Cementitious concrete structures serving in sulfate environments suffer from serious durability challenges caused by chemical sulfate attacks (CSA), which lead to the volume expansion, cracking, and spalling of concrete and the early failure of structures. CSA on concrete involves the behaviors of ion transport, chemical reactions, the crystallization of reaction products, microstructural damage to the cement matrix, and the macroscopic deterioration of concrete, namely the transport-chemo-mechanical behaviors. This paper first introduces the reaction products, such as gypsum, ettringite, brucite, and thaumasite, between sulfate and concrete under different environmental conditions and their formation mechanism. Then, aiming at the ettringite type CSA, the theories of volume increase and crystallization pressure are elaborated to explain it-induced concrete degradation. Additionally, the crystallization pressure theory is used to describe the cracking behavior in the microstructure slurry caused by the ettringite crystal filling pore. Finally, a series of transport-chemo-mechanical models for ettringite type CSA are displaced module by module. It includes the sulfate diffusion-reaction model, the free expansion of concrete, and equivalent expansive force in concrete related to the reaction behavior: the model for chemo-mechanical behavior in concrete caused by CSA. These models can be used to analyze the distribution of sulfate ions and the reaction product content, expansive stress and strain in the concrete, and the cracking and spalling degree of the concrete, which is beneficial to evaluate the durability of concrete structures serving permanently in a sulfate environment.

An accelerated leaching experiment, using 6M ${\mathrm{NH}}_4\mathrm{Cl}$ solution, on specimens of pure cement paste, binary cement paste with different fly ash contents (20, 30, and 50%), and ternary cement paste with the replacement of 10, 20, and 30% fly ash by slag was conducted to investigate the influence of fly ash and its partial replacement by slag on the leaching behavior of blended cement pastes. The leaching depth, porosity, calcium-silicon mass ratio ($\mathrm{Ca}/\mathrm{Si}$ ), phase composition, and microstructure morphology of the specimens were characterized by using the phenolphthalein indicator, saturation-drying weighing, scanning electron microscopy with energy dispersive spectrometer (SEM/EDS), and X-ray diffraction (XRD). Results show that higher fly ash content mixed in pure cement paste causes a greater leaching depth, with slower microstructure deterioration, lower porosity increment, and reduction of $\mathrm{Ca}/\mathrm{Si}$ and calcium hydroxide content. Higher replacement of fly ash by slag in binary cement paste leads to a more compact microstructure and higher $\mathrm{Ca}/\mathrm{Si}$ after complete leaching of calcium hydroxide. The fly ash can reduce the leaching degree of pure cement paste but accelerate its leaching process; however, the partial replacement of fly ash by slag in blended cement paste not only improves its compactness and the stability of its microstructure, but also slows down its leaching process. The ternary cement paste with a water-binder ratio of 0.45, 20% fly ash, and 30% slag has the optimum leaching resistance.

Mineral admixtures (MA), like fly ash (FA), silica fume (SF), and slag (S), are usually added to cement-based materials to improve their compactness and further enhance their mechanical properties, permeability resistance, and durability. In this study, low-field nuclear magnetic resonance (LF-NMR) is adopted to explore the evolution of the early hydration characterization of cement-based materials with MA by testing the transverse relaxation time T2. Meanwhile, the effect of MA on mechanical properties is analyzed by measuring compressive and flexural strength. The results show that, in the early hydration (0–7 days), the T2 distribution shows a trend of gradually moving to a short relaxation time and changes from a double peak to a single main peak. The decrease in T2i (main peak vertex) means that the evaporated water is gradually distributed in smaller pores with more motion constraints. However, the type and content of MA have little effect on T2i. Porosity gradually decreases in the period of early hydration. The addition of MA causes the porosity to decrease, and the order influence is FA > S > SF, i.e., the porosities of cement paste with 0%MA, 10%FA + 10%SF, 10%FA + 10%S, and 20%FA at 7 days are 48%, 44.5%, 40.7%, and 40.2%, respectively. Additionally, the addition of MA to cement-based materials also decreases the early strength, and the influence order is FA > S > SF, i.e., the compression strength values of cement paste with 0%MA, 10%FA + 10%SF, 10%FA + 10%S, and 20%FA at 7 days are 47.8 MPa, 40.1 MPa, 38.6 MPa, and 37 MPa, respectively.