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Sulfate attack is a major cause of the deterioration of concrete structures. Accurately predicting the temporal and spatial distribution of sulfate ions in concrete is the basis for revealing the mechanism of concrete performance deterioration caused by sulfate attack. In this article, based on the pore change caused by sulfate damage to cement mortar, a diffusion model of sulfate ions in cement mortar considering changes in surface concentration was established. Furthermore, a two‐dimensional mesoscopic concrete model composed of cement mortar, coarse aggregate, the intrafacial transition zone (ITZ) and macroscopic defects, which can reflect the real aggregate shape, was constructed to study the diffusion of sulfate ions in concrete. The correctness of the model is verified with existing experimental data, and the model is further studied. The results show the following. (1) The average value of the calculation results of multiple sets of models can be used to eliminate the influence of the randomness of the construction of the two‐dimensional mesoscopic concrete model. (2) With increasing stone content, the corrosion depth increases, but the average concentration at a given location decreases. (3) With increasing sulfate solution concentration and water‐cement ratio, the sulfate ion concentration and the corrosion depth in concrete increase. [ABSTRACT FROM AUTHOR]

Durability is one of the most important considerations for subway structures. However, the cement‐based materials used in subway engineering can be jeopardized by sulfate and the stray current and therefore can be subjected to severe damage during service. This study aims to investigate the degradation process of cement‐based materials exposed to stray current and external sulfate. In particular, the effects of three key variables were evaluated: concentration of external sulfate solution, level of stray current and cement type. After exposure, the visual appearance, compressive strength, SO42− profiles and microstructure of cement‐based materials were analyzed. Moreover, the amounts of degradation products were measured to evaluate the influencing degree of different variables and further explain the degradation mechanisms. Experimental results of this study reveal that (a) sulfate attack was related to the external sulfate concentration, but the relation was changed due to the presence of stray current. One explanation is that sulfate ions can be quickly accumulated in the pores of cement‐based materials under the effect of stray current, which makes the chemical reaction of sulfate attack exchange from diffusion control to reaction control. (b) Stray current could accelerate external sulfate attack of cement‐based materials, and the greater the stray current, the more severe sulfate attack was induced. (c) Compared with P·I type cement, high sulfate resistant (HSR) cement showed a better resistance to external sulfate attack. (iv) And, the main degradation products, ettringite and gypsum, were observed in specimens after exposure. Additionally, stray current might induce the decomposition of ettringite. It should be paid more attention on the durability of cement‐based materials exposed to the coupled conditions of stray current and sulfate. [ABSTRACT FROM AUTHOR]

In this study, the deterioration resistant of concrete with and without multi‐walled carbon nanotubes (MWCNTs) against combined freeze–thaw cycles and sulfate attack was studied, and concretes incorporating MWCNTs (i.e., 0.05, 0.10, and 0.15 wt%) as partial replacement of Portland cement were exposed to 5% sodium sulfate solution under freeze–thaw cycles. The performance, including compressive strength, relative dynamic elastic modulus, mass loss, microstructure, deterioration resistant coefficient of concretes were evaluated. Results show that when exposed to 5% sodium sulfate solution, MWCNTs could enhance the mechanical and durability properties of concrete subjected to combined freeze–thaw cycles and sulfate attack, regardless of the dosage of MWCNTs. The deterioration resistant of MWCNTs concrete was not increase with the increase dosage of MWCNTs. And the best resistance performance was obtained in concrete containing lower dosage of MWCNTs. In addition, MWCNTs could promote formation of hydration and corrosion products, and delay deterioration of concrete was demonstrated by SEM and thermogravimetric analysis. [ABSTRACT FROM AUTHOR]

Condition deterioration of the concrete, especially in corrosive environments, is one of the most common imperfection and disadvantages of reinforced concrete structures. Sulfates' main damages are expansion and loss of strength. Geopolymers are one of the newest structural materials that have been widely developed in recent years for use in sustainable designs. Geopolymers are environmentally friendly and considered as green materials since they are effective in sustainable and eco‐friendly material design concept. One of the most important matters in the sustainability of structural materials is their resistance against sulfates. So far, the durability of perlite‐based geopolymers against sulfates and acids has not been investigated. In this research, the fineness influence, curing conditions, duration of maintaining in the sulfate environment at different weight percentage in solution (i.e., 2.5, 5, 7.5, and 10%) have been investigated. Compressive strength and microstructure changes of geopolymer samples have been assessed in this research. As a result, optimum increase in perlite fineness causes increase in compressive strength and durability of samples against sulfate attack. Also, samples that have been cured by autoclave method have better mechanical properties and better durability. It is hoped that by filling these deficiencies and gaps, a positive step can be taken to accelerate attempts for finding a suitable replacement for ordinary Portland cement. [ABSTRACT FROM AUTHOR]

Lots of concrete structures are located in marine environment, the durability damage of these structures is often the result of the interaction of load, environment, and other factors. In this paper, tests for the stress–strain relation of concrete under the coupled effects of sustained loading, sulfate attack and dry‐wet cycle were conducted. Then, based on the damage stress–strain relation model of concrete under sulfate attack, a calculation method for the stress–strain relation of concrete under the action of sulfate attack and dry‐wet cycle was proposed. Meanwhile, by taking into account the effect of creep on the stress–strain relation of concrete, a time‐dependent stress–strain relation model of concrete was presented. Numerical simulations by the time‐dependent stress–strain relation model demonstrate that the calculated results agree well with the experimental data. Furthermore, the rationality and applicability of the time‐dependent stress–strain relation model were also discussed. [ABSTRACT FROM AUTHOR]