Comparing the Effects of NH4H2PO4 and Na3PO4 on the Properties of Hardened Portland Cement Paste

Phosphates are a commonly used class of inorganic retarders for portland cement. Normally, previous studies focused on the effects of phosphates on the early hydration of cement paste. In this study, the effects of NH4H2PO4 (NHP) and Na3PO4 (NP) with 0.5%, 0.75%, and 1% by weight on the properties of hardened portland cement paste were compared by investigating the macroscopic variations in the mechanical properties and drying shrinkage of portland cement paste, coupled with microstructural analyses utilizing gibbs energy minimization software (GEMS) thermodynamic simulation, thermogravimetric analysis-differential thermogravimetric analysis (TG-DTG), X-ray diffraction (XRD), scanning electron microscopy (SEM), backscattered electron energy-dispersive X-ray spectroscopy (BSE-EDS), and Brunauer-Emmett-Teller (BET) techniques. The research findings revealed that the incorporation of NP resulted in a decrease in the compressive strength and a larger drying shrinkage of the hardened cement paste with the increase of NP dosage. Conversely, the addition of NHP could enhance the compressive and flexural strengths and lead to lower drying shrinkage and the 0.5% by weight NHP showed the best performance. It can be found that the addition of NHP could improve the hydration degree of the cement, increase the Ca/Si ratio of calcium-silicate-hydrate (C─ S─ H) gel and result in a denser microstructure of the cement paste with decreased numbers of larger pores and cumulative total pore volume. Therefore, NHP seems a better inorganic retarder than NP for portland cement with not only a longer retarding capacity but also the ability in improving the properties of hardened portland cement.