process and water resistance mechanism of building gypsum. It was found that the use of calcium oxide to activate slag could effectively enhance the water resistance of building gypsum. It was discovered that when the dosage of slag was 50 % and the dosage of calcium oxide was 2 % of the slag, the softening coefficient of building gypsum after 28 days was increased from 0.26 to 0.93. When calcium oxide was used to activate slag, not only was C-(A)-S-H gel formed, but also more ettringite was produced. The formation of ettringite and C-(A)-S-H gel was greatly influenced by the presence of water. These substances were found to have stronger water resistance compared to gypsum crystals. The structural system built by the interlocking of these substances significantly enhanced the water resistance of the samples, thereby improving the water resistance of building gypsum blocks. Simultaneously, at a lower addition rate (2 % relative to the amount of slag), as the replacement amount of slag increased, the cost of the samples and the carbon emissions were also relatively reduced.