Toward sustainable magnesium phosphate cement: Deciphering the dissolution and reaction mechanisms of magnesium hydroxide in acid phosphate systems

Abstract

Magnesium phosphate cement (MPC) is an inorganic cementitious material formed by the reaction of magnesium raw materials with phosphates, characterized by high early strength and excellent durability. However, conventional MPC relies on dead-burned MgO, which is calcined above 1500 °C, resulting in high carbon emissions (exceeding 1500 kg/t). To reduce the carbon footprint of MPC, the dead-burned MgO was replaced by Mg(OH)₂, the main component of natural brucite, to prepare low-carbon MPC and the reaction mechanisms of Mg(OH)₂ in ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (KDP) solutions was investigated. The results indicated that low-carbon MPC exhibited faster setting and solidification than traditional MPC. The contact efficiency of Mg(OH)₂ with ADP solution was significantly higher than with KDP solution, leading to a primarily surface reaction in ADP solution. Compared to the KDP solution, ADP solution exhibited a lower kinetic parameter N and a higher K in the early stage of the reaction, resulting in the rapid formation of reaction products such as MgNH₄PO₄·6H₂O and MgHPO₄·3H₂O that coated the Mg(OH)₂ surface and hindered further reaction, leaving substantial residual Mg(OH)₂ and ADP. In contrast, Mg(OH)₂ in KDP solution underwent both surface and solution reactions, where lamellar products, Mg₂KH(PO₄)₂·15H₂O and MgHPO₄·7H₂O, gradually transformed into prismatic MgKPO₄·6H₂O as the reaction progressed, leading to near-complete consumption of KDP and a higher reaction extent of Mg(OH)₂ compared to ADP solution. This study provided new insights into the synthesis of low-carbon MPC and established a theoretical foundation for optimizing reaction conditions.