Effect of sucrose on the mechanical properties of magnesium and potassium phosphate cements at low temperatures and its mechanism of action

The inverse relationship between the setting time and high early strength of magnesium potassium phosphate cement (MKPC) affects its practical application in low-temperature environments. However, there has been limited research on adjusting the type and proportion of retarders to achieve a dynamic balance. In this study, we investigated the impact of a composite retarding agent (sucrose, replacing a portion of borax) on the setting time, fluidity, dry shrinkage rate, and mechanical properties of MKPC at different temperatures (20±2°C, 0±2°C, and -10±2°C). The relevant indexes were analyzed and the optimum sucrose dosing range was formulated. Using scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests to observe the hydration products and microscopic morphology of MKPC with varying sucrose dosages at low temperatures. The physical phases were quantitatively analyzed using the Rietveld method. Finding demonstrated that at room temperature, the setting time of MKPC doped with 2.5% and 5% sucrose was extended by 5.58 and 16.72 minutes, respectively, and its stability improved by 19.9% and 24.1% compared to normal MKPC at -10±2°C. Moreover, the stability of MKPC doped with 2.5% and 5% sucrose at -10±2°C improved by 19.9% and 24.1%, respectively, compared to normal MKPC. Furthermore, the MKPC slurry with a sucrose /dead-burned magnesia (S/M) of 2.5% had good dry shrinkage properties, and better long-term strength at low temperatures than conventional MKPC. When the temperature of the specimen was maintained at -10±2 °C and then raised to room temperature, its strength reached 89.6% at 20±2 °C, which was more suitable for the emergency repair and rehabilitation of buildings located in cold regions. However, a higher S/M may inhibit the development of the solid phase in MKPC, resulting in a significant decrease in all aspects of its performance.