Magnesium potassium phosphate cement (MKPC) exhibits rapid setting and hardening, high early strength, and excellent bonding performance, making it a promising material for the rapid repair of concrete structures. It is increasingly utilized in cold regions and deicing-salt environments (NaCl). However, the long-term performance of MKPC in NaCl environments remains unclear. Reported results differ regarding the performance evolution of MKPC in NaCl solutions. While some researchers reported that NaCl exposure increases material degradation, others have observed that, under specific conditions, the late-age strength can be maintained or even improved. This study aims to address the existing knowledge gap regarding the long-term performance of MKPC in NaCl environments.

Among the many factors affecting the durability of MKPC, the water-to-binder ratio (w/b) and NaCl concentration are particularly important. The w/b determines the extent of early reactions and the amount of unreacted residual particles, whereas the NaCl concentration controls ion migration and dissolution processes. In this study, the long-term evolution of MKPC with different w/b values at different NaCl concentrations was investigated.

In this study, the MKPC system was prepared with an MgO/KH₂PO₄ mass ratio of 2. Three w/b ratios (0.16, 0.22, and 0.24) were investigated, while the sand-to-binder ratio was kept constant at 0.8. Borax decahydrate was added as a retarder at 10% (mass fraction) of the MgO mass. After casting, the specimens were precured for 3 d and then cured for up to 360 d in air, DI water, 5% NaCl solution, and 15% NaCl solution, respectively.

To evaluate the long-term durability and volume stability, the linear dimensional change and compressive strength development were measured in different environments. Quantitative X-ray diffraction (QXRD) using the Rietveld method and thermogravimetric analysis (TG-DTG) were used to quantify the transformation of crystalline phases, amorphous content, and hydration products. The transition in hydration-product morphology and elemental distribution was observed by scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS). Finally, mercury intrusion porosimetry (MIP) was employed to analyze the pore structure evolution.

The volume stability results showed that the air-cured specimens underwent shrinkage and gradually stabilized after about 18 d, whereas the specimens immersed in pure water and NaCl solutions exhibited an initial shrinkage followed by expansion. The expansion in the 15% NaCl solution was slightly higher than that in 5% NaCl and DI water. The pH results showed that all immersion solutions remained above 7, indicating that MgO kept dissolving. However, the stable pH followed the order of DI water (10.2) > 5% NaCl (9.9) > 15% NaCl (9.6), suggesting that NaCl altered the reaction process.

The compressive strength of MKPC varied markedly under different conditions. For specimens cured in pure water, a decreasing trend is observed after 28 d of immersion, which is primarily attributed to the gradual dissolution of MgO and struvite-K. Conversely, specimens immersed in NaCl solutions continue to gain strength even after 28 d, indicating that the presence of NaCl promotes strength development of MKPC. After 360 d of immersion, specimens in the 15% NaCl solution reached compressive strengths ranging from 50.99 MPa to 51.34 MPa, higher than those in the 5% NaCl solution (42.32-50.07 MPa). Notably, the w/b = 0.22 specimen achieved the highest strength retention of 95.67% in the 15% NaCl environment.

QXRD analysis showed that struvite-K and MgO were the major mineral phases in all samples. Notably, a new hydrate, hazenite (KNaMg₂(PO₄)₂·14H₂O), was detected only in specimens cured in NaCl solutions, while it was absent in air- and DI-water-cured samples. With increasing curing age and NaCl concentration, the content of hazenite increased whereas the struvite-K content decreased, indicating a phase transformation from struvite-K to hazenite in the NaCl environment.

SEM observations showed that the air-cured specimens were mainly composed of relatively dense plate-like struvite-K, whereas the products in the DI-water-cured specimens were more needle-like. After curing for 28 d in NaCl solutions, plate/cluster-like crystals were formed. EDS results showed that the K: Na ratio in these crystals was approximately 1:1 (hazenite). Compared with the struvite-K, hazenite crystals were larger and filled pores. MIP results further showed that the porosity of the NaCl-cured samples was significantly lower than that of the DI-water-cured samples, and the pore structure was densified at later ages, with the porosity decreasing by about 6%.

The main conclusions of this paper are summarized as follows. The strength retention of the MKPC mortar samples increased from ~81% (in pure water) to about 95% at a w/b of 0.22 after storage in 15% NaCl solution for 360 d. A new phase, hazenite (KNaMg₂(PO₄)₂·14H₂O), increased with NaCl concentration and curing time. XRD/SEM results showed that hazenite (cluster/plate-like configuration) stemmed from struvite-K (needle-like shape) after incorporating Na⁺ and water, which contributed to a reduction in porosity (6%), leading to higher retention of compressive strength.