Existing research on the mechanisms affecting the strength of cementitious materials primarily focuses on the composition and properties of cement hydration products, often overlooking the interactions between different products. This study presents a systematic experimental and theoretical investigation into the mechanical properties of cementitious materials, emphasizing the interactions between crystal and gel products driven by crystallization pressure. A new mechanism based on crystallization pressure is proposed to explain the impact of the interactions between hydration products on the strengths of cementitious materials. Experiments were conducted by immersing specimens in solutions with tailored ion concentrations (including water, isopropyl alcohol, ethanol, and solutions of calcium hydroxide and calcium acetate) to vary the crystallization pressure. The flexural and compressive strengths of these specimens were then tested. An analytical model was developed and validated against the experimental data. Both experimental and calculated results demonstrate a negative correlation between crystallization pressure and strength. Specimens subjected to crystallization pressures of 101.7 MPa and 147.8 MPa showed reductions in flexural strength of 19.34 % and 30.65 %, respectively, and decreases in compressive strength of 10.00 % and 14.41 %, compared to control specimens with zero crystallization pressure. These results suggest that ion concentrations in the pore solution alter the crystallization pressure, which in turn affects the interactions between crystal and gel products and strength of cementitious materials. This study provides insights into the mechanisms of strength degradation due to moisture in porous materials.

Self-healing concrete using encapsulated healing agent has shown great potential in enhancing concrete durability. However, the capsules are expensive to make and can lower the mechanical properties of concrete. In this study, a new type of manufactured aggregate that employs waste-derived fly ash cenosphere as a carrier of healing agent (SH-CS) was designed and produced. The effect of SH-CS incorporation on hydration, engineering properties and self-healing efficiency of cement mortar was systematically evaluated, with a special focus on self-healing mechanism through the analysis of the mineral composition of the healing product. The results indicate that the prepared SH-CS has good stability in and compatibility with cement mortar. The addition of SH-CS has small influence on the fresh properties of cement mortar and less negative effect on compressive strength at the hardened stage compared to the existing study. By replacing 3 wt.% of fine aggregate with SH-CS, up to 71% of the crack opening area of mortar specimens with a crack width of about 0.3 mm was self-healed after 28 days of water exposure. The self-healing behaviour of SH-CS led to a maximal 41% drop in water adsorption and contributed to the recovery of flexural strength. The healing products precipitated on the fracture surface were mainly composed of amorphous C-S-H and Calcite. It can be estimated that incorporating SH-CS in concrete would result in only a moderate (∼29%) rise in cost for C40 concrete.

Pore structure characteristics of cementitious materials play a critical role in the transport properties of concrete structures. This paper develops a novel framework for modeling chloride penetration in concrete, considering the pore structure-dependent model parameters. In the framework, a multi-scale transport model was derived by linking the chloride diffusivities with pore size distributions (PSDs). Based on the three-dimensional (3D) microstructure generated by “porous growth” and “hard core-soft shell” methods, two sub-models were computationally developed for determining the multi-modal PSDs and pore size-related chloride diffusivities. The predicted results by these series of models were compared with corresponding experimental data. The results indicated that by adopting pore size-related diffusivities, even if the total porosities were the same, the proposed multi-scale chloride transport model could better capture the effect of different PSDs on chloride penetration profiles, while the model without pore structure-depended parameters would ignore the differences. Compared with the reference transport models, which adopt averaged chloride diffusivities, the chloride penetration depths predicted by the proposed multi-scale model are in better agreement with experimental data, with 10%–25% reduced prediction error. This multi-scale transport model is hoped to provide a novel computational approach on 3D microstructure generation and better reveal the underlying mechanism of the chloride penetration process in concrete from a microscopic perspective.