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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.

A macro-microscopically chemo-mechanical model has been proposed to numerically investigate the expansion response of hardened cement paste (HCP) under external sulfate attack (ESA). In this model, a saturated HCP specimen and a representative volume element (RVE) are regarded as macro- and microscopic objects. At the macroscale, the sulfate diffusion and expansion response in HCP are described by using Fick’s law and continuum mechanics. At the microscale, the growth of sulfate products and its induced mechanical response in RVE are characterized by using reaction kinetics and microporomechanics. Meanwhile, the macro-micro transition of relevant parameters, such as sulfate concentration, mechanical property, and expansive eigenstrain, is constructed to bridge the relationship between the macro and micro scales. Numerical simulation on expansion response of a HCP cylinder immersed in Na2SO4 solution is performed. Results show that, there exist some differences in the macro- and microscopic expansion responses induced by the growth of sulfate products in the pore, and the microscopic hoop tensile stress is responsible for the initiation and propagation of microcracks in the cement paste.

The coupling action of calcium leaching and chloride attack is an important durability problem for the cement-based materials served in long-termly in the environmental water like seawater or groundwater containing chloride. This paper investigates the influence of calcium leaching on the chloride diffusivity in cement-based materials by the corrosion experiment, in which the hardened cement paste (HCP) specimens are immersed into three corrosion solutions with the same chloride concentration, such as 1 M sodium chloride solution, 1 M ammonium chloride solution and the mixed solution of 1 M sodium chloride and 3 M ammonium nitrate. Some measurements, like the phenolphthalein, saturation-drying weighing, mercury intrusion porosimetry (MIP), Scanning Electron Microscopy (SEM/EDS), X-ray diffraction analysis (XRD) and rapid chloride test (RCT), are used to analyze the leaching behaviors and its influence on the chloride diffusivity of HCP, characterized by leaching depth, pore structure, morphology, calcium-silicon ratio, chloride-silicon ratio, phase composition and free chloride ion content. Results show that, in the calcium leaching process, the compactness, calcium hydroxide content and calcium-silicon ratio of C-S-H gel of HCP decreases with the leaching time. The calcium leaching causes the reduction of the physically chloride-bound ability and the chemically chloride-absorbed ability in HCP, and it results in high chloride diffusivity.

The passive behavior of ductile iron in simulated cement-mortar pore (SCMP) solution with different alkalinity was investigated by electrochemical measurements, such as open circuit potential (OCP), Tafel polarization curve (TP), electrochemical impedance spectroscopy (EIS), and microscopic tests, including scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results show that, with increase in immersion time, the SCMP solution with the high pH value can cause the increase of OCP and capacitive loop of ductile iron surface, a morphology of growing surface film, and the high Fe 2+ /Fe 3+ ratios in its surface Iron-oxide compounds, presenting the formation of passive film on the surface of ductile iron. But in the SCMP solution with low pH value, its OCP, TP curve and EIS show no obvious change, and also produce the corrosion morphology of rust with low Fe 2+ /Fe 3+ ratios, indicating loss of surface protection on the ductile iron.

This paper numerically simulates time-dependent mechanical behaviors, such as stress redistribution and bearing capacity degradation, of concrete subjected to the coupled axial loading and sulfate attack. Based on Fick's law and chemical reaction kinetics, a diffusion-reaction model was proposed to obtain the spatial and time-dependent concentration distribution of sulfate ion, gypsum and ettringite. From previous study, a chemical damage, characterized by the volumetric expansion from ettringite formation, is introduced to reflect the influence of sulfate attack on concrete performance. Furthermore, using the plasticity theory and damage mechanics, a coupled chemo-mechanical damage constitutive model was determined by introducing the chemical damage. Finally, a concrete prism is regarded as a research object, to investigate the time-dependent stress and bearing capacity degradation of concrete structures subjected to the coupled axial compression and sulfate attack.

This paper has numerically investigated the changes of loading-induced stress in concrete with the corrosion time in the sulfate-containing environment. Firstly, based on Fick\'s law and reaction kinetics, a diffusion-reaction equation of sulfate ion in concrete is proposed, and it is numerically solved to obtain the spatial and temporal distribution of sulfate ion concentration in concrete by the finite difference method. Secondly, by fitting the existed experimental data of concrete in sodium sulfate solutions, the chemical damage of concrete associated with sulfate ion concentration and corrosion time is quantitatively presented. Thirdly, depending on the plastic-damage mechanics, while considering the influence of sulfate attack on concrete properties, a simplified chemo-mechanical damage model, with stress-based plasticity and strain-driven damage, for concrete under axial loading and sulfate attack is determined by introducing the chemical damage degree. Finally, an axially compressed concrete prism immersed into the sodium sulfate solution is regarded as an object to investigate the time-varying stress in concrete subjected to the couplings of axial loading and sulfate attack.

This paper uses modelling and experiment to perform a quantitative analysis for the gypsum and ettringite formations in cement pastes subjected to sulfate attack. Firstly, based on Fick\'s law and chemical reaction kinetics, a diffusion model of sulfate ions in cement pastes is proposed, and then the model of the gypsum and ettringite formations is established to analyze its contents in cement pastes with corrosion time. Secondly, the corrosion experiment of the specimens with cement pastes immersed into 2.5%, 5.0% and 10.0% Na2SO4 solutions are carried out, and by using XRD-Rietveld method, the phases of powder samples from the specimens are quantitatively analyzed to obtain the contents of gypsum and ettringite in different surface depth, solution concentration and corrosion time. Finally, the contents of gypsum and ettringite calculated by the models are compared with the results from the XRD experiments, and then the effects of surface depth, corrosion time and solution concentration on the gypsum and ettringite formations in cement pastes are discussed.

In order to investigate the influence of slag on the leaching behavior of cement mortar lined in ductile iron pipe in a flowing solution, two types of ductile iron pipe specimens lined with ordinary- and 40% slag- cement mortar are firstly prepared, then, an accelerated leaching experiment on the prepared specimens is carried out by using a 6 M ammonium chloride solution at a flow rate of 0.3 m/s. The parameters of linings at different leaching times, such as porosity, pore size distribution, phase composition and Calcium-Silicon (Ca/Si) ratio, were analyzed by using the saturation-drying weighing method, mercury intrusion porosimetry (MIP), X-ray diffraction (XRD) and scanning electron microscopy in back scattered electron mode (ESEM-BSE) measurements. Results show that, the mixture of 40% slag in cement-mortar lining can reduce its initial porosity and Calcium-Silicon ratio, and improve its pore size distribution and phase composition. In the accelerated leaching process, the porosity, phase composition and Calcium-Silicon ratio of the slag-cement-mortar lining with 40% slag have smaller change than that of the ordinary-cement-mortar lining without slag, and it indicates that the addition of slag can effectively improve the calcium leaching resistance of cement-mortar lining in ductile iron pipes in water supply engineering. Keywords: Slag, Cement-mortar lining, Calcium leaching, Ductile iron pipe, Flowing solution

In order to obtain quickly the influences of blast furnace slag and water-binder ratio on the leaching behavior of cement-based material and its corrosion resistance in water environments, an accelerated calcium leaching experiment on a slice specimen with slag-cement pastes in 6 M NH4 Cl solution was carried out. Using the saturation-drying weighing method, X-ray diffraction (XRD) and the scanning electron microscope (SEM/EDS) measurement, the influences of water-binder ratio and slag content on the porosity, phase composition, microstructure morphology and Calcium-Silicon ratio of slag-cement pastes in the process of calcium leaching were analyzed. Results show that, compared to cement pastes without slag, the slag-cement paste with the appropriate slag content has a low rate of microstructure deterioration and a good leaching resistance. The optimum slag contents in slag-cement pastes are respectively 40% and 50% corresponding to the low and high water-binder ratio. The porosity, Calcium-Silicon ratio and microstructure morphology of slag-cement pastes exhibit small change in the process of accelerating leaching, and this indicates that the slag-cement pastes with the optimum slag content can provide the best performance of calcium leaching resistance in the soft-water environment.

Expansion of concrete caused by external sulfate attack (ESA) is a multiscale chemomechanical response, mainly including internal stress evolution at the dormant period and significant crac...

Concrete deterioration caused by external sulfate attack (ESA) is a macro-microscopically interactive expansion process, including two stages, namely a dormant period and detrimental duration. Based on ESA-induced deterioration mechanism, an integrated macro-microscopic model has been developed to describe the deterioration of concrete. In this model, a critical characteristic related to crystallization pressure is proposed to recognize two stages of ESA, and a criterion of ESA-induced boundary movement in concrete specimen is established by its total damage associated with micro and macroscopic damages. A diffusion-reaction model is constructed to characterize macroscopic sulfate diffusion and microscopic ettringite/gypsum formation in concrete, which are respectively used to determine the crystallization pressure and volume increase. The macro and microscopic mechanical models, combining with the plastic-damage model, are built to analyze the expansion-induced deterioration of concrete. The integrated model is validated by the experimental results of expansion deformation and damage distribution.

An accelerated leaching experiment, using 6M NH4Cl solution, on specimens of pure cement paste, binary cement paste with different fly ash contents (20, 30, and 50%), and ternary cement pas...

For the volume expansive of cement-based materials subjected to sulfate attack, based on microporous mechanics, the model of representative volume element (RVE) is established to analyze the distribution of microcosmic expansive stress in RVE. By homogenization method, the macroscopic equivalent stress of RVE is analyzed. The results show: on the microcosmic scale, the stress of internal sphere in RVE in all directions is compressive, and the radial stress of spherical shell is compressive, but the hoop stress is tensile; on macroscopic scale, the radial equivalent stress is compressive, the hoop equivalent stress is tensile.