Changes in mechanical performance and pore structure of hardened cement paste under the combined effect of calcium leaching and dry/wet cycles

Abstract

The durability of marine concrete is significantly influenced by the coupled effect of calcium leaching (CL) and dry/wet cycles (DW). This study investigates their combined impact on the Vickers hardness and pore structure of hardened cement paste. The experimental setup includes two groups: Group A (CL followed by DW) and Group B (DW followed by CL), which superimpose the single-factor effects of each factor. Calcium leaching levels were set at 5 %, 10 %, and 15 % loss of sample mass, and dry/wet cycles were varied at 4, 8, and 12 times. A 4 mol/L ammonium nitrate solution was used to accelerate calcium leaching. The results demonstrate that the coupled effect of CL and DW reduces the Vickers hardness by increasing the porosity of the cement paste. Specifically, CL primarily increases the porosity of pores above 200 nm by dissolving calcium hydroxide, and it augments the porosity of 7 ∼ 50 nm pores by removing calcium from C-S-H gels. However, the combined effect of CL and DW on Vickers hardness is less pronounced than the total individual effects. Moreover, Group A exhibits a more substantial impact on the deterioration of mechanical properties in hardened cement paste compared to Group B. In addition, a model is applied for quantitatively predicting mechanical performance based on nominal porosity.