Effect of light-burned magnesium oxide on deformation behavior of geopolymer and its mechanism

For the exploration of a way to compensate the shrinkage of geopolymer, the deformation behavior of geopolymer bars with different contents of MgO was investigated via curing under different conditions (i.e., room temperature and 38°C at relative humidity (RH) of >95%). The composition of the geopolymer was characterized by X-ray diffraction and thermogravimetry. The mechanism of MgO on affecting the deformation behaviour of geopolymer was proposed. The results show that the addition of MgO has an influence on the deformation behavior of geopolymer, and this effect is various with curing conditions. The addition of MgO decreases the shrinkage of geopolymer. The more MgO added, the less shrinkage would be. However, the effect of MgO in shrinkage compensation in geopolymer is not as significant as that in OPC system. MgO is involved in reactions in geopolymer based on different mechanisms. MgO could bond with free [SiO₄]^4- to form magnesium silicate gel at a normal temperature, resulting in a high water absorption capacity. The increase of the curing temperature promotes the geopolymerization, and the reduction of the availability of free [SiO₄]^4- suppresses the formation of magnesium silicate gel.