Effect of MgO Grades and Admixtures on the Rheological Behavior and Reactivity of MgO-Metakaolin Blends

抄録

Magnesium silicate cements are increasingly being studied as they could be a low-carbon alternative to conventional cements, particularly for certain applications such as soil stabilization, hazardous waste immobilization or 3D printing. However, several challenges should be addressed, particularly regarding their rheological behavior. These cements have a high water demand and require high dosages of superplasticizers. This study aims to examine the effect of MgO grades on the rheological behavior and reactivity of magnesium silicate cement and to find a method to reduce the required superplasticizer dosage. Several admixtures were used: sodium hexametaphosphate (NaHMP), commercial polycarboxylate-ether based superplasticizer (PCE), and commercial sacrificial polymer (MS) combined with PCE. Three grades of light-burned magnesium were selected. Different rheological procedures allowing the viscoelastic and flow properties to be measured were performed. Reactivity was characterized through in-situ isothermal calorimetry, TGA and XRD analyses. Uniaxial compressive test on cubic specimens were also carried until 180 days of curing. The results show that the grade of MgO affects strongly the rheological behavior and reactivity of MgO-metakaolin blends. This is linked to their specific surface area which is conditioned by the calcination conditions. Concerning the effect of admixtures, the required dosage of PCE can significantly be reduced (up to 50%) by combining it with a sacrificial polymer (MS) which allows maintaining the steric repulsion of PCE.