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The expansion induced by sulfate attack on cement-treated aggregates (SACA) is a well-known problem that can be solved. It causes obvious heaves in road bases and railway subgrades. In this paper, the effects of the sodium sulfate content, cement content, degree of compaction, sulfate types, attack types, aluminum ion supply, concentration of curing sulfate solution, and temperature on the expansion behavior induced by SACA were investigated over 60 days in the laboratory. Based on the Sobol sensitivity analysis method, the influence of the sensitivity of each factor on the expansion was quantitatively analyzed, and the dominant factor of expansion was proposed. Results show that sulfate content is the domain factor of expansion that is induced by SACA, and it presents a logarithmic function relationship with strain. The 0.5% sodium sulfate content is the minimum sulfate content which causes the expansion that is induced by SACA. When the sulfate content is less than 1%, the expansion induced by SACA is minor. When the sulfate content is between 1% and 3%, the expansion behavior is expressed in four stages as follows: rapid strain increase, followed by a short stagnation period, then a significant strain increase and, finally, constant strain. When the sulfate content is greater than 5%, there are two stages comprising the expansion behavior as follows: the rapid strain increases and constant strain occurs. Greater sulfate content, greater degree of compaction, and lower temperature have positive effects on the expansion induced by SACA. The cement content does not have a consistent effect on expansion behavior. Compared with a sodium sulfate attack, both the reaction rate and expansion of cement-treated aggregates that are attacked by gypsum are smaller, and the attack period is also longer. When the sulfate content is greater than 1%, the addition of kaolin promotes the progression of the expansion induced by SACA. A small amount of water is sufficient for the demand for the sulfate attack. When the sulfate content is at a certain level, the expansion induced by SACA that is under external attack is much smaller than the expansion that is under internal attack. This study is expected to serve as a reference for future research on the mechanics of SACA, and it attempts to provide theoretical support for amending expansions that are induced by SACA. [ABSTRACT FROM AUTHOR]

The use of byproducts as supplementary cementitious materials (SCMs) and recycled coarse aggregates (RCAs) can contribute to the production of more sustainable self-compacting concrete (SCC) while modifying the durability properties of these mixtures. Thus, this study addresses the combined use of concrete waste (CW) as RCA and residue of masonry (RM) as SCM, the CW and RM obtained from construction and demolition waste, to generate SCC mixtures and to evaluate the performance of these mixtures under sulfate attack (5% Na2SO4 and 5% MgSO4 solutions). In detail, a reference SCC (M1) and three SCCs with 25% of RM and different level RCAs (0%, 50%, and 100% replacement by volume) were cast. The hardened properties of the SCC mixtures were assessed in terms of compressive strength and sulfate attack. The results showed that the SCC mixtures with RM and RCA, cured in water, exhibited lower compressive strength results in all the evaluated ages compared with M1 (reference mixture, without RM and RCA). However, the SCC mixture with RM presented the maximum compression strength result among the different mixtures at 180 and 360 days. When mixtures were exposed to sulfate solutions, the SCC mixture with RM showed the best performance, and even the SCC mixtures with RM and RCA exhibited lower linear expansion and loss of compressive strength when compared with M1. The presence and severity of the sulfate attack were assessed via X-ray diffraction and scanning electron micrograph, which confirmed the presence of ettringite and gypsum in all the exposed SCC mixtures in the Na2SO4 and MgSO4 solutions. [ABSTRACT FROM AUTHOR]

In this paper, firstly, the effects of graphene oxide on the mechanical properties of concrete were investigated. Secondly, the degradation and mechanism of the mechanical properties of graphene oxide concrete (GOC) under sulfate attack and a freeze–thaw environment were investigated. In addition, the dynamic modulus of elasticity (MOEdy) and uniaxial compressive strength (UCS) of the GOC were measured under different environmental conditions. According to the test results, the incorporation of graphene oxide in appropriate admixtures could improve the mechanical properties of concrete in these two working environments. It is worth noting that this effect is most pronounced when 0.05 wt% graphene oxide is incorporated. In the sulfate attack environment, the MOEdy and UTS of the GOC0.05% specimen at 120 cycles decreased by 22.28% and 24.23%, respectively, compared with the normal concrete specimens. In the freeze–thaw environment, the MOEdy and UTS of the GOC0.05% specimen at 90 cycles decreased by 13.96% and 7.58%, respectively, compared with the normal concrete specimens. The scanning electron microscope (SEM) analysis showed that graphene oxide could adjust the aggregation state of cement hydration products and its own reaction with some cement hydration crystals to form strong covalent bonds, thereby improving and enhancing the microstructure density. [ABSTRACT FROM AUTHOR]

Recycled aggregates, especially the fine (0–4 mm) fraction, are often contaminated with sulfates coming from gypsum residues on demolition sites. When these aggregates are used in concrete, the sulfates can induce internal sulfate attack which causes expansion of the concrete. European standard EN 206:2013 sets the water-soluble sulfate limit at 0.2% by weight of the aggregate, but other studies suggest this limit could be safely increased. In addition to the sulfate content, other parameters, such as the porosity and alkalinity of the mix, have been shown to influence the swelling results. In this study, different proposed sulfate limits were evaluated on concrete made with 100% fine recycled aggregates (FRA). It was also researched whether the mixing parameters could change the swelling amount regardless of the sulfate content. The results showed that the incorporation of FRA with sulfate contents up to 0.8 wt% is safe when combined with coarse natural aggregates. If coarse recycled aggregates are used in the mix, the sulfate content of FRA can be up to 0.3%. The swelling caused by these sulfate levels was not high enough to be influenced by the porosity or alkalinity of the mix. [ABSTRACT FROM AUTHOR]

External sulfate attack is an important factor causing a decrease in the mechanical properties of cement-based materials. In this paper, a computational prediction model of elastic modulus, considering the characteristics of sulfate corrosion from outside to inside and the influence of the interface transition zone (ITZ), was established to predict the elastic modulus of mortar under the external sulfate attack. Firstly, the backscattered electron (BSE) images of mortar and the algorithm of image threshold segmenting were used to determine a reasonable thickness of corroded ITZ. Secondly, the nanoindentation test was adopted to acquire the microscopic elastic parameters of phases (sand, cement, and ITZ) in corroded mortar. Moreover, the mortar mix proportion and Lu and Torquato's model were adopted to calculate the volume fractions of phases. Finally, a computational prediction model of elastic modulus of mortar under sulfate attack was proposed with homogenization methods. The results indicate that the thickness of corroded ITZ is 20 mm, and the error values of elastic modulus between the theoretical prediction results and the experimental results are within 8%, indicating that the macroscopic elastic modulus of corroded mortar can be precisely predicted by the computational prediction model of elastic modulus. [ABSTRACT FROM AUTHOR]

In order to study the performance of a new cement-based grouting material under the coupling of freeze–thaw cycle and sulfate erosion, tests related to the performance of the new grouting material were designed and carried out to analyze the damage mechanism of the material under the coupling of freezing and thawing and Na2SO4 solution by testing the mass change, relative dynamic elastic modulus, compressive strength loss and mineralogical and microstructural properties of the new grouting material. The test results show that with the increase in the number of freeze–thaw cycles, the mass loss and compressive strength loss of the specimens in 15% Na2SO4 solution gradually increased, and the relative dynamic elastic modulus showed a decreasing trend. When the freeze–thaw cycle number was 30, the mass loss rate, compressive strength loss rate and relative dynamic elastic modulus of the specimens in Na2SO4 solution were 4.17%, 24.59% and 84.3%, respectively, which showed better erosion and frost durability. Mineralogical and microstructural analysis showed that SO42− in solution led to the decomposition of the C-S-H gel and the formation of CaSO4•2H2O inside the specimen, and the internal deterioration was exacerbated by the widening of the crack width being aggravated, suggesting that the rate of material deterioration under the coupling of the two factors increased. [ABSTRACT FROM AUTHOR]

Sulfate attack is one of the main factors affecting the durability of concrete structures. In recent years, multi-walled carbon nanotubes (MWCNTs) have attracted the attention of scholars for their excellent mechanical properties and durability performance. In this paper, the influence of sulfate attack and dry–wet cycles on the performance of multi-walled carbon nanotube–lithium slag concrete (MWCNT-LSC) with varied MWCNT content (0 wt.%, 0.05 wt.%, 0.10 wt.%, and 0.15 wt.%) and varied water–cement ratios (0.35, 0.40, and 0.45) were investigated. In addition, scanning electron microscopy (SEM) and X-ray computed tomography (CT) tests were conducted to analyze the microstructure and pore structure of the concrete. The results showed that concrete incorporated with MWCNTs could effectively mitigate sulfate attack. The resistance to sulfate attack of concrete was negatively related to the water–cement ratio when the dry–wet cycle was fixed. The MWCNT-LSC showed the best compressive strength at the water–cement ratio of 0.35 and 0.10 wt.% MWCNTs. The SEM test results showed that the MWCNTs filled the pores and cracks within the specimen and formed bridges between the cracks, enhancing the resistance to sulfate attack. The CT test results also showed that the addition of MWCNTs could reduce the porosity of concrete, refine the pore size and inhibit the generation and development of cracks, thus optimizing the internal structure of concrete and improving its resistance to sulfate attack. [ABSTRACT FROM AUTHOR]

Sulfate-induced deterioration can reduce the service life of recycled aggregate concrete (RAC). To demonstrate the transport behavior of sulfate ions in RAC, a two-dimensional, six-phase numerical model of RAC is proposed in this study. Sulfate ion diffusion coefficients were defined for different phases [attached mortar, new mortar, interfacial transition zone (ITZ) between attached mortar and original aggregate, ITZ between the new mortar and original aggregate, and ITZ between the new mortar and attached mortar] of RAC. The effects of ITZ width and porosity on ion transport behavior in RAC were analyzed. The results show that the numerical model agrees with the experimental data. Numerical results indicate that the six-phase model of RAC provides more-accurate simulation results because tortuosity is reflected in the model. The width and porosity of ITZs both affect the diffusion of sulfate ions in concrete. However, due to the low volume fraction of ITZ in concrete, just improving ITZ performance has no significant effect on ion attack resistance of concrete. The ITZ is a crucial zone where damage may originate and develop. [ABSTRACT FROM AUTHOR]

The reactive MgO (r-MgO)-microbial curing technology can not only effectively improve the strength of electrolytic manganese residue (EMR) but also repair heavy metal ions. By conducting unconfined compressive strength (UCS) test, heavy metal leaching test, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP), the effects of the type and concentration of sulfate on the strength and repair of heavy metals of the cured EMR under drying–wetting (DW) cycles were studied, and the strength and the evolution mechanism of heavy metal remediation is elucidated. The results indicate the following: With the increase in the number of DW cycles, the dry density of the samples in high concentration of MgSO4 first increased slightly and then decreased gradually and became stable. The dry density of the samples in Na2SO4, low concentration of MgSO4, and water generally showed a downward trend. The pH of the soaking solution slightly decreased, and the leaching concentration of heavy metal Mn2+ ions always remained at a low level. The UCS of the samples in MgSO4 first increased slightly and then decreased gradually, and then became stable. The amount of cured products first increased and then decreased, and the size of internal pores first decreased and then increased. The UCS of the samples in Na2SO4 and water gradually decreased and became stable. The amount of cured products gradually decreased, and the size of internal pores gradually increased. The research results provide a theoretical basis for the evaluation of resistance against the sulfate corrosion of cured EMR under DW cycles. [ABSTRACT FROM AUTHOR]

The fracture toughness (KIC) of Mode I cracked sandstone exposed to sodium sulfate was investigated in this study. The uniaxial behavior of sandstone specimens soaked in different sodium sulfate solutions was determined for different strain rates. The results revealed that the strain rate has a significant and positive influence on the deformation and strength characteristics of sandstone. Compression strength gradually increased with strain rate at an increased slope, but peak strain decreased gradually with an increase in strain rate. The physical–mechanical characteristics and brittleness of sandstone gradually declined with increasing immersion time under sodium sulfate attack, but its plasticity increased significantly. There were holes and pitting corrosions in specimen surfaces exposed to sodium sulfate attack, and the edges and corners were somewhat weakened. Chemical damage variables were proposed to analyze the damage degree of sandstone, based on the change in porosity. Based on the testing results, regression analysis was conducted to determine the relations between ion concentrations (Ca2+ and Mg2+) in corresponding sodium sulfate solutions and for corresponding degrees of damage, and indirectly realize correlations between physical–mechanical characteristics, damage degree, and ion concentration. [ABSTRACT FROM AUTHOR]

Cement–bentonite (CB) slurry cutoff walls are extensively utilized to restrict the horizontal flow of fluids. In this study, cement was replaced by four various contents of 0%, 10%, 20%, and 30% by natural zeolite to evaluate its effects on the unconfined compressive strength, flexural strength, tensile strength, and the coefficient of permeability of cement–bentonite–zeolite (CBZ) specimens. Since underground cutoff walls are always in danger of sulfate attack, the CBZ specimens were immersed in three different containers with three different pH values of 2, 5, and 8 to study the sulfate resistance of this material. Results have indicated that the strength of specimens decreases with an increase in zeolite content at early ages; however, increasing the amount of zeolite up to 20% improves the mechanical properties and permeability of cutoff wall materials at late ages. The bentonite/cement (B/C) ratio equal to 0.2 shows a relatively better response than B/C ratios equal to 0.14 and 0.3. Curing specimens in the sulfate environment increased the mechanical strength of specimens and decreased the coefficient of permeability at first ages. At the late ages, curing in sulfate solutions manifested disadvantageous effects on specimens; however, specimens containing zeolite indicated higher sulfate resistance than specimens without zeolite. [ABSTRACT FROM AUTHOR]

Recycling coal gangue as aggregate to produce concrete in situ is the most effective way to solve the problem of deposited coal gangue in mines. Nevertheless, the mine environment underground is rich in sulfate ions, posing a threat to the durability of coal gangue concrete (CGC). Hence, the degradation process of sulfate-attacked CGC is investigated. A series of tests is performed to evaluate its variation law of mass, dynamic elastic modulus, compressive strength and sulfate ion distribution. Meanwhile, the microstructure and phases of sulfate-attacked CGC are identified by scanning electron microscopy, X-ray diffraction and thermogravimetric analysis. The results indicate that the residual compressive strength ratio of CGC is higher than that of normal concrete after a 240 d sulfate attack, implying a superior sulfate resistance for CGC. Additionally, the higher the sulfate concentration, the more severe the degradation. Except for the secondary hydration of CGC itself, the diffused sulfate ions also react with Ca(OH)2, forming gypsum and ettringite; this plays a positive role in filling the pores at the early stage, whereas, at the later stage, the generated micro-cracks are detrimental to the performance of CGC. In particular, the proposed sulfate corrosion model elucidates the degradation mechanism of CGC exposed to a sulfate-rich environment. [ABSTRACT FROM AUTHOR]

This paper investigated the influence of dry–wet cycles and sulfate attack on the performance of magnesium potassium phosphate cement (MKPC) as well as the effect of waterglass on MKPC. X-ray diffraction (XRD), TG-DTG, and scanning electron microscopy (SEM-EDS) were used to examine the phase composition and microstructure of MKPC. The results showed that the flexural and compressive strength of an MKPC paste increased initially and subsequently decreased in different erosion environments. The final strength of the M0 paste exposed to the SK-II environment was the highest, while that of the M0 paste exposed to the DW-II environment was the lowest. The final volume expansion value of MKPC specimens under four corrosion conditions decreased in the following order: DW-II, M0 > SK-II, M0 > DW-II, M1 > SK-I, M0 > DW-I, M0. Compared to the full-soaking environment, the dry–wet cycles accelerated sulfate erosion and the appearance of damages in the macro and micro structure of the MKPC paste. With the increase in the number of the dry and wet cycles, more intrinsic micro-cracks were observed, and the dissolution of hydration products was accelerated. Under the same number of dry–wet cycles, the strength test and volume stability test showed that the durability in a Na2SO4 solution of the MKPC paste prepared with 2% waterglass (M1) was superior to that of the original M0 cement. The micro analysis indicated that waterglass can improve the compactness of the microstructure of MPC and prevent the dissolution of struvite-K. [ABSTRACT FROM AUTHOR]

The durability of C3S-C3A paste with varied C3A content (0%, 5%, 10%, and 20%) against sulfate attack at various attack ages (3 d, 7 d, 28 d, and 180 d) was investigated in this study through the examinations of corrosion product composition, Ca/Si and Al/Si of calcium-(aluminum)-silicate-hydrate (C-(A)-S-H) gel, formation and evolution of microstructure, migration and transformation of Al containing phase products, and pore structure. The results indicated that sulfate attack can promote the hydration reaction in C3S-C3A paste, thus accelerating the production of C-(A)-S-H gel in the paste. With the increase of C3A content, the acceleration effect becomes more significant. In addition, sulfate attack led to the dealumination and decalcification of C-(A)-S-H gel, resulting in the reduction of the gelling power of C-(A)-S-H gel. The degree of dealumination and decalcification of C-(A)-S-H gel increases with the increase of C3A content. At the same time, free Al and Ca promote the formation of expansive products such as ettringite and gypsum. Finally, under the action of sulfate, the pore characterization of C3S-C3A paste deteriorated, showing a decrease in specific surface area, cumulative pore volume and average pore diameter. [ABSTRACT FROM AUTHOR]

Destructive processes accompanying sulfate corrosion of concrete significantly affect the durability of products and structures based on Portland cement. In the presented study, the long-term effect of sulfate corrosion on the electrical properties of electrically conductive sand concrete was studied. In the course of the study, the following were tested: an electrically conductive composition and a control composition based on plain Portland cement. The analysis of changes in the mineral composition of the samples over the course of time in an aggressive solution was carried out. The results show that during the exposure period of the samples from 28 to 224 days, the absorption of sulfate ions slows down and averages 26% for the control composition and 29% for the electrically conductive composition, of the total volume of absorbed sulfates. At the same time, the course of sulfate corrosion was accompanied by a 6% increase in the density of samples of both compositions, as well as a cyclic change in mechanical strength within 15%. In its turn, the key indicator of the electrical characteristics of the compositions—electrical resistivity—tended to increase throughout the experiment. These results can be recommended for assessing the durability and the nature of the operating conditions of electrical concretes used in aggressive environments. [ABSTRACT FROM AUTHOR]

This study investigated the effect of layered double hydroxides (LDHs) on the deterioration process of cement paste in the sulfate environment. Cement pastes with the addition of original and calcined LDHs at 2.5 wt.% and 5.0 wt.% of cement were exposed to Na2SO4 solution for 360 days. The macroscopic performance of the cement paste was assessed based on mass variation, porosity, compressive strength, and content of sulfate ions. Furthermore, the microhardness, microstructures, and composition of the degraded pastes were examined using Vickers hardness (HV), mercury intrusion porosimetry (MIP), scanning electron microscope (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The results indicate that cement paste incorporated with LDHs can mitigate the corrosion caused by sulfate effectively, especially in the case of calcined LDHs (C-LDHs), which primarily increase the adsorption of sulfate. Compared with the control specimen, the 180 d compressive strength loss ratio of specimens with 2.5 wt.% and 5.0 wt.% of C-LDHs decreased by 63.66% and 80.51%, respectively. Moreover, LDHs can reduce the amount of ettringite crystals, densify the microstructure, and refine the pore structure to mitigate the cement paste's sulfate corrosion significantly. Compared with the control specimen, the 180 d harmful pore volume fraction of specimens laced with 2.5 wt.% and 5.0 wt.% C-LDHs decreased by 43.77% and 54.51%, respectively. In terms of the content of C-LDHs, an optimal content of C-LDHs could ensure the dominant effect of adsorption, while excessive C-LDHs could refine pores. In addition, Vickers hardness has an excellent correlation with compressive strength, which could precisely predict the compressive strength. Moreover, by combining the Vickers hardness distribution and content distribution of sulfate ions, the cross-section of the paste could be classified into four regions to evaluate the deterioration process accurately: the degraded zone, the strengthened zone, the invaded zone, and the intact zone. [ABSTRACT FROM AUTHOR]

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]

Nano-silica (NS) is an effective material to improve the strength and durability of high-performance concrete (HPC), but little information is available regarding its role in HPC response to long-term sulfate attack. In this study, six different dosages of NS (0%, 1%, 2%, 3%, 4%, and 5%) as cement partial replacement were mixed into HPC and the casted specimens were soaked in sulfate solution for different periods (0, 100, 200, and 300 days). The mass change, dynamic elastic modulus, compressive and splitting strength, microstructure morphology, and porosity characteristics of HPC specimens were measured by mass tests, mechanical properties tests, scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) tests. The results showed that the incorporation of NS decreased the mass loss, elevated the compressive and splitting strength, and reduced the porosity formation of HPC in front of sulfate attack. The percentage of 1% NS was among the most effective dosages as, after soaking for 300 days, it decreased the mass loss by 13.5%, elevated the elastic modulus as well as compressive and splitting strength by 50.4%, 31.7%, and 69.8% in comparison of unmodified HPC, respectively. The sulfate attack resistance was delayed in a higher (2–5%) mixed dosage, mainly due to the agglomeration of nano particles, especially after long-term reactions. This study can provide experimental references regarding the performance of HPC mixed with NS in front of sulfate attack. [ABSTRACT FROM AUTHOR]

The sulfate attack (SA) of cementitious materials widely exists, which severely reduces the mechanical properties and durability of cementitious materials. However, few numerical SA damage models of cementitious materials have been comprehensively developed. The understanding and simulation of the chemo-mechanical behavior of cementitious materials are beneficial to better modeling, predictions and designs of underground structures. In this research, a numerical chemo-micromechanical damage model under the SA and loading is proposed to explore the cracking process of the cementitious materials based on the discrete element method (DEM). Some inherent characteristics of the chemical attack can be well considered in the proposed numerical model. The local damage effect can be simulated, as well as the sulfate corrosion degree and the material component. The performance of this proposed model is confirmed and validated by available experimental results. We examine the dependence of the strength and the elastic moduli of the cementitious material on: (a) the content of ettringite, (b) the distribution of ettringite, (c) the mechanical properties of the cementitious matrix, (d) the void ratio of the cementitious materials, (e) the confining pressure, and (f) the expansion coefficient of sulfate corrosion products. The softening and fluctuating phenomena are witnessed in the stress-strain curves. The failure mode of cementitious materials with the SA is different from that of specimens without the SA. The damage parameters of cementitious materials under the SA are examined in depth. The numerical results reveal that the proposed damage model is feasible and valuable in the modeling, predictions, applications and durability of chemo-mechanical sulfate attack problems in cementitious materials. [ABSTRACT FROM AUTHOR]

Sulfate attack is one of the drawbacks of cementitious materials for stabilized soils. In the current study, a durability comparison of stabilized soil with cement (Type IV) and waste paper fly ash (WPFA) was conducted. First, the treated soil's unconfined compressive strength (UCS) was tested. Next, the treated soil was subjected to various wetting/drying cycles with various sulfate concentrations and temperatures for a year. In the meantime, samples were taken for DRX, FTIR, and TGA microstructural analyses. Additionally, samples were manufactured to track swelling over an 800 day period. The outcomes show that WPFA's UCS remained constant. Furthermore, ettringite development can be seen in the microstructural studies, however testing on linear displacement over 800 days revealed no significant changes in swelling. Finally, SEM was used to verify the ettringite formation at 360 days in order to confirm the previous findings. All the results indicated that stabilizing soil with 5% of WPFA and 3% of cement IV is possible even in presence of high sulfate concentrations, while maintaining the durability of the structure. [ABSTRACT FROM AUTHOR]

The cemented gangue backfill body (CGBB) in the backfill mining is not only subjected to the axial compressive stress of overlying strata but also corroded by mine water. To study the mechanical properties of CGBB under the coupling action of axial load and mine water with sulfate, stress corrosion tests were carried out considering the Na 2 SO 4 solution concentration, loading stress level and initial loading age. The changes in surface characteristics, ultrasonic pulse velocity (UPV) and compressive strength were examined, and the strength deterioration model under the influence of multiple factors was established. The microscopic mechanism was analyzed through the testing results of SEM and XRD. The results showed the compressive strength and UPV of eroded CGBB both increased first and then decreased with the increase of soaking age. The deterioration of CGBB increased with the increase of the Na 2 SO 4 solution concentration. The stress level significantly affected the deterioration process of eroded specimens. The sulfate resistance of CGBB gradually increased with the increase of stress levels. When the stress level is 60 %, the sulfate resistance increased with the decrease of the initial loading age. The research results can provide significant guiding value for the design of CGBB. • The impact of multiple factors on the long-term corrosion of CGBB is examined. • The self-designed stress loading device is utilized for long-term loading purposes. • A strength deterioration aging model considering multiple factors is established. [ABSTRACT FROM AUTHOR]

It is necessary to accurately, efficiently, and economically test SO 4 2- in cementitious materials subjected to sulfate attack for the calculation of diffusion coefficient, estimating damage degree and analysis of corrosion mechanism. In this study, the test of free and total SO 4 2- in cementitious materials was investigated by conductivity titration. The results show that the key to test SO 4 2- by conductivity titration is removement of the influential ions such as H+ and OH- with higher molar conductivity. In the test of free SO 4 2-, HCl or CO 2 can be used to eliminate the effect of OH- or OH- and Ca2+. In the test of total SO 4 2-, the combination of Ag 2 O with Ag 2 CO 3 or the combination of Ag 2 O with CO 2 can be used to eliminate the effect of H+, Cl-, and Ca2+ where Ag 2 O and CO 2 is more applicable to eliminate influential ions because CO 2 is easily controlled by CO 2 generator. The ultrasonic can be used to clean the AgCl attached to the surface of Ag 2 O to increase the hydrolysis reaction of Ag 2 O. To eliminate the effect of reaction between Ag+ from hydrolysis reaction of Ag 2 O and Cl- from the conventional titrant of BaCl 2 on titration endpoint, the Ba(NO 3) 2 is chosen as a new titrant. • HCl or CO2 can be used to eliminate OH- and Ca2+ during the test of free SO42–. • Ag2O and Ag2CO3, Ag2O and CO2 can be used to eliminate H+, Cl- and Ca2+ during the test of total SO42–. • The content of Ag2O can be calculated according to the Cl- from HCl during the pre-treatments of titrating total SO42–. [ABSTRACT FROM AUTHOR]

In order to analysis the sulfate diffusion process in polypropylene fiber concrete, the influence of the introduction of polypropylene fiber on the porosity was calculated at first based on the spatial distribution features of polypropylene fiber in concrete, which was classified into three types, penetration type, half through type and embedded type. On the basis of Fick's second law and reaction kinetics equation, the diffusion model of sulfate ions in polypropylene concrete was established. The numerical solution was achieved with the adoption of finite difference method and the concentration of sulfate ions at any position can be calculated. At last, the numerical solution was verified using the results of polypropylene fiber reinforced concrete sulfate attack test. The results show that the relative error between numerical calculation and the actual test results is less than 17.65 %, the accuracy of the model is relatively high. [ABSTRACT FROM AUTHOR]

The stability of the grouting material under the coupling effect of sulfate attack and dry–wet cycles is essential to engineering safety. As a novel grouting material, the stability research of high‐water quick‐setting material (HWQSM) under different corrosive environments is in the initial stage. Moreover, the stratified damage characteristics (SDC) of HWQSM under sulfate attack were not fully explored. To this end, the strength degradation and SDC of HWQSM exposed to three kinds of sulfate solutions (i.e., 10% MgSO4 solution, 10% Na2SO4 solution, and hybrid solution of 5% Na2SO4 and 5% MgSO4) and dry–wet cycles were analyzed by using ETM‐C mechanical testing machine, scanning electron microscopy (SEM), XRD, and AFM. The results show that the strength reduction rate, the thickness of the corrosion layer, and roughness between the corrosion layer and the transition layer of HWQSM are all positively correlated with the concentration of Mg2+ in the erosion solution for the increase in dry–wet cycles. Under the combined action of ion erosion and carbonization, the pores and cracks of the corrosion layer expand, which would reduce the permeability resistance but increase the erosion damage rate. This research furnishes new views into the SDC of HWQSM under sulfate attack and dry–wet cycles, which would deeper our understanding to this special material undoubtedly; the research results are expected to furnish theoretical guidance of HWQSM resistance to different sulfate solutions. [ABSTRACT FROM AUTHOR]

Concrete structures without sufficient durability can be damaged by sulfates in groundwater and from surrounding rock layers. To evaluate the performance of a concrete mixture, precise and performance-oriented test methods are a must. Therefore, a new a performance oriented concrete test procedure based on tensile strength measurements was developed considering experiences reported in international literature and recommendations of state-of-the-art reports. A vast parameter study with approx. 3850 tensile tests on ASTM briquets, 1900 flexural tensile tests on standard prisms and 2100 expansion tests on mortar flat prisms of different ages and with different storage conditions was statistically assessed. Based on the results a performance-oriented test method could be defined which considers not only the chemical, but also the physical resistance of a concrete against sulfate attack. The method was verified by 23 concretes with different cements or cement fly ash combinations and additional field tests. It could clearly be demonstrated that the results represent the performance of a practical concrete in case of sulfate attack. Furthermore, it leads much faster to an evaluation of the sulfate resistance compared to the most other practical oriented methods. [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 coastal areas, concrete of subway infrastructure can be jeopardized by coupled degradation conditions of stray current and sulfate solution. This paper focuses on the effect of stray current on the durability of cement-based materials exposed to sulfate-rich environments. In addition to chemical analysis, mechanical, transport, and microstructural properties of specimens exposed to five conditions were analyzed: (1) control environment with no degradation conditions, (2) stray current, (3) internal mixing sulfate, (4) stray current and internal mixing sulfate, and (5) stray current and external sulfate solution. Experimental results of this study reveal that (1) stray current can exacerbate leaching of cement-based materials, and 40-V stray current results in greater leaching compared to specimens subjected to 20-V stray current; (2) compared to specimens subjected to internal mixing sulfate alone, the combined effect of stray current on the production of gypsum and ettringite is slight due to the increase in particle collision induced by stray current; and (3) stray current has a migration effect on the rate of SO42− ions ingress, which significantly aggravates the external sulfate attack on cement-based materials. The primary degradation products were ettringite and gypsum, which results in the cracking and the maximum decrease in strength of 47.1% after exposure to coupled conditions of 40-V stray current and external sulfate solution for 150 days. [ABSTRACT FROM AUTHOR]

The durability of concrete structures is usually threatened by sulfate attack in their service progress. However, there is still no widely accepted model that can accurately predict the process of sulfate attack on concrete. Combined the diffusion-reaction process, constitutive relationship, and fracture criterion, a mechanistic model is proposed by considering the effects of pore filled by products of sulfate attack on the diffusion coefficient and total expansion strain in this study. The establishment assumptions of model can be observed from the macro- and micromorphology of cement-based materials subjected to sulfate attack. The results show that the route of decohesion goes along the interface between the compressed degraded layer and the tensed non- degraded matrix as the triggered by sulfate attack, and the thickness of decohesion is between 50 and 400 μ m. The time index used to correct the diffusion coefficient is close to −0.41, while the range of pore filling degree is 3.16–6.29% based on the debonding thickness. Compared with the Atkinson model, the degradation rate calculated from the modified model is the closest to the actual test value with an error less than 10%. • Three routes of decohesion could occur in the eroded layer, noneroded matrix, and their interface. • The expansion strain resulting from corrosion products does not contribute to the final expansion strain. • The cement-based materials subjected to sulfate attack can be divided into a spalling zone, expansion zone, decohesion zone and noneroded zone. [ABSTRACT FROM AUTHOR]

This study investigated the performance of 100% recycled coarse aggregate concrete (RAC) containing limestone powder (5% and 10%) and EDTA-2Na (0.4% and 0.8%) in 5% Na 2 SO 4 solution for 360 days. Value engineering and environmental analysis were conducted to determine the optimum proportion of 100% RAC. Results showed that the 28-d compressive strength of RAC decreased by 12.2% when using 100% recycled coarse aggregate. After the sulfate attack, the 100% RAC seriously deteriorated because of the weak interface transition zone. Adding 5% limestone powder refined the pore structure of 100% RAC and improved its sulfate resistance, although the 28-d compressive strength decreased. Adding the proper amount of EDTA-2Na (0.4%) can reduce the mass loss of concrete during the sulfate attack, which is attributed to the promotion of hydration of cement and the decrease in calcium ions by chelating. Besides, the crystals of ettringite in the specimen containing EDTA-2Na was thicker and longer, less damaging to concrete than the thinner needle-like ettringite crystals observed in 100% RAC. The value engineering and environmental analysis showed that applying 100% RAC containing 5% limestone powder and 0.4% EDTA-2Na in a sulfate environment was optimum and had good ecological and environmental effects. [Display omitted] • The 100% RAC was more deteriorated after sulfate attack owing to the weak ITZ. • Appropriate limestone and EDTA-2Na improved the sulfate resistance of RAC. • RAC with 5% limestone and 0.4% EDTA-2Na had good economic and environmental effects. [ABSTRACT FROM AUTHOR]

Rebar corrosion induced by chloride erosion and concrete deterioration caused by sulfate attack are two critical factors that affect bond properties between rebar and concrete in marine environment. However, current studies about above two influencing factors on bond properties either consider rebar corrosion or sulfate attack. The bond properties between rebar and concrete considering the coupled effects of rebar corrosion and concrete deterioration caused by sulfate attack are required to further explored. In this paper, through the accelerate corrosion test, pull-out test and microscopic analysis (including scanning electron microscopy and X-ray diffraction measurements), the bond properties considering rebar corrosion and concrete deterioration caused by sulfate attack were investigated. A total of 88 RC specimens were designed to study the influences of rebar corrosion level, sulfate attack degree, sulfate ions associated cation types (sodium and magnesium ions) and concrete materials types (ordinary concrete and concrete incorporating fly ash and silica fume) on bond properties. The bond strength, bond-slip curve and failure mode were measured, and the effects of cracking width on bond properties were considered. Results show that the rebar corrosion level corresponding to the peak bond strength decreases with the increase of sulfate concentration. The bond strength decreases and the slip increases with the rise of sulfate concentration. Sodium sulfate attack causes the splitting failure, and severer magnesium sulfate attack leads to the transition from splitting failure to pull-out failure. The rebar corrosion level causing the concrete to crack decreases with the increase of sodium sulfate concentration. The higher the magnesium sulfate concentration, the less prone to produce rust-expansion cracks, but the worse cementing properties of RC bonding interface. Magnesium sulfate attack results in the severer attenuation of bond strength than sodium sulfate attack. The bond properties can be significantly improved by incorporating 20% fly ash and 5% silica fume. A prediction model of bond strength variation coefficient considering rebar corrosion level, sulfate ions concentration, sulfate ions associated cation types, and concrete materials types was established and verified. • Reveal bond properties of RC considering rebar corrosion and sulfate attack. • Analyze relationship between rust-expansion cracks and bond properties under sulfate attack. • Compare bond properties under sodium sulfate and magnesium sulfate attack. • Establish bond strength prediction model considering rebar corrosion level and sulfate attack. [ABSTRACT FROM AUTHOR]

Seawater sea sand concrete (SWSSC) is considered a potential solution to the shortage of river sand and freshwater resources in the coastal and island regions. Under the marine environment, the sulfate ions in the seawater and sea sand are detrimental to the properties of concrete. Due to the lack of systematic research on the sulfate attack characteristics of SWSSC exposed to different marine conditions, this study compare the sulfate resistance between SWSSC and freshwater desalinated sea sand concrete (FDC) under full immersion, semi-immersion, and dry-wet cycle exposure through the compressive strength, sulfate ion distribution, and microstructural changes after erosion. The findings showed that among the three different exposure conditions, the damage in SWSSC-F (fully immersion) was more severe than that in SWSSC-S (semi-immersion) and SWSSC-DW (dry-wet cycle). The SO 4 2- concentration in inner SWSSC-F was 9.3% and 21.3% higher than that in SWSSC-DW and SWSSC-S, respectively. The microstructural analysis indicated that the higher amount of sulfate attack products and the leaching of Ca2+/OH- in SWSSC-F significantly damaged its microstructure. Besides, carbonation and physical crystallization in SWSSC-S and SWSSC-DW also refined their pore structure, thus enhancing their sulfate resistance. SWSSC showed better sulfate resistance than FDC under different marine exposure conditions in terms of lower erosion product content and better pore structure distribution after 12 months of erosion. The formation of Friedel's salt in SWSSC suppressed the formation of expansion products and reduced the erosion damage of concrete by SO 4 2-. • Seawater sea-sand concrete (SWSSC) showed better sulfate resistance than ordinary concrete. • Concrete under full immersion showed a more severe sulfate attack than that under semi-immersion and dry-wet cycle conditions. • SWSSC under full immersion generated more expansive products. • Carbonation and physical crystallization in SWSSC under semi-immersion and dry-wet cycle conditions refined the pore structure. [ABSTRACT FROM AUTHOR]

To solve the problem of high carbon emission in RPC production, fly ash (FA) and ground granulated blast furnace slag (GGBS) were used to replace part of cement for the preparation of RPC in this study. The sulfate corrosion resistance of RPC containing mineral admixture was investigated through dry and wet cycle test using a 5% Na 2 SO 4 solution. The influence of different amounts of FA and GGBS replacing cement on the deterioration of the performance of RPC was studied, and the formulas for the mass and compressive strength loss of RPC under dry-wet cycling were developed. The results show that: after the Na 2 SO 4 solution penetrates RPC, it reacts with the cement hydration products to produce erosion products such as gypsum and ettringite (AFt), and due to the expansion effect of erosion products, the gel pores decrease, and the harmful pores gradually increase, causing changes in the strength and quality; the introduction of FA and GGBS serves to fill the pores and diminish the presence of hydrated calcium hydroxide, thus augmenting matrix densification. Strategic substitution optimally harnesses the "superposition effect" of FA and GGBS, resulting in a substantial enhancement of RPC's resistance to sulfate attack. In addition, there is a significant correlation between changes in the microscopic characteristics of RPC and its macroscopic performance. [ABSTRACT FROM AUTHOR]

Supersulfated cement is mainly composed of granulated blast furnace slag, gypsum and a small amount of alkaline activator. The objective of this work is to study the deterioration mechanism of supersulfated cement paste containing phosphogypsum (SSC) exposed to sodium sulfate and hydrochloric acid-sodium sulfate solutions, which was compared to that of ordinary Portland cement (OPC) paste. The mechanical properties of SSC were characterized by compressive strength and its retention rate. Microscopic techniques such as XRD, FTIR, TG-DTG, MIP and SEM were employed to analyze hydration products changes and microstructural alterations in SSC paste, unveiling the deterioration mechanism. Soaked in Na 2 SO 4 solution, OPC generated erosion products ettringite and gypsum, which led to expansion and cracking of the cement paste. In contrast, the hydration products of SSC remained largely unchanged, demonstrating high resistance to sulfate attack. Soaked in HCl-Na 2 SO 4 composite solution, H+ and SO 4 2- reacted with Ca(OH) 2 in OPC leading to gypsum formation and subsequent deterioration of the cement paste. The reaction of SSC with H+ and SO 4 2- in the environment resulted in ettringite decomposition, C-S-H decalcification and gypsum formation, which damaged the structure of SSC paste. Nevertheless, compared to OPC, SSC exhibited the higher ability to maintain mechanical performance under aggressive environment. • Supersulfated cement exhibited excellent resistance to sulphate attack. • SSC had better performance than ordinary Portland cement when exposed to a composite solution of HCl-Na 2 SO 4. • The deterioration of OPC by HCl-Na 2 SO 4 composite solution resulted from the reaction of H+ and SO 4 2- with Ca(OH) 2 to form gypsum. • Ettringite decomposition and C-S-H decalcification resulted in gypsum formation, contributing to the deterioration of SSC. [ABSTRACT FROM AUTHOR]

Durability of concrete exposed to an external sulfate attack is a great concern for the long-term reliability of nuclear waste containment. This paper presents experimental results obtained on cement paste subjected to a low concentration (30 ×10−3 mol/L) external sulfate attack. Samples were characterized with complementary methods (XRD, SEM-EDS, microtomography, microindentation, autoradiography) to follow the evolution of chemical, mineralogical, microstructural and mechanical properties during 8 months. Cement pastes of different C 3 A contents were used to boost either ettringite or gypsum formation during the sulfate attack to assess their respective impact on the degradation. Cracks parallel to the attacked surface occurred in the zone of gypsum formation and portlandite dissolution, which suggests that gypsum has a key effect on the expansion. Reactive transport modeling supported the discussion on the competition between hydrolysis, gypsum and ettringite precipitation as well as the analytical homogenization to estimate Young's moduli. • Multiple characterization tools were used including a synchrotron radiation CT. • Contributions of hydrolysis and sulfate attack on damage were discriminated. • Expansion occurred even for low C3A content and groundwater SO 4 concentration. • Cracking was initiated in the gypsum formation zone. • Ettringite formation was identified by microindentation and homogenization. [ABSTRACT FROM AUTHOR]

Triethanolamine (TEA) is a multifunctional organic additive, applied chiefly to a grinding aid and accelerator for cement. Its effect on cement paste in a sulfate-rich environment remains still unclear and elusive. In this paper, the expansion and mechanical performance of cement paste under external sulfate attack were measured. Thermodynamic model lent itself to estimate the correlation between triethanolamine and generated products content ahead of quantified phase analysis. The transformation of internal component was surveyed by multiple technologies of thermal gravimetric (TG), scanning electron microscope (SEM), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR), while inductively coupled plasma (ICP) was applied to investigate the evolution of interstitial ion concentration. Furthermore, the changes of pore structure over immersion time were separately figured and tabulated by virtue of mercury intrusion porosimetry (MIP). The above analysis shows that when cement paste suffers sulfate attack, triethanolamine intensifies the formation of calcite and exacerbates the decalcification of C-S-H, promoting a large quantity of insoluble precipitation including mirabilite of abundant crystalline water which accounts essentially for severe damage. Determined after comprehensive evaluation, the addition of TEA is conducive to the emergence of physical sulfate attack and an increment of the total porosity and permeability, contributing to an extension of microcracks and a more fragile structure, ultimately makes cement paste appear as a reduction in strength and stiffness. • Triethanolamine correlates adversely with the resistance to sulfate attack of cement paste • The complex of sulfate and aluminum ions occupied by the TEA intensifies decalcification of C-S-H • Triethanolamine loosens internal structure and permeability paths inside cement paste within sulfate attack [ABSTRACT FROM AUTHOR]

Sulfate attack–induced expansion of cement-treated aggregates is a well-known problem to be addressed causing remarkable heave in road and railway subgrades. The specific surface area (SSA) of the materials influences chemical reactions significantly. In this study, cement, sodium sulfate, and basalt powder were ground in a ball mill for 0 (without grinding), 5, 10, 30, and 60 min respectively to achieve different SSA gradients. Under the curing condition of a constant temperature of 10 °C and humidity of 70%, with 5% cement, the effects of sodium sulfate contents and SSA gradients on the expansion of sulfate attack in cement-treated aggregate (SACA) were investigated during 60 days in laboratory. Based on nitrogen absorption methods, microscopic tests, and swelling tests, the results show that, grinding within 60 min, the respective peak value of SSAs of cement, mixture of 3% (or 8%) sodium sulfate and basalt powder, and mixture of 3% (or 8%) sodium sulfate, cement, and basalt powder was 1.867 m2/g, 1.109 m2/g (or 1.393 m2/g), and 1.128 m2/g (or 1.404 m2/g), respectively. The SSAs increase as the aggregates refined, then decrease as the aggregates gathered. The strain variation can be generally divided into four stages with 3% sodium sulfate attack in cement-treated aggregate (3% SACA): rapid strain increase, short stagnation, continuous heave, and constant strain. Two stages of strain variation have been observed with 8% sodium sulfate attack in cement-treated aggregate (8% SACA): rapid strain increase, and a constant strain. Finer needle-like ettringite crystals (average length: 0.5–1 μm) filling in the smaller holes produce the greater expansion induced by SACA. Based on grey incidence analysis, the SSAs of cement, mixture of sodium sulfate and basalt powder (S+B), and the whole mixture of cement, sodium sulfate, and basalt powder (C+S+B) have a significant effect on strain. This work provides theoretical support for amending SACA at the SSA level. [ABSTRACT FROM AUTHOR]

This study investigates the acoustic emission (AE) and physicomechanical properties of concrete under three sulfate attack conditions: continuous immersion, wetting–drying cycles, and cyclic loading coupled with wetting–drying cycles. The characteristics of AE absolute energy during the concrete compression process were statistically analyzed by the histogram method and maximum likelihood (ML) method. The results from this study showed that the concrete specimens experienced two stages: an enhancement stage and a subsequent deterioration stage. Both wetting–drying cycles and cyclic loading accelerated the sulfate attack process. The probability density distribution of AE absolute energy satisfied a power law, and the distribution exponent was related to the degree of sulfate attack. The filling of expansion products during the enhancement stage led to the ML curves displaying a good plateau with lower exponents. Conversely, the cracking occurring during the deterioration stage created upturned ML curves with higher exponents. The results indicate that the exponent can accurately characterize the degree of concrete degradation under sulfate attack. [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]

[Display omitted] • Modified sulfoaluminate cement was prepared as a new kind of cutoff wall material. • Calcined layered double hydroxide is added to cement to improve its properties. • The diffusion distribution curve was fitted using an exponential function. • Optimal ratio selected by anti-corrosion and barrier ability. • The effect of the cutoff wall for sulfate was simulated by Visual MODFLOW. Calcined layered double hydroxide (CLDH) was used to modify sulfoaluminate cement (SAC) mortar to develop a new sulfate-resistant barrier material. Through investigations into mechanical properties, permeability, microstructure, and migration characteristics, the results demonstrated that the addition of CLDH improved the mechanical properties and pore structure of SAC mortar and inhibited the diffusion of SO 4 2-. Based on its anti-corrosion and barrier ability, the optimal CLDH dosage in SAC mortar was determined to be 2 % of the cement mass. Service life calculations and Visual MODFLOW simulations demonstrated that the SAC-CLDH cutoff wall has significant durability and barrier effect on sulfate. Compared to the site without the SAC-CLDH cutoff wall, the effective dispersion distance and area of sulfate decreased by 88.79 % and 93.43 % during 4000 days. This finding provides a new reference for the containment and management of high-concentration sulfate-contaminated sites. [ABSTRACT FROM AUTHOR]

• A coupled PD model of sulfate chemo-diffusion-mechanical is developed. • The PD pairwise force function considering the expansion effect is constructed. • A distortion-based damage model is developed to identify the bond damage fracture. • The interaction between crack propagation and sulfate diffusion is considered. • The results of the PD model coincide well with the analytical and experimental results. Sulfate attack is an important cause of concrete durability deterioration. A multi-field peridynamic (PD) model is developed to investigate the expansion and cracking behaviour of concrete under sulfate attack. The deformation at the material point is decomposed into expansion and distortion, and an expression for expansion is derived in the PD framework. A novel pairwise force function is constructed to capture the expansion effect. Additionally, a distortion-based damage criterion is established to determine bond failure in expansion-dominated deformation. The mutually reinforcing effects between crack propagation and sulfate diffusion are explored. Several numerical examples are performed to verify the validity and accuracy of the PD model. The results of the coupled PD model exhibit in good agreement with those obtained from the analytical solutions, finite difference method, and experimental findings. The developed model further elucidates the profound interplay between concrete expansion cracking, chemical reaction and expansion process during sulfate attack. [ABSTRACT FROM AUTHOR]

[Display omitted] • Applied electric fields were found to expedite sulfate attack in cementitious materials significantly. • In soil environments, electric fields increased sulfate ion ingress by 2–4 times versus diffusion only. • Ettringite formed at low sulfate levels, while gypsum prevailed at higher concentrations. • The extent of degradation followed the order: semi-immersion > full immersion > full immersion in soil. Stray currents in urban subway systems pose a severe risk as they have the potential to corrode both the reinforced concrete structure and cementitious materials within concrete. This study simulated urban rail transit conditions to assess their impact on cement-based materials. Various analytical techniques, including X-ray diffraction (XRD), compressive strength, ion distribution and scanning electron microscopy (SEM/EDS) were employed to examine the degradation of samples. The research findings revealed that electric fields accelerated the migration of sulfate ions in soil conditions. In the presence of an electric field, the migration of SO 4 2- ions was approximately 2–4 times higher than in non-electric field conditions. Additionally, the corrosion resistance coefficient of the samples exposed to electric fields was reduced by up to 0.45, whereas the samples without electric field exposure experienced a reduction of up to 0.12. Notably, the full immersion in a soil environment resulted in less degradation than in a sulfate solution. Although the extent of degradation varies across different conditions, the mechanism governing sulfate attack on the samples remained consistent. These findings underscore the elevated severity of sulfate attack in urban rail transit systems, deepening our understanding of durability challenges in underground constructions. [ABSTRACT FROM AUTHOR]

• Fibre-reinforced sulfoaluminate cement-based composites were developed for 3D printing. • The mechanical and sulfate-attack resistance of 3DCP was experimentally assessed. • The damage anisotropy of 3DCP exposed to sulfate attacks was elucidated. • The influence of interfaces on the sulfate resistance was uncovered from meso-/microscales. 3D concrete printing (3DCP) technology has grown substantially in recent years and expanded from use in the development of architectural landscapes to the manufacture of load-bearing structures. The durability performance of 3D-printed concrete materials is crucial for the application of 3DCP in engineering structures. In this study, polypropylene (PP) fibre-reinforced sulfoaluminate cement-based composites were prepared for 3D printing, and their mechanical and durability performances after exposed to a sulfate-attack environment were experimentally assessed. The surface damage, mass loss, dynamic modulus, and residual strength of the prepared 3D-printed concrete were quantified during 150 cycles of sulfate attacks. The damage anisotropy of 3DCP exposed to sulfate attacks was introduced and elucidated from both the meso- and microscales. The pore characteristics of the matrix and interfaces were investigated through computerised tomography (CT) scanning and scanning electron microscopy (SEM) to uncover the influence of interfaces on the sulfate resistant performances. The results indicate that the damage anisotropy of 3DCP after exposed to sulfate attack was reduced due to the self-healing properties of sulfoaluminate cementitious composites. [ABSTRACT FROM AUTHOR]

The performance of cementitious materials made with Recycled Concrete Aggregates (RCA) and exposed to sulfate-laden environments is not yet fully understood. There are few studies on this topic, and for a full description of the mechanisms of external sulfate attack (ESA), several influencing parameters should be considered. The replacement of natural aggregates by RCA modifies the physical and chemical properties of the new material and introduces additional variables into the already complex mechanism. The high porosity of RCA contributes to accelerating the sulfate penetration from the surface, while the attached mortar increases the mineral supply that can react with the incoming sulfate. This paper describes a study on the performance of mortars made with RCA exposed to different ESA conditions. Design and exposure parameters include three cement, two specimen sizes, and two sulfate media (soil and solution). Visual inspection and measurements of expansion and weight change were used to determine the performance of the mixes. From a microstructural point of view, thermogravimetric analyses were carried out on samples obtained from the mortar bars after exposure. These results are then used to infer on the thermodynamic and kinetic aspects of ESA progression in recycled mortar. Finally, SEM/EDS analyses were also performed to further describe sulfate penetration and its relationship with cracking. The results show a limited influence of fine recycled aggregate on the mortar performance against ESA and no proportionality to its content. [ABSTRACT FROM AUTHOR]

Presented is a direct-comparison sulfate resistance study of mortars containing 3% or 6% cement replacement with either colloidal nanosilica (nS) or microsilica (mS), exposed to 6 months of full submersion in a 5% sodium sulfate (Na2SO4) solution. Mortar bar samples measured for linear expansion per current standards indicated that at 6% cement replacement, colloidal nS exhibited on average 75% of the expansion of the microsilica-containing counterpart. At 3% replacement, either form of silica reduced the sulfate attack–related expansion to a similar degree (by 35%±2%) in comparison with the control mixture. Supplemental rapid sulfate permeability (RSPT) and absorption testing current standards supported the expansion results. The 6% nS mortar mixture exhibited the least charge passed and smallest permeable pore volume, which indicated that the nS-containing mortars were physically more impermeable and more resistant to ion transport. Increasing nS replacement (i.e., from 3% to 6%) decreased a mixture's permeable pore volume, whereas increasing mS had an inverse effect. This countered the benefits of increasing the pozzolanic content in the sulfate attack test. Mercury intrusion porosimetry (MIP) testing also revealed evidence of paste and paste-to-aggregate interfacial zone densification, as well as pore-size refinement, which was further supported by higher increases in compressive strength in mixtures with nS over those with mS. [ABSTRACT FROM AUTHOR]

Sulfate attack and accelerated carbonation tests were conducted on five concrete mixes made with 50% recycled concrete aggregate (RCA) and 30% fly ash (FA) blended with 70% ordinary portland cement (OPC). The graphene oxide (GO) and graphene oxide ball-milled (GObm) added in the concrete mixes varied by 0.05% and 0.1% by weight of binders. The 28-day cured specimens of the mixes were exposed to sodium sulfate solution and accelerated carbonation chamber under a controlled environment. The change in mass increased but the residual compressive strength reduced with progressing exposure to sulfate solution. The carbonation depth decreased with an increase in graphene oxide inclusion in the mixes but with increasing exposure to CO2 , the carbonation depth increased. The X-ray diffraction analysis (XRD) was conducted on samples after 28-day exposure to sulfate solution to confirm the delayed ettringite formation and gypsums. The thermogravimetric analysis (TGA) was performed on 28-day exposure to accelerated carbonation in order to understand the variation in the degradation process. The porosity and pore size distribution for the mixes with GO and GObm addition showed lesser porous compared to the mix without graphene oxide, which was confirmed from the mercury intrusion porosimeter (MIP) analysis conducted on 28-day cured specimens. [ABSTRACT FROM AUTHOR]

Durability of cement-based materials for marine/coastal structures is an increasingly challenging problem. Sulfate ions in seawater can react with aluminate in cement to form erosion products, causing cracks and spalling. When cement is used to stabilize loose erodible sand in coastal areas, the resistance to sulfate attack is questionable. In this study, four cements with different aluminate contents were used to stabilize sand. Cement stabilized sands were immersed in 5% Na2SO4 solution for 300-days to simulate long-term sulfate attack process. The deterioration of engineering performance was evaluated based on expansion ratio, mass change, uniaxial compressive strength, and ultrasonic velocity. The deterioration mechanisms were analyzed through mineralogical and microstructural observations including X-ray diffraction, EDS, scanning electronic microscopy, and nuclear magnetic resonance. The results showed that the development of macro-scale mechanical performances could be divided into two stages (initial stage and erosion stage) when subjected to 300-days immersion in 5% Na2SO4 solution. Sand stabilized by low-aluminate-content cement displayed better engineering performance especially at the erosion stage. Mechanistically, more ettringite was formed in high-aluminate-content cement stabilized sand, leading to swelling and cracking. The formation of ettringite and gypsum were accompanied with the consumption of portlandite, leading to further strength loss. [ABSTRACT FROM AUTHOR]

A type of sealed steel box was designed to simulate sulfate attack and hydraulic pressure on concrete. Cup-shaped concrete specimens with water/cement (w/c) ratios of 0.4, 0.5, and 0.6 were fabricated. During the 270 days corrosion time, corrosion behavior on concrete surfaces was observed every 30 days. The extent of damage on concrete was calculated on the basis of the results of periodic ultrasonic testing. After the completion of experiments, X-ray fluorescence analysis was conducted on the element contents at different depth locations of partial specimens. Experimental results showed that hydraulic pressure accelerated the sulfate attack on concrete, and high pressure yielded serious damage. In comparison with zero pressure, the accelerating coefficients of 0.1, 0.2, and 0.3 MPa hydraulic pressures were 1.48, 1.75, and 2.35, respectively. Hydraulic sulfate corrosion can bring additional sulfate ions into concrete and cause chemical and physical sulfate corrosions on concrete simultaneously, thereby resulting in serious damage on concrete. The reduction of w/c ratio and incorporation of fly ash are beneficial for concrete to resist hydraulic sulfate attack. [ABSTRACT FROM AUTHOR]

• That chloride ions have no corrosion actions on concrete was confirmed. • Friedel's salt is the key of Cl− ions inhibiting concrete sulfate attack. • Chloride ions can only partially inhibit sulfate attack on concrete. To explore the key inhibitory mechanism of external chloride ions on concrete sulfate attack, long-term partial- and full-immersion experiments of concrete in chloride solutions and chloride and sulfate composite solutions were performed. As corrosion time begins, visual inspection, compressive strength, and ultrasonic velocity were periodically observed or tested. Moreover, the micro-morphology, the contents of Cl and S elements in concrete, and the transport speeds of Cl− and SO 4 2− ions through concrete were investigated by using scanning electron microscope, X-ray fluorescence spectrum, and chemical analysis. Chloride ions neither have corrosive effects on concrete nor effects on concrete physical and mechanical properties, which is the prerequisite for chloride ions in inhibiting concrete sulfate attack. In the case of combined solutions, the transport speed of Cl− ions into concrete is evidently higher than that of SO 4 2− ions. Then, chloride ions enter concrete early and form Friedel's salt with AFm or with C 3 A and Ca(OH) 2 in concrete, which inhibits the transport of SO 4 2− ions into concrete and reduces the formation of expansive corrosion products of ettringite or gypsum. Thus, the sulfate attack on plain concrete is alleviated. Based on the inhibitory mechanism of Cl− ions on concrete sulfate attack, the presence of Cl− ions cannot completely avoid concrete sulfate attack but can partially reduce sulfate risk on concrete. [ABSTRACT FROM AUTHOR]

• Concrete with a recycled aggregate content greater than 50% has a significant deterioration in its antifreeze performance and cannot meet its frost resistance requirements. • The Weibull model can better describe the damage variation of recycled concrete. • When the recycled aggregate content is 100%, it is only 40% of the antifreeze life of ordinary concrete. • It is suggested that the proportion of recycled aggregate should not be more than 30%. To study the frost resistance of recycled concrete in a sulfate-rich environment, we prepared recycled concrete with 0%, 30%, 50%, and 100% recycled aggregate replacement rate. The physical and mechanical properties of the recycled concrete in the coupled action of freeze-thaw cycles and a sulfate environment for different replacement rates of aggregate was then studied. We analyzed the micro-structure and the reaction products of the recycled concrete. We used a two-factor Weibull distribution model to establish the damage equation, then predict the service life of the recycled concrete. The results show that when the replacement rate of recycled aggregate increases, the physical and mechanical properties of the recycled concrete gradually deteriorate in the later stages of the freeze-thaw test. When the number of freeze-thaw cycles increases, ettringite and gypsum are gradually formed due to the reaction with sulfate. The Weibull model accurately describes the damage variation in the recycled concrete. Through the prediction of the service life of the recycled concrete, we found that the durability was reduced. Given our analysis, we found that the proportion of recycled aggregate should not exceed 30% to meet the requirements for durability design of a concrete structure in China (GB/T 50746-2008). [ABSTRACT FROM AUTHOR]

• The driving damage mechanism for partially immersed cement mortars was examined. • Lowering the RH would accelerate physical sulfate attacks in drying parts. • An intermediate temperature of 35 °C causes more serious degradation than 20 and 50 °C. Concrete structures in salt lakes of western China and South China Sea deteriorate dramatically because of the aggressive salt, high temperature and low relative humidity (RH). In order to study the deterioration mechanism of cement-based materials in these severe environments, the damage progress of mortar specimens that partially exposed to sulfate solutions and under different ambient RH and temperatures was examined. The results indicate that the driving deterioration mechanism for cement mortars that partially exposed to Na 2 SO 4 solution is physical attacks in drying portions, while it is chemical attacks in immersed portions for cement mortars that partially exposed to MgSO 4 solution. Besides, lowering the ambient RH (e.g., from 80% to 50% and 30%) would promote a narrow but supersaturated evaporation interface zone, which accelerates the salt crystallization and eventually surface scaling. While the relatively high ambient RH (80%) causes a broad diffusion zone where the chemical related expansion and cracking formed. Moreover, an intermediate temperature of 35 °C is proved to produce the most severely deterioration when compared to the temperature of 20 and 50 °C. [ABSTRACT FROM AUTHOR]