Sulphate Erosion Resistance of Cementitious Materials Modified with CaCO₃ Nanoparticles

Saline soils are widely distributed in Xinjiang, China, with locally high SO₄^2– concentrations. Under the complex service environment, conventional cementitious materials are often difficult to effectively resist sulphate erosion, resulting in the insufficient durability. It is thus necessary to enhance their durability through modification techniques. Nanoparticles of calcium carbonate (NC) as an economical, environmentally friendly and high-performance nano-material can provide an effective way to optimize the performance of cementitious materials. The NC promotes the formation of hydration products and improves the microstructure of cementitious materials through microaggregate filling, nucleation, and chemical reaction activity, thus enhancing the performance of the materials. However, the mechanism of NC on cementitious materials under sulfate erosion environment still needs to be analyzed. Also, the diffusion pattern of SO₄^2– in the materials and its correlation with the microstructure lacks a systematic quantitative analysis. In this study, the effect of NC dosages (i.e., 0–3%) on the sulphate erosion resistance of cementitious materials was investigated by a multi-scale method combining macroscopic performance test, X-ray diffraction (XRD), ethylene diamine tetraacetic acid (EDTA) titration and numerical simulation, revealing the modification mechanism of NC and determining the optimal NC dosage.

P·O 42.5R cement and analytical pure NC were used as raw materials to prepare cement mortar specimens at different mix ratios. After 28-d standard curing, the specimens were subjected to wet-dry cyclic erosion tests in 5% sulfate solution. The cyclic system was defined as, soaking for 16 h-drying at 80 ℃ for 6 h-cooling for 2 h, with a 24 h cycle period, totaling 150 cycles. The flexural/compressive strength, mass loss rate, and dynamic elastic modulus of specimens were measured for every 30 d. The phase composition and microstructure were characterized by X-ray diffraction and scanning electron microscopy. specimens The SO₄^2– content at different depths of specimens was determined by the EDTA titration method. Based on Fick's second law, a chemical erosion model was proposed to simulate the SO₄^2– concentration distribution in the specimens for each 30 d period and compare with the measured data for validation.

The impact of calcium carbonate nanoparticles (NC) dosage on the mass loss rate, relative dynamic elastic modulus, and flexural/compressive strength loss coefficients of cementitious materials is investigated under dry-wet cyclic sulfate erosion. At 150 d erosion, the NC1 group with 1% NC dosage exhibits an optimal erosion resistance. The smallest mass loss rate is, the highest relative dynamic elastic modulus will be. The flexural/compressive strength loss coefficients are 19.11% and 23.7% lower than those of the NC0 group (without NC), respectively. This indicates that an appropriate NC dosage (i.e., 1%) can significantly enhance the long-term durability of the materials, highlighting a critical role of NC dosage in the performance optimization. Conversely, NC dosage of exceeding 1% triggers agglomeration effects, creating a more penetration pathways for the erosion medium and leading to a pronounced material degradation. The XRD patterns reveal that NC1 retards the excessive growth of ettringite (AFt), provides additional nucleation sites for C-S-H gels to enhance their stacking density, and forms a unique structure where ultra-high-density C-S-H gels wrap around high-density C-S-H gels. This process optimizes the microstructure and phase composition of C-S-H gels, while modulating the growth orientation of calcium hydroxide (CH) crystals-transitioning from hexagonal plate-like to prismatic morphology. The synergistic effect between crystal orientation optimization and C-S-H gel densification mutually reinforces a multiphase microstructural stability of cementitious materials. The SO₄^2– concentration at the same material depth increases with erosion duration, while it firstly decreases and then increases with increasing NC dosage (i.e., NC0 > NC3 > NC2 > NC1). At 150 d, the SO₄^2– concentration in the 15–20 mm depth of NC1 is 34.3% lower than that of NC0, and the maximum diffusion coefficient (D_max) is only 43% of the NC0 group, demonstrating that a low NC dosage (i.e., 1%) inhibits SO₄^2– diffusion. In contrast, the excessive dosages (i.e., 2% and 3%) weaken this inhibition due to the NC agglomeration. The simulation results show that the SO₄^2– concentration profile of NC1 aligns well with the experimental data. As erosion progresses, SO₄^2– penetrates from the surface to the interior, reducing the concentration gradient, with the internal concentration consistently lower than the surface. Note that the NC1 group maintains lower SO₄^2– concentrations than NC0 at all depths, further confirming a critical role of NC1 in enhancing the material erosion resistance.

The moderate incorporation of NC could improve the sulphate attack resistance of cementitious materials, as evidenced by the reduction of mass loss rate, the increase of relative dynamic elastic modulus, and the significant optimization of the loss coefficients of flexural strength and compressive strength. At the NC dosage of 1%, the optimum material performance could be obtained. The NC effectively optimized the microstructure of the material via inhibiting the excessive growth of AFt, promoting the high density of C-S-H gel, and regulating the growth orientation of CH crystals, as well as significantly inhibiting the diffusion of SO₄^2–, thus effectively improving the durability and mechanical properties of the material. However, the agglomeration effect of nanoparticles led to a significant decrease in the material properties with the further increase of NC dosage.