Reaction characteristics and compressive strength of magnesia-phosphate cement at negative temperatures

Abstract Magnesia-phosphate cement (MPC) is considered the most promising inorganic cementing material for the rush-repair construction of concrete structures, pavements, and airport runways in negative-temperature environments. To prepare MPC repair materials that are suitable for negative temperatures, we studied and quantitatively analyzed the influences of dead-burned magnesia/phosphate (M/P), borax/dead-burned magnesia (B/M), and water/MPC (W/C) ratios on the reaction rate, early strength, and long-term strength of MPC prepared and cured at negative temperatures. The ice point of the mixed water decreased to −10 °C because of the double hydrolysis role of monoammonium phosphate and borax; therefore, MPC could set and harden at temperatures higher than −10 °C, and its 2 h compressive strength was > 15 MPa. The setting and reaction rate of MPC decreased steeply with decreasing temperature, and the 2 h compressive strength was as low as 3.0 MPa at − 20 ± 2 °C. The early strength of MPC enhanced on increasing the magnesium ion concentration in the MPC slurry, and the 2 h compressive strength of MPC at − 10 ± 2 °C increased by approximately 50% when the M/P ratio increased from 3 to 5. The early strength of MPC at negative temperatures was more sensitive to changes in the B/M and W/C ratios than at room temperature (20 ± 2 °C). When the temperature of the frozen MPC specimens that were maintained at − 10 ± 2 °C and − 20 ± 2 °C was increased to room temperature, the long-term strength of the MPC increased to approximately 80.9% and 67.6%, respectively, than that of the MPC maintained at room temperature, owing to the secondary reaction in the MPC.