This paper aims to investigate the effect of fine aggregates on properties of magnesium potassium phosphate cement (MKPC) mortar. River sand (RS), limestone sand (LS), and mixed aggregates (RS/LS mixture) were used to prepare MKPC mortar, respectively. Then, the fluidity, mechanical strength, shrinkage deformation, and hydration temperature of different MKPC mortars were tested. Compared with MKPC mortar prepared using RS, MKPC mortar prepared using LS has significantly higher 60-day flexural strength, slightly higher 60-day compressive strength and 60-day shrinkage strain, and lower 60-day flexural strength residual ratio under water curing condition. Compared with MKPC prepared using LS, MKPC mortar prepared using mixed aggregates has higher 60-day compressive strength, higher strength residual ratio under water curing condition, and lower 60-day shrinkage strain. Also, the effect of the ratio of aggregates to a binder (MKPC) on properties of MKPC mortar is investigated. Results show that the optimal aggregates to binder ratio for MKPC mortar prepared using mixed aggregates ($W_{\mathrm{LS}}$ to $W_{\mathrm{RS}}$ ratio is $2:1$ ) is $1.5:1$ , resulting in high strength and low binder content of MKPC mortar. When $W_{\mathrm{LS}}$ to $W_{\mathrm{RS}}$ ratio is $2:1$ and aggregates to binder ratio is $1.5:1$ , 60-day flexural and compressive strengths of MKPC mortar are greater than 40% higher than that of MKPC mortar prepared using RS; 60-day flexural and compressive strength residual ratios of it are above 80%, and its shrinkage deformation is close to that of MKPC mortar prepared using RS.

For the repair of narrow cracks in concrete, the potassium magnesium phosphate cement (MKPC)-based material paste should have high fluidity and self-compacting ability, making it convenient for pouring and compacting. A self-compacting MKPC paste that meets the index requirements recommended by the European Federation of National Associations Representing for Concrete (EFNAFC) was prepared by increasing the water–cement ratio and adding water glass and fly ash (FA). Specimens of self-compacting MKPC paste were subjected to long-term water corrosion tests, which found that those high-fluidity MKPC paste specimens (reference sample M0) that were produced with only an increased water–cement ratio lost 15–30% of their strength. The residual ratio of folding to compression was 84.6%, and the volume expansion rate was 7.78 × 10⁻⁴ after immersion in water for 560 days. The strength residual rate of MKPC slurry (M1) modified by sodium silicate and fly ash is over 90% after 560 days of immersion in water, and the residual rate of flexural-compressive ratio is 101.3%, which meets the requirements of hydraulic cement-based materials. The volume expansion rate of M1 is 5.19 × 10⁻⁴, which is 67% of the reference sample M0 with the same water immersion age.

Concrete structures can be degraded by exposure to environmental stressors such as freeze–thaw cycling and salt corrosion. Magnesium potassium phosphate cement (MKPC) mortar is useful for the rapid repair of such structures but must acquire environmental resistance rapidly. In this study, the freeze–thaw resistance of MKPC mortar specimens of different hydration ages was tested in water and a 5% Na₂SO₄ solution. The strength, volume deformation, and water absorption rates were compared with those of full-age MKPC mortar specimens (28 d). The phase composition and microscopic morphology of the MKPC mortar specimens before and after corrosion were observed, and the corrosion-resistance mechanism was analyzed. After 225 freeze–thaw cycles in water and sulfate solution, the strength residual rates of the early-age specimen (1 d) were higher than those of the full-age specimen (28 d). The degree of strength attenuation in the 1 d specimen was lower in the sulfate environment than in the water environment. After 225 freeze–thaw cycles, the volume expansion rates of 1 d specimens in water or sulfate were 0.487% and 0.518%, respectively, while those of 28 d specimens were 0.963% and 1.308%. The comparison shows that the 1 d specimen had significantly better deformation resistance under freeze–thaw than the 28 d specimen. After 225 freeze–thaw cycles, the water absorption rates of 1 d specimens were 1.95% and 1.64% in water and sulfate solution, respectively, while those of 28 d specimens were 2.20% and 1.83%. This indicates that freeze–thaw cycling has a greater effect on the pore structure of fully aged mortar than on early-age mortar (1 d). Therefore, MKPC mortar is suitable for the rapid repair of concrete structures in harsh environments. The results form a theoretical basis for winter emergency repair projects. They also further the understanding of the application of MKPC-based materials in extreme environments.

Concrete is the material of choice for constructing hydraulic structures in water-related buildings, and its mechanical properties are crucial for evaluating the structural damage state. Machine learning models have proven effective in predicting these properties. However, a single machine learning model often suffers from overfitting and low prediction accuracy. To address this issue, this study introduces a novel hybrid method for predicting concrete compressive strength by integrating multiple soft computing algorithms and the stacking ensemble learning strategy. In the initial stage, several classic machine learning models are selected as base models, and the optimal parameters of these models are obtained using the improved metaheuristic-based gray wolf algorithm. In the subsequent stage, the lightweight gradient boosting tree (LightGBM) model and the metaheuristic-based optimization algorithm are combined to integrate information from base models. This process identifies the primary factors affecting concrete compressive strength. The experimental results demonstrate that the hybrid ensemble learning and heuristic optimization algorithm achieve a regression coefficient of 0.9329, a mean absolute error (MAE) of 2.7695, and a mean square error (MSE) of 4.0891. These results indicate superior predictive performance compared to other advanced methods. The proposed method shows potential for application in predicting the service life and assessing the structural damage status of hydraulic concrete structures, suggesting broad prospects.

To study the sulfate corrosion behavior of potassium magnesium phosphate cement (PMPC) paste, the sulfate content, strength, and length of PMPC specimens were measured at different corrosion ages under 5% Na₂SO₄ solution soaking conditions, and the phase composition and microstructure were analyzed. The conclusion is as follows: In PMPC specimens subjected to one-dimensional SO₄²⁻ corrosion, the relation between the diffusion depth of SO₄²⁻ (h) and the SO₄²⁻ concentration (c (h, t)) can be referred by a polynomial very well. The sulfate diffusion coefficient (D) of PMPC specimens was one order of magnitude lower than Portland cement concrete (on the order of 10⁻⁷ mm²/s). The surface SO₄²⁻ concentration c (0, t), the SO₄²⁻ computed corrosion depth h₀₀, and D of FM2 specimen containing 20% fly ash (FA) were all less than those of the FM0 specimen (reference). At 360-day immersion ages, the c (0, 360 d) and h₀₀ in FM2 were obviously smaller than those in FM0, and the D of FM2 was 64.2% of FM0. The strengths of FM2 specimens soaked for 2 days (the benchmark strength) were greater than those of FM0 specimens. At 360-day immersion ages, the residual flexural/compressive strength ratios (360-day strength/benchmark strength) of FM0 and FM2 specimens were all larger than 95%. The volume linear expansion rates (S_n) of PMPC specimens continued to increase with the immersion age, and S_n of FM2 specimen was only 49.5% of that of the FM0 specimen at 360-day immersion ages. The results provide an experimental basis for the application of PMPC-based materials.