Development of Magnesium Phosphate Cement Based on Low-Grade MgO

Magnesium phosphate cements (MPCs) are a class of inorganic cements that have gained significant attention in recent years due to their exceptional properties and diverse applications in the construction and engineering sectors, particularly in the confinement of radioactive waste. These cements set and harden through an acid–base reaction between a magnesium source (usually dead-burnt magnesia) and a phosphate source (e.g., KH2PO4). The dead-burnt MgO (DBM) used is typically obtained by calcining pure MgCO3 at temperatures between 1600 and 2000 °C. The present work explores the possibility of using low-grade magnesia (≈58% MgO), a secondary waste product generated during the calcination of magnesite for sintered MgO production. Low-grade magnesia is a by-product from the calcination process of natural magnesite. In this manner, the cost of the products could be substantially diminished, and the cementitious system obtained would be a competitive alternative while enhancing sustainability criteria and recyclability. This paper also evaluates the effect of the M/P ratio and curing conditions (especially relative humidity) on the mechanical, microstructural, and mineralogical development of these cements over a period of up to one year. Results indicate that low-grade MgO is suitable for the preparation of magnesium potassium phosphate cements (MKPCs). The presence of minor phases in the low-grade MgO does not affect the precipitation of K-struvite (KMgPO4·6H2O). Moreover, the development of these cements is highly dependent on both the M/P molar ratio and the RH. Systems prepared with an M/P ratio of 3 demonstrated good compressive strengths, low total porosity, and stable mineralogy, which are essential parameters for any cementitious matrix that aims to be considered as a potential confiner of radioactive waste.