ZHAO Guipeng，WEI Xin，KOU Zhipeng，ZHANG Chong，ZHANG Xiuzhi，DUAN Guangbin

School of Materials Science and Engineering， University of Jinan， Jinan 250022， China

Abstract

Objective Magnesium potassium phosphate cement (MKPC) exhibits rapid setting, high early strength, and stable volume, making it highly promising for emergency repair applications. However, its limited water resistance restricts its wider use in underwater engineering, as humid environments may induce strength retrogression and structural deterioration. In this study, silica fume and nano-silica are employed as modifiers to investigate their effects on the water resistance of MKPC, determine optimal dosages, and explore underlying mechanisms, thereby providing technical support for the application of MKPC in humid and underwater engineering projects.

Methods The experiment adopted a controlled variable method. The matrix mix ratio of MKPC was kept constant, and specimens were prepared by incorporating different ratios of silica fume (0%，3%，5%，7%) and nano-silica (0%, 0.5%, 1%, 1.5%), respectively. Two curing conditions were established: air curing (20±2 ℃, RH 60±5%) and water curing (20±2 ℃ in distilled water). Three parallel specimens were tested for each group. Compressive strengths at 3, 7, and 28 d were measured using a universal testing machine, and pore structure characteristics were determined by mercury intrusion porosimetry, serving as core evaluation indicators.

Results and Discussion The experimental results showed that both modifiers could improve the water resistance of MKPC, but optimal dosages differed. With 5% silica fume content, the best performance was achieved. Under this condition, the strength after 7 d of water curing increased by 32% compared with the baseline group, and the strength attenuation reduced from 45% to 18%. The modification effectiveness was more pronounced when the nano-silica content was 1%, with the 7 d water-cured strength reaching 48.2 MPa. This represented an increase of 57% over the baseline group and even exceeded the strength of air-cured specimens (45.6 MPa) at the same curing age, thus breaking the strength retrogression trend. Pore structure tests showed that the total porosity of the 1% nano-silica sample decreased to 12.3% (41% lower than that of the baseline group), indicating superior pore refinement effectiveness. Through pore structure analysis, X-ray diffraction analysis, and scanning electron microscope observation, the microscopic mechanism behind the improved water resistance of MKPC was further revealed. The total porosity of MKPC modified by adding 1% nano-silica was 27.26% after water curing for 7 d, and the proportion of large pores (>1 000 nm) was reduced by 4.42% compared with that of adding 5% silica fume.

Conclusion Silica fume can fill the internal pores of MKPC, while nano-silica promotes hydration via a nucleation effect and physically refines the pore structure. Optimal dosages are identified as 1% nano-silica and 5% silica fume, with the former exhibiting better performance. This study clarifies the parameters and mechanisms of modification, resolves the water-resistance issue of MKPC, and provides theoretical and technical support for its underwater application. The research findings also provide practical guidance for the application of MKPC in water environments. The optimization of Mg/P ratio and the addition of micro-nano silica effectively improve the compressive strength and pore structure of MKPC, resulting in excellent water resistance.

Keywords：magnesium potassium phosphate cement; water resistance; silica fume; nano-silica; pore structure