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In order to effectively assess mechanical property of concrete exposed to sulfate solution under drying-wetting cycles, the degradation rule of mechanical property of concrete was studied under sulfate attack and drying-wetting cycles. Based on the damage mechanics and Ottosen failure model, the damage failure model of concrete under sulfate attack and drying-wetting cycles was established. The experimental results show that the axial compressive strength and elastic modulus of concrete decrease with increase of drying-wetting cycles, and the tensile and compressive meridian of concrete move to the hydrostatics compressive axis. Concrete failure surface shrinks gradually with the increase of damage and the crack-resistant property of concrete reduces. At the early stage of the sulfate attack, the shrinkage degree of concrete failure surface is smaller. With corrosion time increases, the shrinkage degree of concrete failure surface is accelerated, and the degradation degree of concrete increases. The tensile and compressive meridian of concrete is significantly dependent on the types of sulfate solutions. Compared with concrete in sodium sulfate solution, the shrinkage degree of concrete failure surface under magnesium sulfate solution is more obvious, indicating that the concrete deterioration by magnesium sulfate covers more aggressive corrosion. [ABSTRACT FROM AUTHOR]

Degradation of cementitious systems exposed to sulfate salts is the result of sulfate transport through the pore system, chemical reaction with the hydration product phases present, generation of stresses due to the creation of the expansive reaction products, and the mechanical response (typically spalling and cracking) of the bulk material due to these stresses. Each component of this process plays a unique role in the ultimate response of the concrete; change the material properties relevant to any one component and the concrete performance can change dramatically. Therefore, laboratory tests of "sulfate attack'' that are based primarily on submerging the specimens in sulfate solution and then measuring some physical property, such as expansion, are effectively lumping all of these mechanisms into a single test. The result is a test that characterizes how a particular concrete performs under specific conditions. If the field conditions are different, the performance of the concrete can also be different. Therefore, to predict the resistance to sulfate attack of a concrete, it is necessary to develop a protocol that takes into account the type of exposure and separates the various mechanisms. The major degradation processes that the concrete is likely to encounter are sulfate attack, corrosion of reinforcing steel, alkali-aggregate reactions, carbonation and leaching by neutral or acidic ground waters. These degradation processes often involve the transport of moisture, ions and dissolved gases into concrete by diffusion, convection and capillary forces. Rates of degradation will be often controlled by the rate of intrusion of moisture and dissolved salts and gases into concrete. [ABSTRACT FROM AUTHOR]

29Si and 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy techniques were employed towards evaluation of hydrated cement paste and concrete subjected to accelerated sulfate attack. Quantitative analyses of the 29Si NMR spectra of specimens subjected to different periods of sulfate attack indicated that the chain length CL of SiO4=AlO4 tetrahedra was lowered by sulfate attack. 27Al NMR spectra indicated that sulfate attack produced a significant rise in the intensity of ettringite and a sharp drop in the concentration of monosulfate hydrate, which are some primary indications of sulfate attack on concrete. Fourier-transform infrared (FTIR) spectroscopy method was employed in order to verify the result of NMR spectroscopy. [ABSTRACT FROM AUTHOR]

Portland cement concrete (PCC) suffers sulfate attack in sulfate-rich environments. On the other hand, industrial by-products such as fly ash and slag are routinely added to PCC to improve its properties. However, the impact of fly ash on concrete durability is not always conclusive and it is time- and energy-consuming to test concrete durability in the laboratory or through field observation. This study presents a numerical simulation of the durability of Portland cement concrete made with fly ash. The simulation of sulfate attack on fly ash concrete was performed based on ion transport mechanism. The chemical reactions among chemical compounds of hydrated cement paste and sulfate ions were considered. Additionally, the pozzolanic and hydration reactions of fly ash were incorporated as well to incorporate the impact of fly ash. The pozzolanic and hydration reaction were simulated through the extended version of an existing numerical procedure. Field observation data from the United States Bureau of Reclamation (USBR) was employed to validate the extended simulation program. After pozzolanic and hydration reactions of fly ash were incorporated, the prediction agreed well with the field measurements. The simulation shows that pozzolanic reaction of fly ash can significantly slow down sulfate attack on concrete, thus increasing the sulfate resistance of PCC and extending the service life of concrete infrastructures. The major factors that affect the sulfate resistance of concrete were explored through the numerical simulation as well. [ABSTRACT FROM AUTHOR]

The paper presents and discusses the results of resistance to sulfate aggression cement mortars containing calcalerous gaize. Were tested cements containing 15-35% gaize (CEM II A, B) and the reference cement CEM I (no additive). Using DTA/TG, XRD investigated the effect of gaize on the hydration of cement. Linear change mortars were tested according to the procedure in PN-B-19707, complement the research was to determine the strength of the mortar as a result of following long-term storage of samples in corrosive environments. Mortar microstructure studies were performed using mercury porosimetry and SEM/EDS. The results of the study allow us to conclude that already 15% of the content of Gaize making cement resistant to corrosive environments rich in sulfate ions. [ABSTRACT FROM AUTHOR]

The damage process and failure mechanism of shotcrete under the combined actions of sulfate attack and drying–wetting cycle were investigated. The results indicated that shotcrete was inherently more inhomogeneous, the compact area and defect region exhibited different erosion mechanisms and failure forms. The connected and loose defects could accelerate the intrusion of external sulfate ions and the growth of corrosion products. The localization and directivity of defects would lead to the localized distribution of expansive stress. Hence, the crack in defect region developed significantly faster and was the key factor that limited the durability of shotcrete under sulfate attack. [ABSTRACT FROM AUTHOR]

Properties of concrete (with additional 20% fly ash) subjected to freeze–thaw cycles in water and in sulfate solutions were investigated in this paper. The corrosion solutions include two types, namely, 5% sodium sulfate solution and 5% magnesium sulfate solution. Through the experiment, visual examination was conducted to evaluate the surface damage. The deterioration considering the weight loss, relative dynamic modulus of elastically (RDME) loss and compressive strength loss of concrete under the coupling effect were also investigated. To identify the products formed by sulfate attack, analytical techniques, including X-ray diffraction, scanning electron microscopy and thermal analysis were performed on the selected samples after freeze–thaw circulations. Test results show that freeze–thaw cycles and sulfate attack affected each other. On the one hand, a lower temperature during freeze–thaw cycles slows down the diffusion of sulfate ions in concrete. On the other hand, sulfate attack accelerates the formation of microcracks in concrete, which leads to more severe damage under freeze–thaw cycles. The rate of damage in concrete is significantly dependent on the types of sulfate solutions, and the concrete deterioration by magnesium sulfate covers the most aggressive corrosion subjected to freeze–thaw cycles. Furthermore, the compressive strength loss and RDME loss of concrete in sodium sulfate solution is less than that in water during the initial freeze–thaw cycles, but the damage is more severe in further test period. [ABSTRACT FROM AUTHOR]

The effect of chloride on the performance of cement mortars made with CEMI and CEMI blended with 10% limestone filler and subjected to sulfate attack was investigated. The specimens were exposed to the combined action of chloride (0%, 0.5%, 1.0% and 2% of Cl − ) and sulfate (0.6% SO 4 2 − ) for 630 days at 5 ± 0.5 °C. The performance of the mortar cubes was assessed by means of visual inspection and loss of mass. The mineralogy of the deterioration products was determined by infra-red spectroscopy and X-ray diffraction, and scanning electron microscopy was used to examine the microstructure of the specimens. Chemical analysis of the test solutions was also carried out at 540 days of immersion in order to assess the end concentrations of calcium, sulfate and magnesium ions. All the specimens suffered from the thaumasite form of sulfate attack, except those stored in the combined sulfate and 2.0% chloride solution, which showed better performance within the 630-day study. The greatest deterioration occurred in mortars made with CEMI blended with 10% limestone filler. Based on visual and mass assessments, it was found that the effect of chloride on thaumasite degradation depended on chloride concentration, accelerating the damage at 0.5% and mitigating it at 2% in the conditions investigated. This is probably related to the reduction in calcite solubility and the increase in Friedel’s salt deposition observed as the chloride level increased in solution. [ABSTRACT FROM AUTHOR]

Two newly developed cement-reduced eco-friendly concretes with high limestone powder content and low water/powder ratio were tested for sulfate resistance. Mortar samples with a paste composition of eco- as well as conventional concretes were immersed in 30 g l −1 Na 2 SO 4 and saturated Ca(OH) 2 reference solutions for 200 days at 8 °C. To evaluate the reaction mechanisms of progressing sulfate attack a combined approach of mechanical, mineralogical, and microstructural methods was applied. Gypsum and bassanite neo-formations related linearly to the expansion during sulfate exposure, except for one sample where ettringite co-precipitated. Thaumasite formation was not observed in spite of potentially favorable conditions. This is considered to be related to the evolution of the experimental solutions, kinetic effects, and the competing formation of CaCO 3 polymorphs triggered by the usage of superplasticizer. Both eco-friendly mixes exhibited a better sulfate resistance than their corresponding reference samples and are therefore suggested to be applicable in low sulfate-loaded environments according to DIN EN 206-1. Eco-friendly concrete based on CEM III/B performed superior against sulfate attack and is expected to withstand even severe sulfate exposure despite a much higher water/cement ratio than required by the standard. [ABSTRACT FROM AUTHOR]

Concrete exposed to sulfates in hot and arid regions can severely suffer from salt weathering. The resulting damage is typically localized at the above ground portion of concrete members. This process highly depends on the pore structure of the concrete surface through which salty water rises by capillary action. When water evaporates, salt crystals grow in the concrete surface pores leading to concrete damage. Thus, protecting the concrete surface can potentially enhance its durability to salt weathering. However, the vast variety of surface treatment compounds available makes the selection of an adequate material challenging. This is particularly true for concrete exposed to physical sulfate attack due to the lack of pertinent data in the open literature. Therefore, this study focuses primarily on assessing the effectiveness of different commercially available surface treatment materials in mitigating physical sulfate attack on concrete. [ABSTRACT FROM AUTHOR]

The objective of this study is to examine the influence of cation type on deterioration process of cement paste exposed to external sulfate attack. The experiment was carried out by micro-hardness test (Vickers hardness), compressive strength test and expansion test. Solutions respectively containing Na 2 SO 4 , K 2 SO 4 , (NH 4 ) 2 SO 4 and MgSO 4 with sulfate concentration of 5.0 wt% were chosen as the sources of sulfate to attack the cement paste. The pH values of solutions containing different sulfate salts were measured to support the analysis. The deterioration process of the cement paste specimens immersed in different attacking solutions was analyzed. The results shown that, the reducing rate of Vickers hardness of cement paste samples immersed in solutions containing different sulfate salts were generally sequenced as Na 2 SO 4 ≈ K 2 SO 4 > MgSO 4 > (NH 4 ) 2 SO 4 . With the addition of exposure time, the deterioration process exerted at different rate with the variation of sulfate salts. The expansion of specimens immersed in solutions containing Na 2 SO 4 , K 2 SO 4 , MgSO 4 and (NH 4 ) 2 SO 4 occurred in two stages within a certain range of exposure time. Besides, compressive strength appeared to be well correlated with Vickers hardness for different kinds of sulfate salts. [ABSTRACT FROM AUTHOR]

Context and purpose Treatment of clay with cement has been used for various ground improvement and geoenvironmental applications including vertical barriers to isolate contaminated areas. An important issue with cement-treated clays is their durability especially when applied in chemically aggressive environments. The purpose of this study is to evaluate the impact of chemical degradation on engineering properties of cement-treated clay. Methodology The mechanical and hydraulic behaviors of a cement-treated clay in contact with water and an aggressive sodium sulfate solution were investigated. Bender elements were installed in a flexible-wall hydraulic conductivity cell to simultaneously monitor the small-strain shear modulus ( G 0 ) and the hydraulic conductivity ( k ) of the cement-admixed clay. Results During permeation with clean water, an increase of G 0 and a gradual decrease of k with time were observed due to cement hydration. Conversely, after permeation with the sulfate solution, a sudden decrease of G 0 and a gradual increase of k were recorded. Conclusions G 0 measurements provided evidence of immediate deterioration of the sample as it comes in contact with sulfates. However, in order to have significant impact on the permeability, a network of interconnected preferential flow paths had to first develop through the whole sample. Monitoring of G 0 was shown to provide valuable additional information to study the impact of chemical degradation of cement-treated clays. [ABSTRACT FROM AUTHOR]

• Carbonation treatment mitigates the deterioration of HFA to sulfate attack. • Carbonation treatment of HFA consumes f-CaO and capture CO 2 effectively. • Carbonation treatment and curing improve resistance to freeze–thaw and sulfate attack. High-calcium fly ash (HFA) is available in large quantity produced by coal-based electrical power plants. Up to now, the utilization of HFA in construction materials is limited. In this research, carbonation treatment of HFA in association with carbonation curing was applied for stabilization of hazardous residues and CO 2 sequestration. The improvement on durability of carbonation-cured cement mortar incorporating carbonated HFA against coupled freeze–thaw and sulfate attack was investigated. The results show that the synergy of carbonation treatment and carbonation curing mitigated the deterioration of relative dynamic elastic modulus and compressive strength caused by coupling action of freeze–thaw and sulfate attack. The collaborative carbonation treatment and carbonation curing can also reduce the content of expansile free-CaO, as well as the components that would expand under sulfate attack conditions, such as CH and aluminates. On the other side, the results of MIP show that carbonation curing improved the pore structure and decreased the permeability of cement mortars specimens. [ABSTRACT FROM AUTHOR]

• The fracture properties of concrete subjected to drying-wetting cycles in different types of sulfate solutions were investigated. • With the proposed method, the fracture toughness and the evolution of FPZ were studied. • The horizontal strain and displacement fields in the FPZ obtained by the DIC technique were analyzed. The fracture properties of concrete subjected to drying-wetting cycles in different types of sulfate solutions were investigated. Five exposure conditions, i.e. 5% and 10% sodium sulfate solutions, 5% and 10% magnesium sulfate solutions (by mass) and water were prepared. Three-point bending tests were carried out at the corrosion periods of 1, 2, 3, 5, 9 and 15 months with the digital image correlation (DIC) technique. The initial and unstable fracture toughness were calculated by the proposed method, and the evolution of the fracture process zone (FPZ) was studied. Results indicate that (1) with the increase of corrosion time, an initial increase stage and a followed decrease stage for the fracture toughness were noticed. The values of unstable fracture toughness obtained by the analysis method agreed well with those obtained by the experimental method. (2) The FPZ length increased linearly with the crack propagation until a full FPZ development, and then decreased gradually. The maximum FPZ length decreased slightly with the corrosion time. (3) Using DIC technique, the fracture path can be easily identified from the horizontal strain field, and the evolution of the FPZ can be determined by the horizontal displacement with high accuracy. [ABSTRACT FROM AUTHOR]

Due to several similarities in deterioration behavior between concrete and other porous materials, sulfate salt weathering is regularly considered to be the degradation mechanism causing the concrete deterioration in evaporation zone of partially buried concrete elements in the sulfate environment. This issue has received increasing attention in recent years. However, according to an extensive literatures review on long term field and laboratory tests in this paper, the experimental results are clearly illogical and contradict with the classic theory of salt weathering of porous materials, such as (1) the sulfate crystals cannot be identified by means of micro-analysis methods in the damaged concrete as direct evidences for sulfate crystallization in concrete; (2) concrete is susceptible to deterioration in a high RH environment; (3) a mere change of cement compositions shows significant influence on concrete damage, however concrete damage is immune to pore structure change; and (4) the damaged concrete part does not contain the highest salt content, and so on. Instead, the experimental results support that the chemical sulfate attack should be still the mechanism causing the concrete deterioration. Therefore, an adequate understanding of deterioration mechanism of concrete in the evaporation zone of partially buried concrete elements is quite urgent. In this paper, several issues aimed at the contradictions deduced from field and laboratory tests are proposed to attempt to seek the truth of sulfate salt weathering of concrete. [ABSTRACT FROM AUTHOR]

This work was divided into two phases. Phase one included the validation of neural network to predict mortar and concrete properties due to sulfate attack. These properties were expansion, weight loss, and compressive strength loss. Assessment of concrete compressive strength up to 200years due to sulfate attack was considered in phase two. The neural network model showed high validity on predicting compressive strength, expansion and weight loss due to sulfate attack. Design charts were constructed to predict concrete compressive strength loss. The inputs of these charts were cement content, water cement ratio, C3A content, and sulfate concentration. These charts can be used easily to predict the compressive strength loss after any certain age and sulfate concentration for different concrete compositions. [ABSTRACT FROM AUTHOR]

In this paper, we consider the mechanical effect of the sulfate attack on concrete. The durability analysis of concrete structures in contact to external sulfate solutions requires the definition of a proper diffusion-reaction model, for the computation of the varying sulfate concentration and of the consequent ettringite formation, coupled to a mechanical model for the prediction of swelling and material degradation. In this work, we make use of a two-ions formulation of the reactive-diffusion problem and we propose a bi-phase chemo-elastic damage model aimed to simulate the mechanical response of concrete and apt to be used in structural analyses. [ABSTRACT FROM AUTHOR]

Purpose – The purpose of this study was to reuse cement kiln dust (CKD) in cement products and report the results of determining the long-term compression and flexural tensile strengths of mortars containing CKD as a partial replacement of sulfate-resistant cement (SRC). During the manufacturing of Portland cement, voluminous quantities of the byproduct dust are produced, which is commonly known as CKD. In the past decade, according to environmental requirements, many researchers have attempted to reuse CKD in cement products. Design/methodology/approach – The long-term compression and flexural tensile strengths of mortars containing CKD as a partial replacement of SRC were tested. The replacement ratios in this study were 0, 5, 10, 15 and 20 per cent. The specimens were exposed to a highly saline environment after normal curing in water for a 28-day period. Findings – The results indicated a slight increase in the strength of CKD–SRC mortar containing 10 per cent CKD and moderate sulfate resistance when the CKD ratio reached 20 per cent, as compared to the reference mortar. In addition, CKD did not adversely affect the properties of SRC mortar subjected to sulfate exposure, even after one year. Originality/value – The tests were inducted for the first time on SRC, and the new results can be used to produce an environmental-friendly concrete. [ABSTRACT FROM AUTHOR]

Over the vast Northwest China, arid desert contains high concentrations of sulfate, chloride, and other chemicals in the ground water, which poses serious challenges to infrastructure construction that routinely utilizes portland cement concrete. Rapid industrialization in the region has been generating huge amounts of mineral admixtures, such as fly ash and slags from energy and metallurgical industries. These industrial by-products would turn into waste materials if not utilized in time. The present study evaluated the suitability of utilizing local mineral admixtures in significant quantities for producing quality concrete mixtures that can withstand the harsh chemical environment without compromising the essential mechanical properties. Comprehensive chemical, mechanical, and durability tests were conducted in the laboratory to characterize the properties of the local cementitious mineral admixtures, cement mortar and portland cement concrete mixtures containing these admixtures. The results from this study indicated that the sulfate resistance of concrete was effectively improved by adding local class F fly ash and slag, or by applying sulfate resistance cement to the mixtures. It is noteworthy that concrete containing local mineral admixtures exhibited much lower permeability (in terms of chloride ion penetration) than ordinary portland cement concrete while retaining the same mechanical properties; whereas concrete mixtures made with sulfate resistance cement had significantly reduced strength and much increased chloride penetration comparing to the other mixtures. Hence, the use of local mineral admixtures in Northwest China in concrete mixtures would be beneficial to the performance of concrete, as well as to the protection of environment. [ABSTRACT FROM AUTHOR]

Abstract: We develop and apply in this study a chemo-transport-mechanical model for simulating the external sulfate attacks in Portland (CEM I) cement pastes and mortars. Basically, this degradation consists in the simultaneous decalcification of the hydrated phases resulting from leaching processes, and the migration of sulfate ions within the material and its subsequent interactions with these phases. The sulfate uptake leads generally to ettringite precipitation mainly from monosulfate, which in turn may produce intense macroscopic expansions and cracking. In our approach, crystallization pressures arising from the restrained growth of monosulfate crystals due to the confinement of the surrounding C–S–H matrix are assumed to initiate the observed macroscopic expansions. A macroscopic strain tensor evaluated from the volume fraction of supplementary precipitated ettringite is further introduced in the mechanical behavior law for explicitly reproducing the macroscopic expansions. Analytical homogenization schemes are applied to estimate both mechanical and diffusive properties from the local volume fraction of solid phases. The numerical platform Alliances is then used for solving both reactive transport and mechanical coupled problems, and is applied to the simulation of laboratory tests consisting in prismatic mortar specimens immersed in solutions containing sodium sulfate and subjected to free expansions. Comparison of the numerical results with experimental ones in terms of phase assemblage profiles, evolutions of mass changes and expansions shows a correct agreement. Finally, the extension of the model towards cases of restrained displacement conditions is discussed and some modifications regarding the kinetics of ettringite precipitation are proposed for such situations. [Copyright &y& Elsevier]

Highlights: [•] It investigates the evolution of surface hardness of concrete under sulfate attack. [•] It reveals reinforcing effect and weakening effect in concrete during erosion. [•] Reinforcing and weakening effect are all induced by delayed ettringite and gypsum. [•] It uses an integral method to analyze the attenuation tendency of surface hardness. [ABSTRACT FROM AUTHOR]

Abstract: One of the available tests that can be used to evaluate concrete sulfate resistance is USBR4908. However, there are deficiencies in this test method. This study focuses on the ANN as an alternative approach to evaluate the sulfate expansion. Three types of cement combined with FA or SF, along with variable W/B were study by USBR4908. ANN model were developed by five input parameters, W/B, cement content, FA or SF, C3A, and exposure duration; output parameter is determined as expansion. Back propagation algorithm was employed for the ANN training; a Tansig function was used as the nonlinear transfer function. It was clear that the ANN models give high prediction accuracy. In addition, The engineer can avoid the use of the borderline 2.5–5% C3A content in severe sulfate environments and borderline 6–8% C3A content in moderate sulfate environments, specially with W/B ratio greater than 0.45. [Copyright &y& Elsevier]

Highlights: [•] Efficiency of FRP composites in restoring shear strength of sulfate-damage beams. [•] Evaluating anchorage of FRP strips using FRP sheets. [•] Analytical predictions of shear capacity of beams matched experimental results. [•] Use of FRP strips and sheets helped restoring original mechanical properties. [•] FRP composite enhanced crack resistance, and delayed the onset of initial cracking. [Copyright &y& Elsevier]

• Effect of initial curing period on the behavior of mortar under sulfate attack was systematically investigated. • Relationships between compressive strength and other mechanical properties considering initial curing age are proposed. • Initial curing for 14 days can ensure the development of mechanical strength, mass growth rates and linear expansion rates. The infrastructures in the ocean and salt lake areas are usually subjected to sulfate attack shortly after casting. But the influence of early exposure is unclear yet. This study aims to evaluate the influence of early exposure on mortar properties under sulfate attack and to identify the degradation mechanism under this condition. The mortar specimens with different initial curing periods (1, 3, 7, 14 and 28 days) were immersed in 10% Na 2 SO 4 solution for equal time periods (60, 90, 150, 240 and 360 days). Evaluation of the influence of early exposure on mortar behavior under sulfate attack was based on changes in physical properties and mechanical strengths. The degradation mechanism of the mortar specimens was analyzed by X-ray diffraction (XRD), and scanning electron microscopy (SEM) with energy-dispersive spectrometry (EDS). The results showed that the shortening of the initial curing period led to more sulfate ions penetrating into the specimen, resulting in more serious surface spalling, higher expansion rate and mass growth rate, lower mechanical strength. Microscopic images showed that the cement matrix of the early exposed specimen was much looser. In addition, the relationships between compressive strength and other mechanical strengths considering the initial curing period were established and fitted reasonably well with other data sources. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing may be induced easily in a cement-based structure in a sulfate-rich environment, which threatens engineering safety. In order to investigate the evolution of critical water pressure, a series of hydraulic fracturing tests and splitting tensile strength tests on the cement mortar under different sulfate-exposure periods are performed. The critical water pressure of the cement mortar under sulfate attack experiences an initial increase stage and a subsequent decrease stage. A stress intensity factor is modified by two proposed damage variables which are crack length and fracture stress. Then, the relationship between the critical water pressure and the tensile strength is established. Moreover, an evolution model of the critical water pressure is proposed, which reveals that the matrix tensile strength and porosity of cement mortar strongly affect the critical water pressure evolution. Additionally, an empirical formula is suggested to describe the critical water pressure evolution of the cement mortar under sulfate attack, and its validity is verified by experimental results. [ABSTRACT FROM AUTHOR]

• The effects of slag/fly ash ratios, Na 2 O% and Ms of activators on the sulfate attack of AACs are studied. • Upon the Na 2 SO 4 attack, the C-A-S-H gel structures of AACs do not change basically. • The MgSO 4 attack on AACs is severe due to the dissolution of alkali and the migration of Mg2+. • Gypsum is the main product in AACs upon the MgSO 4 attack. • The effect of Ms on the sulfate resistance of AACs is related to the slag/fly ash ratios. In this work, the effects of slag/fly ash ratios, Na 2 O concentration, and silicate modulus (Ms) of activators on the reaction mechanism of sulfate attack on alkali-activated slag/fly ash cements are elucidated by XRD, FTIR, and TG/DTG analysis. To accelerate the sulfate attack process, the alkali-activated cements (AACs) were crushed into particle samples before immersion in the Na 2 SO 4 and MgSO 4 solutions. The results show that upon the Na 2 SO 4 attack, the C-A-S-H gel structures of AACs do not change basically. However, the MgSO 4 attack on AACs is severe due to the dissolution of alkali and the intrusion of Mg2+. Gypsum is the main product in AACs upon the MgSO 4 attack, and the amount of brucite decreases with the incorporation of fly ash. The NaOH-activated slag (Ms = 0) has a relatively higher MgSO 4 resistance compared with the Na 2 SiO 3 -activated slag, while it is the opposite when the fly ash replacement level increases to 80%. [ABSTRACT FROM AUTHOR]

In this study, laboratory tests were conducted to evaluate the sulfate resistance of three types of mortar containing palm oil fuel ash (POFA) collected from different palm oil mills. The compressive strength of three different concretes containing different types of POFA was also investigated. For comparison purpose plain Portland cement mortar and concrete were used as a reference. Ground POFA was used to partially replace Portland cement type I, 20 % by weight of binder. For testing the durability of mortar exposed to sulfate attack, the expansion of standard mortar bars immersed in 5 % sodium sulfate NaSO and magnesium sulfate MgSO solutions for up to 15 weeks was determined. Moreover, FESEM micrograph and EDS analysis were carried out. The results of 28 days compressive strength of two concretes containing POFA showed higher compressive strength than that of plain Portland cement concrete. The expansion levels of blended mortars were in general lower than plain Portland cement mortar. The FESEM and EDS analysis showed good correlation to the results of expansion tests. [ABSTRACT FROM AUTHOR]

Abstract: One of the available tests that can be used to evaluate the sulfate resistance of concrete is a procedure for length change of hardened concrete exposed to alkali sulfates (USBR4908). However, there are deficiencies in this test method including a lengthy measuring period, insensitivity of the measurement tool to the progression of sulfate attack. Moreover it is difficult to obtain experimental expansion due to time and cost limitations. A reasonable prediction for the expansion in USBR4908 is basically required. This study focuses on the artificial neural network (ANN) as an alternative approach to evaluate the sulfate resistance of concrete. A total of 273 different data for three types of Portland cement combined with fly ash (FA) or silica fume (SF) concrete mixes, along with different w/c ratios of 0.35, 0.45 and 0.55 were collected from the experimental program. ANN models were developed. The data used in the ANN model consisted of five input parameters which include W/B ratio, cement content(CC), FA or SF content, tricalcium aluminate content (C3A), and exposure duration (D). Output parameter is determined as expansion (E). Back propagation (BP) algorithm was employed for the ANN training in which a Tansig function was used as the nonlinear transfer function. Through the comparison of the estimated results from the ANN models and experimental data, it was clear that the ANN models give high prediction accuracy. In addition, the research results demonstrate that using ANN models to predict the expansion in concrete cylinders is practical and beneficial. [Copyright &y& Elsevier]

Abstract: The function of tunnels is degraded due to sulfate ambient, which has spread widely in Western China. The characteristics of tunnel concrete damage, development of cracks, structure deformation, and the safety factor of tunnel liner suffering from sulfate attack are studied in this work using field investigation and laboratory tests. The main damage styles of tunnel lining structure included structure cracking, spalling, leakage filtration, degradation, and corrosion. These indicate that either physical damage by crystal erosion or chemical dissolution damage – or both – may have occurred. They appeared in the liners with weak waterproof ability, poor compaction, and high hydraulic gradient. The crack of the tunnel liner first developed at the vault, and there were more cracks at the vault than in other places when the load was high. Crack width and structure deformation increased with load and erosion time as exponential functions, whereas the safety factor was reduced as a negative exponential function. [Copyright &y& Elsevier]

Abstract: The experiment for examining the dynamic properties of cement mortar under sulfate attack is carried out and reported in this paper. By using the ultrasonic technique, the evolution of the dynamic elastic modulus of samples is derived. A new “1sV+nε” method combining advanced Hopkinson technique with pathline-based Lagrangian reverse analysis is firstly presented to obtain stress–strain relations. It was found that the variation of dynamic modulus involved an initial increase stage followed by a substantial decrease stage. The turning points from increase to decrease obtained from the ultrasonic and advanced Hopkinson techniques were found to be different, reflecting the difference between the erosional and mechanical damage. [Copyright &y& Elsevier]

Abstract: Two factors which affect concrete’s durability were investigated, including the effect of the mineral admixtures used, as well as the effect of chlorides on concrete’s deterioration due to the thaumasite form of sulfate attack. Concrete specimens were prepared with Portland limestone cement as well as by replacing a certain amount of limestone cement with natural pozzolana, fly ash, blastfurnace slag or metakaolin. The specimens were immersed in two corrosive solutions (chloride–sulfate; sulfate), and stored at 5±1°C. Visual inspection of the specimens, mass measurements and compressive strength tests took place for 24months. The partial replacement of limestone cement with mineral admixtures retards and inhibits concrete’s deterioration. In the case of limestone cement concrete without mineral admixtures, chlorides mitigate the corrosive effect of sulfates. Regarding concrete containing mineral admixtures, the concomitant presence of chlorides amplifies the detrimental effect of sulfates and leads to a worse level of damage. [Copyright &y& Elsevier]

Abstract: The primary factors affecting concrete sulfate resistance are the chemistry of Portland cement and the replacement level of mineral admixtures [1]. In order to investigate the effect of those on the sulfate attack the testing program in the literature involved the testing of different mortar mixes using the standardized test, ASTM C1012-95. ASTM C1012-95 has been widely used by researchers to study the sulfate resistance of cement based materials by exposing 1*1*11 in. mortar specimens to 50g/L Na2SO4 or MgSO4 solution [2]. However, there are deficiencies in this test method including lengthy measuring period, insensitivity of the measurement tool to the progression of sulfate attack, the effect of curing and the effect of the pH change during the time in the solution [3–5]. In this research a study is presented to build a model by ANN equivalent to ASTM C1012-95. The input parameter was obtained from 16 different mortars according to ASTM C1012-95. Plain Portland cement mortars, mortars with cement combined with fly ash (FA), and mortars with cement combined with slag (GGBFS) were tested by using ASTM C1012-95. Four cements, two ratio of FA, and one GGBFS were obtained from the literature. ASTM C1012-95 modeling techniques can help us understand the influence of aggressive environments on the concrete performance more readily, faster, and accurately. Such an understanding improves the decision making process in every stage of construction and maintenance and will help in better administration of resources. [Copyright &y& Elsevier]

Pyrites and pyrrhotites are the most abundant minerals of the iron sulfide group in nature and may be found in the aggregates used to produce concrete. In the presence of water and oxygen, such compounds generate expansive reactions that may have severe structural damages. This is especially critical in concrete dams, given the large volume of material used and the restrictions in movement imposed by the surroundings. In these cases, the definition of adequate rehabilitation programs depends on the prediction of the expansive reaction evolution and the future behavior of the structure. Although models that describe solid particle-gas reactions may be used with this purpose, none were specifically developed to simulate this phenomenon in dams. This paper introduces a new kinetic model based on the unreacted-core model for pyrrhotite oxidation. The comparison of the results obtained with this new model and with the direct application of the unreacted-core model show significant differences. Following an extensive parametric study, a simplified constitutive equation is proposed to estimate the pyrrhotite oxidation kinetics in concrete dams. The estimations performed with this constitutive equation agree with the experimental data obtained in the tests of different particle sizes. [ABSTRACT FROM AUTHOR]

Abstract: In this work the effect of mineral admixtures on the thaumasite form of sulfate attack in limestone cement concrete is studied. Additionally, the effect of the type of sand (calcareous or siliceous) and the storage temperature is investigated. Limestone cement, containing 15% limestone, was used. Concrete specimens were prepared by replacing a part of cement with the studied minerals. The specimens were immersed in a 1.8% MgSO4 solution and stored at 5°C and 25°C for 3years. A well designed concrete made with limestone cement and fly ash, blastfurnace slag or metakaolin seems to have the ability to withstand thaumasite form of sulfate attack. The addition of natural pozzolana presented only a limited improvement of concrete’s sulfate resistance. The type of the sand and its cohesion with the cement paste has a remarkable effect on the performance of concrete at low temperature. Finally, no damage was observed in the specimens exposed to sulfate solution at 25°C. [Copyright &y& Elsevier]

Abstract: This paper presents a theoretical and experimental study on the attenuation coefficient of the ultrasonic expansion wave in concrete specimens made of different mixes and suffering from sulfate attack. The relationships between the attenuation coefficient, relaxation time, and the wave velocity of the elastic expansion wave were derived. By using simple ultrasonic techniques, the variation of the attenuation coefficient with time during the process of sulfate attack was determined experimentally. The results showed that the magnitude of the attenuation coefficient of concrete increased with time, reflecting damage evolution in the material caused by sulfate attack. This suggested that the damage caused by sulfate attack in concrete can be determined by using simple ultrasonic testing methods. Also, it was found that the concrete specimens with added fly ash had a better performance (reduced sulfate attack) than those with added fine ground slag. [Copyright &y& Elsevier]

Abstract: Sulfate attack is one of the major durability problems in concrete structures. Sulfate ions present in concrete can react with cement hydration products to form gypsum and ettringite, and this may affect the pore structure and cause structural damage. In this paper, an experimental study was performed on the diffusion of sulfate ions in concrete. Based on the experimentally obtained results, a new diffusion model for sulfate ions was developed, taking into account the influence of the evolution of sulfate-induced damage on the diffusion of ions. The evolution of damage with immersion time and concentration of sulfate ions can be determined using ultrasonic tests. A numerical method was utilized to solve the nonlinear parabolic partial differential equation describing diffusion. A comparison between numerical results and experimental data is provided to validate the proposed models. [Copyright &y& Elsevier]

Abstract: The damage process of concrete exposed to sulfate attack under flexural loading and drying–wetting cycles are investigated in this paper. ESEM, MIP, and XRD were used to investigate the changing of microstructure and corrosion products of interior concrete. The results indicate that compared with the single damage process of sulfate attack, flexural loading and drying–wetting cycles can both accelerate the damage process of concrete subjected to sulfate attack. While compared with sulfate attack and drying–wetting cycles, the effect of flexural loading on the deterioration of concrete depends on the stress level. It was also found that the addition of admixtures can improve the ability of sulfate resistance of concrete when it is also subjected to mechanical loading and drying–wetting cycles. [Copyright &y& Elsevier]

Abstract: This paper describes an experimental study on the expansion and subsequent damage of concrete specimens immersed in pure water, sulfate solutions of different sulfate concentrations, and mixed solutions of different sulfate and chloride concentrations. Based on experimentally obtained results, a theoretical expansion model is proposed for predicting the stresses, strains and volume expansion developed during the process of sulfate attack. The model proposed is based on the assumption that concrete may be treated as a porous visco-elastic material, and was developed by using a micromechanics approach method. The internal expansion force produced by delayed ettringite and gypsum is considered as the main cause of the expansion. The expansion model was validated using the experimental results obtained. [Copyright &y& Elsevier]

Abstract: In this study, white cement CEM I-32,5 & white limestone cement CEM II-A/LL, with 15% limestone substitution, were investigated. Mortars were made from these types of cement and the durability of these mortars against sulfate attack as well as the effect of temperature (50°C) were investigated. During the experiment, the mortar samples were being observed visually on a regular basis using the Heine Delta 20® LED Illuminated Loupe 10× device. After a period of 90days, the mortar samples’ compressive strength was determined and the deterioration products which affected the durability were identified. The identification of the deterioration products was made through means of X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM-EDAX), Thermogravimetry (TG), Infra-Red Spectroscopy (FT-IR) and Mercury Porosimetry (MP). The results of this study clearly displayed that the rate of sulfate attack to the cement with limestone substitution was lower compared to that of pure cement. Furthermore, it was shown that this type of cement is three times more durable than pure cement of the same category. It was also identified via different methods of analysis that the cause of the deterioration was the development of microcrystalline ettringite which caused microcracks and degradation of the specimens. In conclusion, it is shown that in the case of same class cement with limestone substitution, such a microstructure is developed which does not allow the penetration by sulfate ions. This is demonstrated through the higher resistance of this specific mortar despite the adverse conditions in which these mortars were exposed to. [Copyright &y& Elsevier]

Abstract: Three cements were examined: (i) Portland cement, (ii) limestone cement containing 15% limestone and (iii) limestone cement containing 30% limestone. Concrete specimens, prepared with siliceous or calcareous sand, were immersed in a 1.8% MgSO4 solution and cured at 5°C and 25°C for 3years. Limestone cement concrete is susceptible to thaumasite form of sulfate attack at 5°C. In concrete with siliceous sand the damage is greater, the higher the limestone content of the cement. In concrete with calcareous sand the sulfate resistance seems to be independent on the limestone content. Concrete with calcareous sand showed better performance than concrete with siliceous sand. No damage was observed in the specimens exposed to sulfate solution at 25°C. [Copyright &y& Elsevier]

Abstract: In order to evaluate numerically the volumetric expansion of concrete under sulfate attack, models are proposed to calculate the expansive volume strain of concrete exposed to sulfate solutions. Firstly, a one-dimensional differential equation related to the diffusion–reaction behavior of sulfate ions in concrete is presented; secondly, based on chemical reactions between sulfate and calcium aluminates in concrete, the expansive volume strain caused by ettringite growth is obtained; thirdly, numerical analysis is carried out to investigate the volume expansion of concrete when it is exposed in sulfate solutions. Numerical results provide the evolution response of concrete volume expansion with the diffusion time, sulfate concentration, and calcium aluminates dissipation. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental study on the interaction between sulfate and chloride attack and mechanical loading on concrete specimens made with two different water-to-cement ratios (w/c). The resulting damage owing to these combined effects was investigated using ultrasonic techniques. The experimental results showed that high strength concrete had a strong resistance to sulfate attack. Flexural loading accelerated damage to the concrete, whereas the damage owing to sulfate attack is diminished in the presence of chlorides. In general, sulfate attack initially has a positive effect on concrete performance but this reverses with time and significant deterioration can occur in concrete suffered from a long term sulfate attack. [Copyright &y& Elsevier]

Abstract: The sulfate resistance of a white Portland cement (WPC) containing 0%, 50% or 70% granulated blast-furnace slag as a partial cement replacement was investigated. The use of Na2SO4 as a chemical activator in the 50% slag blend was also studied. The performance of the blended cements was monitored by exposing the prepared mortar specimens to a 5% Na2SO4 solution for 2years according to ASTM C1012/04 and using TG/DTA, DRX and SEM/EDX analyses of the paste samples. The same blends composed a high early strength Portland cement (PC) were also used. The results showed the benefits of slag in both cements, and an increase in its percentage increased sulfate resistance. Chemical activation reduced the expansion compared to those mixtures without it. For long-term exposure, all of the WPC blends showed less expansion than the corresponding blends with PC. A microstructure analysis identified ettringite and gypsum as the main degradation products. [Copyright &y& Elsevier]

Abstract: In order to give confidence to those responsible for the management of infrastructure, it is necessary to investigate the structural effects of the thaumasite form of sulfate attack (TSA) on buildings and structures. Whereas TSA is less common than other forms of structural concrete deterioration, its consequences can be quite serious. To investigate TSA deterioration of concrete within a reasonable time scale, it is generally necessary to accelerate TSA in the laboratory. Using such an accelerated testing procedure, this paper is concerned with predicting the rate of TSA depending on varying mix designs and aggressive solutions. A methodology was developed to estimate the annual deterioration rate of TSA. It was shown that a combined carbonate and sulfate solution is less aggressive than a pure sulfate solution, and that deterioration rates of up to 1.8mm/year are possible in commonly used concrete. [Copyright &y& Elsevier]

Abstract: When concrete elements are partially exposed to sulfate rich environment, the upper part of concrete in contact with air will be deteriorated more severely than the underground part. Fly ash additions seem to accelerate the collapse of concrete in such an environment. Although concrete technologists attribute concrete damage mainly to salt crystallization or physical sulfate attack, the influence of chemical sulfate attack cannot be neglected and should also be studied. The objective of this paper is twofold. First, pore solution expression test was conducted to squeeze pore solution of different parts of cement paste partially exposed to Na2SO4 solution. The sulfate concentration and pH value of pore solution were measured. Results showed that the sulfate concentration of the pore solution in the upper part of paste in contact with air was much higher than in the lower submerged part. Fly ash additions could draw more sulfates into the paste in a shorter time, forming a higher concentration sulfate pore solution than in normal concrete. The second test was designed to simulate the effect of severe exposure condition on reactive products of cement paste. Pure cement and cement+fly ash (25% dosage) pastes were immersed in 5%, 15% and 30% at 30°C and 15% at 40°C Na2SO4 solutions. Thermogravimetric analysis was used to analyze the reaction products of the paste. The results indicate that more ettringite and gypsum were formed in cement+fly ash paste than pure cement paste. [Copyright &y& Elsevier]

Abstract: Concrete structures in service may be suffer from sulfate attack, which may lead to the deterioration of both static and dynamic properties of concrete, and therefore jeopardize their performance in earthquake events. To predict the seismic performance of sulfate-damaged concrete structures, the rate-dependent strength, deformation behavior and energy absorption capacity of concrete specimens under sulfate attack were investigated. First, half of the specimens were initially cured under sulfate attack for 4months and the other half of the specimens were cured in a room environment for comparison. Then axial compressive experiments for concrete specimens with different strain rates were carried out. Experimental results show that sulfate attack had a significant impact on the dynamic properties of concrete. Sulfate attack makes concrete strength more sensitive to strain rate. With increasing strain rates, the peak strain and energy absorption capacity of control and sulfate attacked concrete tend to increase, but they are greatly decreased by disruptive expansion and chemical deterioration induced by sulfate attack. Finally, the failure phenomenon was shown and the damage mechanism of specimens was discussed. [Copyright &y& Elsevier]

Abstract: Many degradation processes in cement based materials include the diffusion of one or more chemical species into concrete and consequent chemical reactions which alter the chemical and physical nature of the microstructure. External sulfate attack is mostly described by a coupled diffusion–reaction mechanism which leads to the decomposition of hardened cement constituents and cracking of the paste. This paper discusses the significance of diffusion properties and chemical changes in external sulfate attack in blended cement based composites. A method based on Particle Induced X-ray Emission (PIXE) was developed to measure the diffusion properties in a non-destructive test method. Quantitative Energy Dispersive Spectrometry (EDS) and micro-hardness technique were also used to study the chemical and mechanical changes from sulfate attack. Diffusion coefficients and rates of reaction were determined for paste and mortar mixtures, showing higher diffusion rates and lower hardness values in mortar compared to paste for control mixtures. Partial replacement of cement with fly ash improved the transport properties and reduced the level of damage in exposure to sulfate attack. [Copyright &y& Elsevier]

Abstract: The concrete lining of a vertical ventilation shaft exposed to sulfate-bearing groundwater for 45years was investigated. The concrete was characterized at several sites with different degrees of damage. Apart from strength measurements, optical and electron microscopy were the main tools used to assess the degree of damage and alteration of the concrete. The occurrence of damage appears to be related to the availability of sulfate-bearing groundwater on one side and the presence of poor-quality concrete on the other side. Most of the damage in the form of severe spalling is caused by ettringite formation. The impact of thaumasite formation with disintegration of the cement paste seems to be minor as it occurs after the concrete is already severely damaged. The mineral assemblages and the sequence of mineral formation caused by sulfate attack agree with the findings of laboratory studies and thermodynamic modeling. [Copyright &y& Elsevier]

This study investigates the effectiveness of metakaolin (MK) on the improvement of durability of concretes to sulfate attack. Experimental parameters of the study were MK replacement levels, water to cementitious materials ratio, initial curing procedure in terms of air curing and water curing, and type of the sulfate exposure regimes such as continuous and drying-immersion cyclic exposures. The degree of sulfate attack was evaluated by the reduction in compressive strength (CS) of concrete. The tests were conducted at specified periods up to 365 days. At the end of initial curing, concrete specimens were divided into three groups such that the first group was transferred into tap water to be used as control in the assessment of CS reduction while the other groups were immersed in 10% Na2SO4 solution for the period of 365 days under continuous or cyclic exposures. The results indicated that inclusion of MK as modifying agent increased the resistance of concrete against sulfate attack depending mainly on the MK replacement level, w/cm ratio, and initial curing procedure adopted. [ABSTRACT FROM AUTHOR]

Abstract: A numerical methodology is proposed in this paper to simulate the degradation of cementitious materials under external sulfate attack. The methodology includes diffusion of ions in and out of the structure, chemical reactions which lead to dissolution and precipitation of solids, and mechanical damage accumulation using a continuum damage mechanics approach. Diffusion of ions is assumed to occur under a concentration gradient as well as under a chemical activity gradient. Chemical reactions are assumed to occur under a local equilibrium condition which is considered to be valid for diffusion controlled reaction mechanisms. A macro-scale representation of mechanical damage is used in this model which reflects the cracking state of the structure. The mechanical and diffusion properties are modified at each time step based on the accumulated damage. The model is calibrated and validated using experimental results obtained from the literature. The usefulness of the model in evaluating the mineralogical evolution and mechanical deterioration of the structure is demonstrated. [Copyright &y& Elsevier]