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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]

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]

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]

This study analyzed changes in the durability characteristics of cement mortar incorporating carbon nanotube (CNT) and the electrical properties subjected to deterioration induced by sulfate attack. Powder types of multi-walled or single-walled CNTs were used and added to the composites with 1.0% and 2.0% mass fraction, and the specimens were immersed in 5% and 10% sulfuric acid solutions to investigate the durability of CNT cementitious composites. Although mechanical performance decreased due to relatively large pores (370–80 μm) caused by CNTs, specimens incorporating CNTs exhibited enhanced resistance to sulfuric acid as CNTs, which offered strong resistance to acid corrosion, and prevented contact between the cement hydrate and the sulfuric acid solution. Therefore, it is expected that self-sensing performance was exhibited because there were no significant differences in the electrical properties of cement mortar subjected to the deterioration by sulfate attack. [ABSTRACT FROM AUTHOR]

Dry–wet cycle conditions have significant effects on the corrosion of concrete under sulfate attack. However, previous studies have only applied them as a method for accelerating sulfate attack and not systematically studied them as an object. In order to explore the impact of sulfate attack with different dry–wet cycle periods on concrete, in this study, four dry–wet cycle periods (3, 7, 14, and 21 days) were selected. The flexure strength, relative dynamic modulus, and mass were tested, and the microstructures of the eroded specimens were also analyzed. The intensity and depth of sulfate erosion were influenced by the wet–dry cycle period. The results show that the deterioration of concrete first increased and then decreased with an extension of the dry–wet cycle period. Microstructural analysis indicated that, with an increase in the dry–wet cycle period, the corrosion depth of sulfate attack increased. Moreover, the erosion products such as ettringite and gypsum were greatly increased, in agreement with the macroscopic variations. However, excessively prolonging the dry–wet periods does not significantly further the deterioration of concrete's performance. Therefore, considering the strength and depth of corrosion caused by sulfate attack, it would be appropriate to employ dry–wet cycle periods of 7–14 days under natural dry conditions in studies on concrete. [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]

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 on concrete structures is a major durability concern wherein concrete interacts with marine water, swamp water, groundwater, sewage water, freshwater, etc. In this study, the supplementary cementitious materials such as fly ash (FA) and nanoparticles are together incorporated into conventional concrete aiming to enhance the resistance of concrete against the penetration of sulfates. The present work is focused to understand the degradation in FA concrete modified with nanoparticles by surface sulfate attack. Concrete mix such as FA and FA modified with 2 wt% nano-TiO2 (FAT), nano-CaCO3 (FAC), and 1:1 ratio of nano-TiO2 to nano-CaCO3 (FATC) was fabricated. The specimens were exposed in 3% of ammonium and sodium sulfate for 90 days. The deterioration effects and changes in microstructural properties in all the specimens were comparatively studied. Results showed FAT, FAC, and FATC concrete have been deteriorated in ammonium and sodium sulfate solution compared with FA concrete. Partial replacement of cement with fly ash decreases the quantity of freely available reactive aluminates. Consumption of free lime by the fly ash prevents to react with sulfate. The enhanced properties of fly ash concrete against sulfate attack could be achieved with less C3A content thus reducing the available Ca(OH)2 and reducing the possibility of development of deleterious ettringite and gypsum. [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]

The previous researches on the degradation process of concrete under sulfate attack mainly focus on non-damaged concrete. It may lead to an excessive evaluation of the durability of the structure, which is detrimental to the safety of the structure. In this paper, three different damage degrees of concrete specimens with non-damaged (D0) and initial damage of 10% (D1) and 20% (D2) were prefabricated and subjected to sulfate attack and wetting–drying cycles. With the increase of sulfate attack cycles (0, 30, 60, 90, 120, 150 cycles), the changes in mass loss, relative dynamic modulus of elasticity, and the stress–strain curve were studied. The results show that the mass of the D0 specimen had been increasing continuously before 150 sulfate attack cycles. The mass of D1 and D2 had been increasing before 60 cycles, and decreasing after 60 cycles. At 150 cycles, the mass loss of D0, D1, D2 were − 1.054%, 0.29% and 3.20%, respectively. The relative dynamic modulus of elasticity (RDME) of D0 specimen increases continuously before 90 sulfate attack cycles. After 90 cycles, the RDME gradually decreases. However, for D1 and D2 specimens, the RDME began to decrease after 30 cycles. The damage degree has an obvious influence on the compressive strength and elastic modulus. For the D0 specimen, the compressive strength and elastic modulus increased continuously before 90 cycles and decreased after 90 cycles. The compressive strength and elastic modulus of D1 and D2 specimens began to decrease after 30 cycles. The stress–strain curves of concrete with different initial damage degrees were established, and the fitting results were good. Finally, based on the analysis of experimental data, the degradation mechanism of concrete with initial damage under the sulfate wetting–drying cycle was discussed. [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]

The deterioration of cement composites containing nanosilica partially exposed to sulfate attack was studied, and the microstructure change of the composites was analysed by a scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results showed that nanosilica–cement composites had better sulfate resistance compared to plain cement composite under partial exposure to sulfate attack, and their sulfate resistance increased as the nanosilica content increased (in the range of 0 to 5 wt% replacing cement by weight). The main sulfate products were gypsum and ettringite within the surface and inner parts, respectively in both the immersed and evaporation portions of the nanosilica–cement composites, which was consistent with the plain cement composite. Thus, the incorporation of nanosilica did not change the distribution characteristics of the sulfate products within the composites partially exposed to sulfate attack. However, the addition of nanosilica reduced the amount of sulfate products in both the immersed and evaporation portions, and their amount decreased with the increase of nanosilica content. The evaporation portions of the composites suffered chemical sulfate attack rather than sulfate salt crystallization. Nanosilica–cement composites could be applied in real partial exposure environments containing sulfate ions. [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]

Usually the regulation defines the concentration of sulfates to consider aggressive soil or water. There are different standards to evaluate the resistance of concretes to the external sulfate attack. Most of these tests are based on the immersion of samples in solutions with high concentration of sulfates. But in the field, the concretes are also exposed to wet–dry cycles and the crystallization of salts can occur. In this work, 65 concretes manufactured with sulfate-resisting cements are tested. The cores were tested during 1 year submerged in sodium and calcium solutions for analyzing aspect, expansion, and variation of weight. After this, the cores were exposed to the same solutions semi-immersed for 2.5 years. After being semi-submerged for 1 year, some of the concretes revealed/showed alterations. After 2.5 years, around 15% of cores showed cracks and loss of material, due to formation of ettringite and thaumasite. [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]

ASTM C452 accelerated sulfate resistance tests were performed using 10 ASTM Type V portland cements mixed with sufficient gypsum to obtain 7% sulfur trioxide content by mass of cement. The purpose was to compare the test results (optional test in ASTM C150 for Type V cements) to the tricalcium aluminate contents calculated by the Bogue equations. Tricalcium aluminate contents were also measured using X-ray diffraction with Rietveld refinement. It was found that the Bogue equations alone were not sufficient for identifying sulfate-resistant portland cement that exhibited good performance in ASTM C452. In addition, ASTM C452 expansions were compared with ASTM C1012 mortar bar expansions in sodium sulfate solution and ASTM C1038 mortar bar expansions in water. It was found that cement with a high MgO content appeared to increase 12-month C1012 expansions, likely due to formation of brucite, thus interfering with interpretation of test results. [ABSTRACT FROM AUTHOR]

Sulfate attack on concrete considerably affects the lifetime of concrete structures. The critical content of ettringite at which a considerable decreasing of compressive strength is created in this research. An experimental program on cement paste and concrete was carried out. To accelerate the magnesium sulfate attack, magnesium sulfate salt was added to the paste and concrete at the mixing stage. A theoretical chemical method was suggested to calculate the ultimate ettringite content. The experimental cement paste stage, with 24 mixtures, was tested to verify the results of this method. The ettringite content was measured using X-ray diffraction testing and compared with the theoretical method. Fifteen concrete mixtures were tested to study the effect of accelerated ettringite on mechanical properties and expansion. The studied variables included, C3A contents and relative magnesium sulfate addition. From this study, there is a good agreement between the proposed method and experimental results. The critical ettringite content ranges from 45 to 66 kg/m3 and depends on C3A content. [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]

[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]

• 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]

• New model captures sulfate ions distributions at different sulfate attack ages. • A study of effective diffusivity and its effect on sulfate transport model is presented. • An effective transport model of sulfate attack in concrete is presented. This paper presents both an experimental and theoretical investigation of the consumed and accumulated rates of sulfate ions in concrete. The authors analyze the effects of sulfate solution concentration and the water-cement ratio on the sulfate ions distribution. Based on the diffusion theory, a sulfate transport model for concrete was established considering the effect of surface sulfate concentration. On this basis, an effective transport model of sulfate attack is presented, which relies on effective diffusivity. This model can be applied to predict the sulfate ions distribution; moreover, its predicted values are in good agreement with the experimental ones. [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]

• Numerical model is capable of describing the diffusion process of concrete cast-in-situ pile exposed sulfate attack. • Numerical solution and stability were presented for the diffusion equation in the cylindrical coordinates. • Damage evolution model was proposed for the cast-in-situ pile under sulfate attack. • Pile radius and w/c showed an obvious impact on the durability for the cast-in-situ pile. This paper presented a numerical model and a damage evolution model for cast-in-situ concrete piles under external sulfate attack. Based on Fick's second law and chemical reaction kinetics, a diffusion-reaction model for describing the time-dependent diffusion behaviour of cylindrical piles exposed to external sulfate was obtained. Then, in order to maintain numerical stability, an improved numerical method using the Crank-Nicolson scheme was presented. Experiments using concrete cylinders exposed to sulfate were executed to validate the diffusion model and numerical method and revealed that the numerical solution was capable of ensuring the stability of the numerical computation and obtaining values that were in agreement with the experimental data. Finally, a damage evolution model was introduced for the chemical damage caused by the ettringite growth. Moreover, the proposed damage model was employed to predict the compressive concrete strength responses to changes in relevant factors, including the pile diameter, sulfate ion concentration and water to cement ratio. [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]

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Highlights • Continuous propagation of corrosion crack in concrete under sulfate attack is detected. • Temporal and spatial distribution of expansion stress and delayed ettringite are analyzed. • Propagation rate of cracks is directly proportional to the growth rate of delayed ettringite. Abstract The corrosive damages of plain concretes in marine environments are mainly attributed to delayed ettringite generated by liquid and solid phase reactions caused by sulfate ions entering concrete pores. This study investigated the correlation of corrosive crack propagation rate and growth rate of delayed ettringite. Cement mortar with water/cement ratio of 0.45 was employed as test sample and was immersed in 8% sodium sulfate solution. First, the correlation of corrosive crack length and corrosion time was obtained. Then, a temporal and spatial model considering the diffusion of sulfate ions and chemical reactions involved was established for the growth of delayed ettringite. The obtained results indicate that the growth of delayed ettringite is directly proportional to crack propagation. Based on that, we proposed a crack propagation model under sulfate corrosion. Also, sensitivity analysis of the diffusion model of sulfate ions was conducted and the obtained results indicated that damage played a key role in sulfate ions transmission. [ABSTRACT FROM AUTHOR]

Highlights • Pioneer study with sulfate attack on compressed stabilized earth blocks. • Wetting time and sodium sulfate concentration were highlighted. • Failure mechanism associated with thenardite and mirabilite precipitation. • A procedure to test CSEB against external sodium sulfate attack was recommended. Abstract Cementitious materials can be damaged by external sulfate attack due to ingress of sulfate ions from external environment. This process causes loss of cohesion, increase in porosity, local expansion, spalling and cracking. The objectives of this work were to study the influence of the capillary absorption time and of the sulfate ions concentration on compressed stabilized earth blocks (CSEB) exposed to sulfate attack. The CSEB were submitted to cycles of capillary absorption of sodium sulfate solution and drying. Two sets of procedures were used to evaluate the wear of the samples. In the first set of procedures, the capillary absorption time was varied. In the second one, the sodium sulfate concentration was varied instead. The CSEB used had the same dosage of earth, cement and water and for each procedure 3 samples were tested. The masses were measured after taking the samples out of the solution and after a 2-week drying time, being that a cyclic process for each procedure. The room temperature and relative humidity allowed crystallization of thenardite and mirabilite. Granulometry, XRD, XRF and SEM tests were used to identify the materials. The results showed that capillary absorption time and sulfate concentration influences the wear of the CSEB. [ABSTRACT FROM AUTHOR]

Highlights • The effects of different sulfate concentrations on the antifreeze properties of recycled concrete were studied by rapid freeze-thaw test of the specimens. • Based on the analysis of the microstructure of recycled concrete, combined with the rapid freeze-thaw test, the damage mechanism under the joint action of freeze-thaw and sulfate erosion was deeply analyzed. • Based on the theoretical analysis and experimental results, the damage model under the combined action of freeze-thaw cycle and sulfate erosion was established. Abstract The freeze-thaw of the 3% Na 2 SO 4 , 5% Na 2 SO 4 and 10% Na 2 SO 4 (mass fraction) solutions and water was tested using the quick freezing method for recycled concrete, with a water-binder ratio of 0.45 and a recycled aggregate content of 30%. The damage-deterioration law of recycled concrete was studied from the aspect of changes in the weight of recycled concrete, relative dynamic elasticity modulus, loss of compressive strength, thickness of the damage depth, and compressive strength of the damaged layer concrete. Combined with scanning electron microscopy, the composite damage mechanism of recycled concrete under sulfate and the freeze-thaw cycles was analyzed. The results revealed that the relative dynamic elasticity modulus of recycled concrete presents three stages: slow declining stage, rapidly declining stage, and accelerated declining stage. Compressive strength exhibited a slow declining stage, and accelerated the loss of two stages. With the increase in freezing and thawing times, the ultrasonic velocity in the damaged layer of recycled concrete decreases, the thickness of the damaged layer increases, the compressive strength of the recycled concrete in the damaged layer decreases, and the damage degree of the recycled concrete increases. The freeze-thaw damage of recycled concrete in the 5% Na 2 SO 4 solution was serious, and the frost resistance was the worst. In the 3% Na 2 SO 4 solution, the degradation degree of recycled concrete in the early stage of the freeze-thaw cycle was smaller than that of water, and the degradation degree of recycled concrete was more than that of water after 200 cycles of freezing and thawing. The deterioration of recycled concrete in the 10% Na 2 SO 4 solution was greater than that of the 3% Na 2 SO 4 solution and water. Through theoretical analysis and experimental data regression, the freeze-thaw damage model of recycled concrete with different sulfate concentration is established. [ABSTRACT FROM AUTHOR]

Abstract Accelerated laboratory tests on resistance of concrete exposed to sulfate attack have generally been conducted in high-concentration sulfate solutions or under drastic drying-wetting cycle conditions. However, these accelerated regimes radically alter the nature of sulfate attack mechanisms on concrete under real field exposure situation. To obtain reliable information on the long-term behaviors of concrete under real field conditions, the behaviors of concrete samples under three different exposure regimes, i.e., continuous full immersion, full immersion with general use drying-wetting cycles and full immersion with natural drying-wetting cycles, were investigated in this research. Three different concentrations of sulfate sodium solutions, i.e., 0% water for control, 2.1% for field-like condition and 15% for high-concentration condition, were considered. Physical and mechanical properties, such as mass, expansion, permeability and compressive strength, were tested at regular time intervals during the whole exposure period to describe the associated evolution laws. Microanalysis was also carried out to identify the underlying mechanisms. Results from this study showed that the exposure regime of full immersion in 2.1% sulfate sodium solutions subjected to natural drying-wetting cycles can well reproduce the field exposure condition of concrete under certain sulfate-rich environments. Both concentration and exposure type affect the nature of sulfate attack mechanism on concrete, along with the evolution of physical and mechanical properties. A three-stage evolution model, consisting of the enhancement stage, the incubation stage and the degeneration stage, was observed in the property evolution of concrete samples under the field-like exposure condition. Meanwhile, a distinct coupling physicochemical damage on concrete samples was detected when subjected to drying-wetting cycle exposure. In addition, the effects of water-to-binder ratio and binder type were duly studied. [ABSTRACT FROM AUTHOR]

Abstract The resistance of concrete to sulfate attack is often analyzed using laboratory test methods on mortar samples and expansion measurements. In rare cases the loss of strength is measured by bending or tensile strength tests. To reduce the effort of sample preparation for tensile strength testing, briquet specimens according to ASTM C307 were used in this research work. Sodium sulfate solutions at sulfate concentration levels of 3.000 and 6.000 mg SO 4 2−/l were utilized at a temperature of 5 °C to achieve a realistic, yet slightly accelerated attack. The impact of the sulfate solution is studied by measuring the tensile strength and characterizing the ingress of sulfates by XRD and carbon/sulfur analysis. Significant tensile strength loss due to sulfate attack could be observed in the course of time, especially for Portland cement with a high C 3 A content. The results show a promising new approach to test the sulfate resistance of concrete. [ABSTRACT FROM AUTHOR]

Highlights • 70% slag improved the sulfate resistance of steam-cured concrete. • 50% slag accelerated the sulfate damage of steam-cured concrete. • Steam curing increased the deterioration of slag concrete. • The variation of mass change lagged behind than that of open porosity. Abstract This paper mainly investigated the sulfate resistance of the slag incorporated concrete under steam curing. Concrete samples with different replacement ratios (0%, 20%, 50% and 70%) of slag under steam curing were exposed to 5% Na 2 SO 4 solution for up to 540 days. Mass changes, open porosity and sulfate-ion contents of concrete were determined separately. Moreover, XRD, TGA, SEM and MIP were performed on the cement paste samples to investigate the landscape of their microstructures. The results showed that the variation of mass change lagged behind than that of open porosity. Steam curing increased the sulfate penetration and deterioration of the slag concrete. Interestingly, incorporation of 50% slag improved the sulfate resistance of standard-cured concrete and incorporation of 70% slag improved the sulfate resistance of steam-cured concrete. This was because a hydrotalcite type phase in the slag paste hindered the ettringite formation. However, the steam-cured concrete prepared with 20% and 50% slag experienced much worse deterioration, which was mainly due to the combined formation of ettringite and gypsum. [ABSTRACT FROM AUTHOR]

Cement-stabilized pavement layers are subject to sulfate attack (SA) in sulfate-abundant regions due to internal expansion–induced damage throughout its expected service life. SA is a coupled physical-chemical-mechanical damage process for cementitious materials involving complicated chemical reactions between sulfate and components of cement composite. SA contains intricate interactions among porous media, moisture transport, and heat transfer. Engineering mechanics has been used to explain the failure process of the internal expansion caused by this coupled physical-chemical ingress phenomenon. Existing studies have considered only heat transfer or moisture content–dependent modeling. The literature lacks a comprehensive model that considers coupled interaction of temperature and humidity upon sulfate ingression. Thus, a chemomechanical (CM) model has been developed for capturing the true failure process of cement-stabilized pavement subgrades under SA. In this paper, a set of governing equations are developed, and a unique expression for a moisture-dependent and heat-dependent sulfate diffusion coefficient is proposed. Consequently, the equations are solved using the finite-element method. The results conform well with experimental results. The model has been validated to be accurate enough compared with previously implemented models. It is capable of evaluating and predicting SA-induced expansive failure in unsaturated cement-stabilized pavements. Two different models were combined to estimate the mechanical behavior of cement-stabilized subgrades subject to SA. Finally, the Drucker-Prager (DP) failure criterion was used for determining the damage zone. [ABSTRACT FROM AUTHOR]

Highlights • Accelerated aging can damage the soil cement specimens especially for the most clayey soils. • Immersion in sulfate solution causes swelling and a slight decrease in compressive strength. • The W/D cycles causes a progressive microcracking according to the P-wave velocities. • Above 10% of clay in the soil durability problems could arise depending on the cement dosage. • Increased the cement content can delay, but not prevent, the damage due to accelerated aging. Abstract There is a growing interest and technically demanding for Deep Soil Mixing methods. Therefore, new issues have emerged and it has become crucial to ensure the service life of the material, notably for permanent structural works. In this context, the durability potential of different soil-cement mixes was studied in the laboratory by using accelerated aging tests, i.e. wetting/drying (W/D) cycles and external sulphates attacks. The experimental design proposes to follow the evolution of mass, P-wave velocity, permeability and mechanical properties according to the exposure duration. Critical thresholds relative to clay and cement content were defined. The mechanical damage was the result of micro cracking and ettringite expansion. [ABSTRACT FROM AUTHOR]

Fly ash, a supplementary cementitious material, is normally used in conjunction with portland cement to improve the durability properties of concrete in the field, such as sea-crossing bridges, high-speed railways, and mass concrete in dams. Currently, these concrete structures are frequently exposed to sulfate environments. The object of this paper is to investigate the relationship among content of calcium hydroxide (CH), formation of gypsum, and compressive strength, as well as the relationship among the formation of ettringite, pore structure, and expansion properties in blend cement paste at long-term sulfate attack, up to 1,110 days. Mercury intrusion porosimetry (MIP) and thermo gravimetric (TG) analysis were used to evaluate the pore structure and content of CH, respectively. The results show that there is a significant negative relationship between CH content and the coefficient of compressive strength during sulfate attack as more gypsum formation is contributed for the loss of strength. Compared to the reference sample, the addition of 20% and 40% fly ash decreased the CH content by 16.7% and 25.1% through the pozzolanic reaction, which reduced the formation of gypsum and prevented the decalcification of calcium silicate hydrate (C-S-H). Crystallization pressure, due to the formation of ettringite in a limited space, is discussed to explain the cracking of cement-based materials. Several harmful pores (>0.1 µm) were formed in the reference sample due to crystal pressure with the formation of ettringite, which caused the expansion and exacerbated the diffusion of sulfate ions. The pozzolanic reaction of fly ash also improved the pore structures and reduced the degree of supersaturation of ettringite by preventing the ingress of external sulfate ions. [ABSTRACT FROM AUTHOR]

Highlight • The addition of fibers effectively increases erosion resistance to mortar. • The increase depends on fiber types and dosages. • Polypropylene fiber is superior to steel and micro polypropylene fiber. • Submersion in 5% w/w sodium sulfate decreases erosion resistance of mortar. • Fibers cannot reduce the adverse effect of sulfate attack on mortar. Abstract This research was conducted to study the combined effect of sulfate attack and underwater abrasion on fiber reinforced cement mortar. Compared to plain mortar mix, cement mortars reinforced with three types of fiber, i.e. steel fibers, polypropylene fibers, and micro polypropylene fibers were investigated. Each fibers was used individually and the micro polypropylene was additionally hybridized with steel and polypropylene fibers. Test specimens were prepared as per ASTM C1138. Their abrasion resistance were examined after submersed in 5% sodium sulfate (Na 2 SO 4) solution for 4 and 8 months. The experimental results showed that the Na 2 SO 4 solution increased the weight loss of all specimens. The longer the period of submersion, the higher the weight loss. In the specimens reinforced with fibers, the abrasion resistance was enhanced. The highest resistance was found in the mortar mixed with the polypropylene fiber at 1% volume fraction. Moreover, in abrasion resistance point of view, the addition of micro polypropylene fiber had little or no benefit. [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]

• The content of bound sulfates in FCPs is mainly attributed to their chemically bound sulfate content. • The presence of chlorides can reduce the content of chemically bound sulfates in FCPs. • The influence of chloride concentration on the sulfate attack was discussed. • The addition of FA can inhibit the physical binding of sulfates and chlorides in C-(A)-S-H. Sulfate-induced concrete deterioration is associated with not only its material compositions but also environmental conditions. This study has performed some experimental investigations on the influence of fly ash (FA) and chlorides on the behavior of sulfate attack in blended cement pastes, and they were quantitatively characterized by the contents of bound sulfates and chlorides. Firstly, FA-blended cement paste particles (FCPs) with different FA content were prepared, and then they were performed a series of immersion experiments. Finally, free sulfate and chloride concentrations in the solutions were measured by using ultraviolet spectrophotometry and silver nitrate titration, respectively, while the change of phase and chemical compositions in FCPs were analyzed by quantitative X-ray diffraction (QXRD), scanning electron microscopy and energy spectrometry (SEM-EDS). Results showed that, the addition of FA can not only increase the aluminum content in FCPs, improving the formation of sulfates-contained phases in Na 2 SO 4 solution, but also influence the content and chemical compositions of C-(A)-S-H, resulting in the decrease of physical binding capacity of sulfates. The content of physically bound sulfates can be released by the sulfate-induced decalcification of C-(A)-S-H. After the sulfates-contained FCPs were immersed into NaCl solutions, the presence of chlorides can release the bound sulfates from the sulfates-contained phases, resulting in the formation of chlorides-contained phases. Additionally, the releasing effect of chlorides on the physically bound sulfates becomes more pronounced than that on the chemically bound sulfates, especially in the chloride solution with a high concentration. Eventually, these conclusions were consistent with the SEM-EDS results. [ABSTRACT FROM AUTHOR]

• The performance of mortar with different SCMs in sulfate environments were investigated under constant and variable temperature conditions. • Expansion rate, compressive strength, visual inspection, XRD, and SEM of mortar in sulfate attack were tested. • Mortar with added silica fume has the best performance in resisting sulfate attack at different temperatures. • Metakaolin is extremely unfavorable to sulfate resistance of cement-based materials under high temperature environment. Sulfate attack on cement-based materials has been extensively studied, but the effect of temperature variation on the rate of sulphate corrosion needs to be further discussed, especially when the vast majority of concrete structures are long exposed to variable temperature. In this paper, four scenarios, including three constant temperature environments (10 °C, 20 °C, and 40 °C) and one variable temperature environment, are investigated for the temperature effect. Six supplementary cementitious materials (SCMs) were added to the mortar specimens respectively, including fly ash, limestone powder, steel slag powder, metakaolin, silica fume, and blast furnace slag. The mortar specimens were immersed in 5% Na 2 SO 4 solution for 330 days, and the resistance of the specimens to sulfate attack was explored using expansion, compressive strength, visual inspection, X-ray diffraction (XRD), and scanning electron microscope (SEM). The high-temperature environment (40 °C) is highly unfavorable to the mortar with added metakaolin, while the low-temperature environment (10 °C) is harmful for the mortar with added limestone powder. The corrosion damage of mortar with added SCMs is less at high temperature than that at low temperature, such as fly ash, limestone powder and blast furnace slag. [ABSTRACT FROM AUTHOR]

Influences of limestone powder on the resistance of concretes to the chloride ion penetration and sulfate attack with a constant water/binder ratio and a constant 28-day compressive strength were studied. The sensitivity of the properties of concrete to the initial moist curing time was also explored. The results indicate that, under a constant water/binder ratio condition, the resistance to sulfate attack of concrete deteriorates with the increasing of limestone powder content, and the resistance to chloride ion penetration decreases when the replacement ratio of limestone powder is 24%. As the initial moist curing time declines, the reducing magnitude of the properties of concrete containing limestone powder is larger than that of plain cement concrete. Nevertheless, lowering the water/binder ratio of concrete containing limestone powder can significantly reduce the sensitivity of the properties of the concrete to the initial moist curing time. No matter how long the initial moist curing time is, replacing cement with 8% limestone powder can improve the resistance of concrete to sulfate attack. Concrete with up to 24% limestone powder addition can still obtain a resistance to chloride ion penetration and sulfate attack similar to plain cement concrete on the premise of a constant 28-day compressive strength. Moreover, it was found that both the crystallization of sodium sulfate and the formation of ettringite collectively result in the deterioration of concrete subjected to the sulfate solution. [ABSTRACT FROM AUTHOR]

Sulfate attack is one of the major durability problems in concrete structures, but there is less research on sulfate attack of concrete with initial damage. In this paper, the deterioration law of concrete with damage degrees under combined actions of sulfate attack and drying-wetting cycles were investigated through mass change and relative dynamic elastic modulus (Erd). The pore structure and corrosion products of the concrete exposed to sodium sulfate solution under drying-wetting cycles were also investigated by MIP and SEM/EDS, respectively. The results revealed that the mass change and Erd of concrete with damage were greater than that of concrete without damage; the high content of slag in concrete can increase the resistance sulfate attack and the low content of slag reduce sulfate resistance of concrete. As time increases, the porosity and the number of large pores of concrete with damage increased. The internal corrosion products of concrete with high water-to-cement ratio were mainly gypsum, and the internal corrosion products of concrete with low water-to-cement ratio were mainly ettringete. [ABSTRACT FROM AUTHOR]

The damage process of cement pastes exposed to different sulfate solutions (sodium sulfate solution, magnesium sulfate solution, a mixed solution of sodium sulfate and sodium chloride) was in situ quantitatively investigated by X-ray computed tomography (X-ray CT). The X-ray diffraction (XRD) was also employed to determine the content of corrosion products. The results show that the three-dimensional crack distribution of cement pastes for different immersion times can be extracted by using X-ray CT. When the solution contains the sulfate and magnesium ions, the specimen deteriorates faster. The presence of chloride ions alongside sulfate ions significantly reduces the degree and speed of sulfate attack. The porosity and pore number of specimens markedly decrease with the increase of immersion time when the pore volume exists between 0.01 and 0.1 mm 3 . Gypsum and ettringite are formed to the least extent in the mixed solution of sodium sulfate and sodium chloride, and most evident for the magnesium sulfate solution. In addition, the relationship between the crack volume fraction and the erosion time of specimens in different sulfate solutions is established, respectively, which all conforms to a quadratic function relationship. [ABSTRACT FROM AUTHOR]

A recent microstructural model for simulating external sulfate attack on cement paste is used to calculate the driving force for local expansive growth of AFt phase in terms of crystallization pressure, and the strain and stress fields are tracked within the microstructure with micrometer-scale resolution using a linear elastic finite element model. Damage induced by expansion modifies both the local effective transport properties and linear elastic properties of each microstructure at different depths, and thereby potentially alters the rates of sulfate ingress and expansion. Simulations give insight about sulfate attack mechanisms, which are investigated in more detail by separating the individual influences of sulfate concentration and pH of the pore solution. Especially when soluble carbonates are present, reductions in the pore solution pH, which often accompany ingress of sulfates, significantly destabilizes calcium monosulfoaluminate and accelerates AFt growth. In fact, within a narrow pH range some calcium monosulfoaluminate can spontaneously transform to AFt without any additional sulfate addition. Therefore, the progress of phase transformations and expansion from the surface to the interior of the porous material is dictated by the rate of ingress of concentration fronts of both sulfate ions and pH, which do not necessarily coincide. [ABSTRACT FROM AUTHOR]

In this study, mortar samples were exposed to sulfate attack for 1 year and then healed with the polymer that contains phosphazene. After this healing, the microstructure and mechanical properties of these mortars were investigated. In the experimental work, the percentage of phosphazene in the monomer (0, 1%, 2%, 3% and 4%), the curing time (30, 60, 90, 180 and 365 days), the cement dosage (300, 350, 400, 450 and 500 kg/m 3 ) and percentage of sulfate solution (0%, 1%, 2%, 4% and 6%) were selected as experimental parameters. Due to the high number of experimental parameters, experiments were reduced using the Taguchi L 25 (5 6 ) orthogonal array as the test plan. After the design of the experiment, 50 × 50 × 50 mm cubes and 25 × 25 × 285 mm prismatic samples were prepared. The produced samples were exposed to the sulfate attack for the predetermined curing times. The samples were dried at 105 ± 5 °C for 24 h after sulfate attack. Following this procedure, the monomer containing phosphazene was impregnated to samples at atmospheric conditions for 24 h. 1% benzoyl peroxide was used as the monomer initiator. The samples were heated to 60 °C for 4 h to polymerize the monomers. The compressive strength, weight changes and length changes of mortar specimens healed using polymer containing phosphazene after sulfate attack were investigated. Furthermore, SEM, EDX and XRD analyzes were conducted on the samples. [ABSTRACT FROM AUTHOR]