External Sulfate Attack Research Articles

A Review on the Transport-Chemo-Mechanical Behavior in Concrete under External Sulfate Attack

Cementitious concrete structures serving in sulfate environments suffer from serious durability challenges caused by chemical sulfate attacks (CSA), which lead to the volume expansion, cracking, and spalling of concrete and the early failure of structures. CSA on concrete involves the behaviors of ion transport, chemical reactions, the crystallization of reaction products, microstructural damage to the cement matrix, and the macroscopic deterioration of concrete, namely the transport-chemo-mechanical behaviors. This paper first introduces the reaction products, such as gypsum, ettringite, brucite, and thaumasite, between sulfate and concrete under different environmental conditions and their formation mechanism. Then, aiming at the ettringite type CSA, the theories of volume increase and crystallization pressure are elaborated to explain it-induced concrete degradation. Additionally, the crystallization pressure theory is used to describe the cracking behavior in the microstructure slurry caused by the ettringite crystal filling pore. Finally, a series of transport-chemo-mechanical models for ettringite type CSA are displaced module by module. It includes the sulfate diffusion-reaction model, the free expansion of concrete, and equivalent expansive force in concrete related to the reaction behavior: the model for chemo-mechanical behavior in concrete caused by CSA. These models can be used to analyze the distribution of sulfate ions and the reaction product content, expansive stress and strain in the concrete, and the cracking and spalling degree of the concrete, which is beneficial to evaluate the durability of concrete structures serving permanently in a sulfate environment.

A Three-Phase Model to Evaluate Effects of Phase Diffusivity and Volume Fraction upon the Crack Propagation in Concrete Subjected to External Sulphate Attack

This study models concrete as a multi-phase system that comprises the mortar, coarse aggregates, and interfacial transition zones (ITZs). The diffusivity and the volumetric fraction of these phases are considered to propose a three-phase diffusion–reaction model to simulate the external sulphate attack. Furthermore, the parametric analysis alongside the sensitivity analysis is carried out to quantify the effect of these phases on the expansive cracking in concrete when exposed to a sulphate-rich environment. The results show that mortar dominates the sulphate ingress and the ensuing distress, while the ITZ is found to be least significant. Due to its significantly low permeability, the coarse aggregate may act as a “deceleration strip” or a “dam”, which in turn obstructs the sulphate penetration. More importantly, this effect is further noted to evolve with a decrease in the diffusivity and a rise in the volumetric fraction of coarse aggregates. As for ITZ, its volume fraction may play a more dominant role than its diffusivity on sulphate attack in concrete.

Influence of hoop restraint on microstructure and phase composite of cement paste filled steel tube under external sulfate attack

The deterioration of cement-based material caused by external sulfate attack (ESA) is a major durability problem affecting concrete structures. This study aims to investigate the effect of hoop restraint of steel tube on microstructure and phase composition of cement pastes prepared with ordinary Portland cement (OPC) and high sulfate-resistance Portland cement (SRPC) under ESA for 540 days. The effects of hoop restraint on the change in micromorphology, phase composition and microstructure of cement hydration products under ESA were investigated by SEM-EDS, XRD, 29Si and 27Al MAS NMR spectroscopy and low-field NMR. Moreover, the effects of hoop restraint on local mechanical properties of the cement pastes under ESA were evaluated by micro-hardness measurement. Results indicate that hoop restraint significantly inhibits the formation of micro-cracking and limits the permeation rate of sulfate ions, leading to a reduction in the amount of ettringite and gypsum crystals. 27Al NMR analysis shows that the hoop restraint has significant “delay effect” on the transfer of Al phase transition from monosulfoaluminate (AFm) and third aluminate hydrate (THA) to ettringite. 29Si NMR test reveals that hoop restraint can significantly slow down decalcification and dealuminization of C-(A)-SH gel, leading to a slow reduction in Al[4]/Si ratio of C-(A)-SH, together with the lower mean chain length (MCL) of the silicate chain. The MCL reduces by 34.7 % and 9.0 %, and Al[4]/Si ratio increases by 41.9 % and 8.3 % for OPC and SRPC pastes with steel tube compared to the equivalent free ones, respectively. Additionally, hoop restraint results in better quality pore structure of cement pastes with steel tube than the equivalent free ones.

Mineralogical Evolution and Expansion of Cement Pastes in a Sulfate-Confined Environment

The geologic disposal of radioactive waste could lead to confined conditions in which cementitious materials impose a moderate alkaline pH, in which the rocks supply sulfate ions at rather low concentrations. In this context, the purpose of this work was to study the degradation of cement pastes under such conditions using a non-renewed 30 mmol/L Na2SO4 solution without pH regulation. Samples were investigated through laboratory testing and numerical modeling. XRD, SEM–EDS, and micro-indentation acquisitions were performed to follow the evolution of chemical, mineralogical, and mechanical properties during the weak external sulfate attack. Reactive transport modeling was performed with the HYTEC code. Based on these results, the Young’s moduli of the degraded zones were estimated using analytical homogenization. Decalcification occurred despite the high pH value of the solution and significantly affected the mechanical properties. Macroscopic swelling and cracking were caused by the formation of expansive sulfate minerals after 60 days, despite the low sulfate content of the tank solution. The modeling supported the discussion on the evolution of the mineral fronts (ettringite, portlandite, and gypsum).

A new look at the resistance of recycled aggregate concrete (RAC) to the external sulfate attacks: The influence of the multiple mesoscopic material phases

Recycled aggregate concrete (RAC) prepared with recycled coarse aggregate (RCA) has more complex material phases on the meso scale compared to natural aggregate concrete (NAC), including the old virgin aggregate, the new/old mortar, and the three types of interfacial transition zone (ITZ). These mesoscopic material phases would influence the RAC to external sulfate attacks in different ways. This paper provides a new look at the performance of RAC subjected to external sulfate attacks by considering such influence. The quality and quantity of the multiple mesoscopic phases were changed by taking varying water-to-binder ratios (i.e., w/b) and volumetric fractions of RCA (i.e., VRCA), respectively. The changes in properties of both the material phases on the meso scale and concrete on the macro scale subjected to external sulfate attacks were experimentally investigated. The old mortar and ITZ phases (regardless of the ITZ type) were identified as the weaker phases against external sulfate attacks as their microhardness deteriorated faster than that of the new mortar. Improving the quality of the new mortar/new ITZ phases by taking a lower w/b can greatly enhance RAC's resistance to external sulfate attacks: the ingress of sulfate ions was retarded, meanwhile the degradation in compressive strength and dynamic elastic modulus were postponed obviously. Increasing the volumetric fraction of old virgin aggregates by taking a higher VRCA can also help RAC sustain better under sulfate attacks, even though it was accompanied by the simultaneously increasing quantity of the old mortar and ITZ which were identified as the weaker phases against sulfate attacks. Basically, changing the quality of the multiple mesoscopic material phases had more remarkable effects on RAC's sulfate resistance than changing the relative quantity ratios. This paper thereby suggests that RAC's sulfate resistance should be better enhanced by the strengthening or removal of the old mortar phase, or by the improvement of the new mortar and ITZ phases, rather than adjusting their quantity ratios in concrete.

Simulation of internal and external sulfate attacks of concrete with a generic reactive transport-poromechanical model

Chemical reactions can lead to the expansion of the geomaterial because of the strong precipitation of minerals in the pores of the medium. The quantity and the variability of these reactions led to the development of several chemo-mechanical models. A generic chemo-poromechanical model is proposed to predict both the material degradation induced by various concrete pathologies and the environmental multi-factor impact on the macroscopic swelling. The model is based on the coupling between reactive transport (species diffusion and chemical reactions) and poromechanics. The mineralogy and the poromechanical properties are estimated by a hydration-micromechanics model which allows to consider the material effects. The main applications are dedicated to External Sulfate Attack and Delayed Ettringite Formation of cementitious material at mesoscale. The studies highlight the ability of the model to deal with complex and various phenomena such as coupling chemical reactions with calcium leaching, sulfate sorption and ettringite precipitation, and to consider their impact on the macroscopic expansion. Results also bring out the accuracy of the chemo-poromechanical model to simulate different swelling pathologies. The characteristic crack paths induced by differential strain, material impact, and swelling at the macroscopic scale, fit with experimental observations.

A Critical Review of Existing Test-Methods for External Sulfate Attack.

External sulfate attack (ESA) of cementitious materials has been studied worldwide for a very long time. This physical/chemical interaction between sulfate ions and the cement hardened elements affects the long-term durability of concrete structures: cracking, spalling or strength loss of concrete structures. To study these damaging phenomena, some standardized and non-standardized accelerated aging tests are used to evaluate the performance of cements in sulfate-rich environments. However, these existing methods do not adequately predict field performance and some shortcomings or deficiencies still exist: change of degradation mechanisms when using high concentrations of sulfate, variable boundary conditions and small specimens compared to the real concrete structures. In this work, a critical review of some existing test methods and foreign national standard methods for ESA are presented, analyzed, and discussed. This results in some proposed recommendations for improving these methods to meet the needs of structure managers.

Spalling rate of concretes subject to combined leaching and external sulfate attack

A spalling rate model is established to estimate the damage of concretes subject to the combined action of leaching and external sulfate attack (ESA). Two sub-models are developed to consider the different damage patterns. The profiles of sulfate and hydroxide ions are obtained from the solution of a dual moving boundary problem (MBP). The supersaturation of ettringite crystal is evaluated from these profiles, the effective stress is calculated through poromechanics for pore crystallization, and the spalling depth is determined using the fracture criterion. The experimental validation confirms the proportionality between the spalling depth and the square root of exposure time, and the models give conservative results but on the correct magnitude order for spalling depth. The parametric analysis shows the foremost impact of aluminate content and sulfate diffusivity on damage rate. The main assumptions in models lead to conservative predictions, but yet the model results are meaningful for engineering use.

Optimization of mixture parameter for physical and mechanical properties of reactive powder concrete under external sulfate attack using Taguchi method

• Taguchi analysis has been used to determine optimum mix design parapeter of RPC exposed to different Na 2 SO 4 solutions and exposure conditions. • Optimized level for RPC mixture with different water to binder ratio, steel fiber contents, and silica fume contents depends on different external sulfate attack exposure conditions. • RPC appears to be sustainable and durable material under different external sulfate attack exposure conditions. Reactive powder concrete (RPC) is defined as a cementitious composite material with an optimized size of granular constituents, very low water-to-binder ratio (w/b), pozzolanic materials like silica fume (SF), and discontinuous fiber reinforcement. RPC applications include bridge decks and girders, seismic columns, wind turbine towers, and pile foundations. Especially, a durable and robust RPC pile foundation with long service life is essential in building construction because continuous maintenance is impossible. Moreover, natural in-situ conditions such as water table, temperature, and sulfate concentration in soil to which the pile foundation is exposed are critical and related to deteriorating the pile foundation. Therefore, the Taguchi design of experiments (DOE) was used in this research to determine the optimal RPC mixture with beneficial characteristics against external sulfate attack (ESA). Mixture design parameters included steel fiber content (0, 1 and 2 %), w/b (0.16, 20 and 0.24), and SF content (15, 20 and 25 %). In contrast, environmental conditions of ESA exposure contained three different sodium sulfate (Na 2 SO 4 ) concentrations (0.35, 1.05, and 3.15 M), cyclic and continuous exposure at normal temperature (20 °C), and continuous exposure at elevated temperatures (40 °C and 60 °C). The analytical and statistical investigation evaluated mass change, compressive strength, and modulus of rupture in RPC mixtures exposed to these conditions for 52 weeks. Test results show that cyclic exposure and increased solution temperature in ESA facilitate RPC damage. If measured after air drying, RPC mixtures subjected to cyclic ESA are especially susceptible to failure. Taguchi analysis indicated the optimum parameters’ level for mass change as: 0 % steel fiber, w/b = 0.16, and 20 % SF content; for the compressive strength: 2 % steel fiber, w/b = 0.24, and 20 % SF content; and, for modulus of rupture: 2 % steel fiber, w/b = 0.20, and 25 % SF content. In conclusion, RPC appears to be sustainable and durable material under different ESA exposure conditions.

Calcium leaching from cement hydrates exposed to sodium sulfate solutions

Calcium leaching from cement hydrates to pore solution increases the porosity and reduces the bonding strength of cement hydrates, accelerating the degradation of concrete. Calcium leaching can be quantified by solid–liquid equilibrium curves, which have been studied in deionized water or ammonium nitrate. The research of solid–liquid equilibrium curve of calcium under sulfate attack is limited and its mechanism is poorly understood. Reported here provide insights into the dissolution process of calcium from cement hydrates exposed to the sodium sulfate solution. The experimental programme examines the effects of sulfate ion concentration and temperature. An external sulfate attack (ESA) model considering the influence of calcium leaching is established and validated. The results show that, compared to deionized water, sulfate ions impact strongly the leaching process. Qualitatively, the dissolution of calcium in cement hydrates is accelerated by increasing the concentration of sulfate ions and decreasing the environmental temperature. Quantitatively, the presence of sulfate ions modifies the equilibrium curve describing calcium leaching in deionized water. In addition, the prediction results of the ESA model considering the influence of calcium leaching are in good agreement with the experimental results.

Experimental and modelling study on the deterioration of stabilized soft soil subjected to sulfate attack

• A green stabilizer was confirmed in improving the properties of softy soil. • Experimental and numerical study was done on sulfate resistance ability of stabilized soil. • Less ettringite and gypsum were formed in the green stabilizer treated soil. • Numerical model was proposed to predict crystallization pressure by ettringite and gypsum. Developing an alternative for traditional cement in soil stabilization has long been an attractive topic in order to improve the overall performance and reduce cement consumption. Considering the long term safety, a novel green soil stabilizer mainly consisting of industrial wastes, i.e., ground granulated blast furnace slag, steel slag, desulfurization gypsum was evaluated by a series of tests involving the unconfined compressive strength (UCS) under wet-dry cycling and sulfate attack. X-ray diffraction and backscatter scanning electron microscope were performed to reveal the deterioration mechanism. The results show that the addition of green stabilizer shows an increment effect on UCS of soil compared with neat cement. The UCS after 15 cycles of wetting–drying and 30 days of external sulfate attack are both highly retained for the soil stabilized with green stabilizer, while the porosity and expansive gypsum and ettringite are less. A computational model was implemented to investigate the deterioration by determining the crystallization pressure contributed by ettringite and gypsum, and then compared with the tensile strength obtained in experimental works. It indicates that the internal stress in the soil stabilized with green stabilizer is less than that with cement, and the former one show a better durability under sulfate attack. The method and materials presented in this work is helpful for the design and application of stabilized soil considering the complex environmental effects.

Influence of Ca2+ on Early Degradation of Cast-In-Situ Mortar Induced by Sulfate-Magnesium Multiple Combined Attack.

Early degradation of cast-in-situ concrete induced by multiple internal–external sulfate combined attacks significantly affects the development of concrete strength. An experimental study regarding the effects of Ca2+ on the early degradation of cast-in-situ mortars subjected to internal–external sulfate and magnesium combined attacks is investigated in this paper. In particular, a specific method for accurately simulating the degradation of cast-in-situ structures was proposed in this experiment. Physical properties (including weight, size changes, and porosity), mechanical properties (including flexural strength and compressive strength), sulfate concentration, and microstructural properties were monitored during 28 days of immersion. The results show that an internal sulfate and magnesium combined attack (ISA-IMA) obviously retards the development of early strength and accelerates the degradation induced by external sulfate attack (ESA). The diffusion path of sulfate ions from outside is blocked by flake-shaped magnesium hydrates, delaying the penetration of external sulfate attacks. However, it is far from neutralizing the strength loss induced by an internal magnesium attack (IMA) at an early age. Premixed excessive Ca2+ would improve the strength development and pore structure of concrete or mortar, enhancing durability against corrosive conditions.

Natural Resources Exploitation in Sulfate-Resisting Portland Cement Manufacturing: Towards Quality Improvement and Reduction of Environmental Impact.

Sulfate-resisting (SR) Portland cement is commonly used in building works to improve concrete’s durability against external sulfate attack. This attack is considered a very serious chemical aggression that causes damage and cracking of concrete structures. These special cements have a very particular mineralogical composition, C3A ≤ 3% and (2C3A + C4AF) ≤ 20%, which makes the cementitious matrix resistant to sulfate attack. This kind of product is very difficult to manufacture since low alumina (C3A) necessitates the use of a high kiln temperature in order to keep a sufficient liquid phase necessary to maintain the stability of the cement manufacturing process. In this context, this study aims to optimize SR Portland cement raw meals using natural materials collected from different regions in Tunisia, mainly ordinary limestone, siliceous limestone, black marl, grey marl, iron ore, and natural fluorapatite. The collected specimens were characterized by an X-ray fluorescence spectrometer in order to determine its elemental chemical composition. The optimization of the SR Portland cement raw meal combinations was done by means of a calculation tool based on the chemical composition of each used raw material and the variation of burning modules (LSF, SIM, and ALM). It has been found that natural fluorapatite integration (0%–15%) in raw mix preparation leads to the raw meals required for the SR Portland cement standard (C3A ≤ 3% et 2 C3A + C4AF ≤ 20%). Moreover, it was shown that the estimated SR raw meals ensure the cement manufacturing process stability (acceptable burning modules “LSF = 100; SIM = 3; ALM = 0.91 and sufficient liquid phase) and decrease the CO2 emissions in cement production.

Roles of fly ash, granulated blast-furnace slag, and silica fume in long-term resistance to external sulfate attacks at atmospheric temperature

This paper presented the role of supplementary cementitious materials (SCMs), including fly ash (FA), granulated blast-furnace slag (GBFS), and silica fume (SF) in long-term resistance to external sulfate attack (ESA) of cement mortar at atmospheric temperature. The cement mortars were immersed in 5% sodium sulfate solution at a temperature of 20 °C for 36 months. The results showed that the amount of deterioration products and pore characteristics were the two critical factors affecting sulfate resistance. The pore characteristics played more integral roles in influencing the long-term sulfate resistance than the amount of deterioration products. FA maintained excellent sulfate resistance after exposure to ESA for 36 months. SF exhibited higher sulfate resistance than FA because it primarily supported total porosity and percentage of harmless pore, rather than the portlandite removal by the pozzolanic reaction. On the contrary, the addition of GBFS accelerated the deterioration process and exhibited the worst sulfate resistance because the total porosity and percentage of harmless pore occurred massive changes.

Reliability of the damage rating index to assess the condition of concrete affected by external sulfate attack

The damage rating index (DRI) has proven to be a quite reliable microscopic technique enabling the condition assessment of concrete affected by internal swelling reactions. Yet, very little research has been conducted on the use of the DRI to appraise induced deterioration caused by physical salt attack (PSA) and external sulfate attack (ESA). This study aims to assess the condition of concrete subjected to different salt solutions (i.e. sodium sulfate (Na2SO4), sodium chloride (NaCl), seawater and limewater), and exposure conditions (i.e. partially and fully immersed) through the DRI. Concrete specimens displaying two distinct water/cement ratios (i.e. 0.45 and 0.65) were manufactured and exposed to the above salt solutions for 12 months. Then, the samples were removed from the exposure conditions and prepared for microscopic analysis. The DRI was shown to be an effective technique to assess the condition of PSA- and ESA-affected concrete since different degrees of damage and features were captured for the various exposure conditions studied.

Durability analysis of concrete foundations exposed to external sulfate attacks in the south of Santa Catarina, Brazil

Durability analysis of concrete foundations exposed to external sulfate attacks in the south of Santa Catarina, Brazil

External Sulfate Attack on Cementitious Binders: Limitations and Effects of Sample Geometry on the Quantification of Expansion Stress.

The hollow cylinder method was used to estimate the expansion stress that can occur in concrete due to the crystallisation pressure caused by the formation of ettringite and/or gypsum during external sulphate attack. Hardened cement paste hollow cylinders prepared with Portland cement were mounted in stress cells and exposed to sodium sulphate solutions with two different concentrations (3.0 g L SO42− and 30.0 g L SO42−). Microstructural analysis and finite element modelling was used to evaluate the experimental observations. The expansion stress calculation was verified for a range of diameter/length ratios (0.43–0.60). Thermodynamically predicted maximum expansion stresses are larger than expansion stresses observed in experiments because the latter are affected by the sample geometry, degree of restraint, pore size distribution and relaxation processes. The results indicate that differences in self-constraint at the concave inner and convex outer surfaces of the hollow cylinder lead to an asymmetric expansion stress when ettringite is formed. This leads to macroscopic longitudinal cracks and ultimately failure. Heavy structural components made of concrete are likely to support larger maximum expansion stresses than observed by the hollow cylinder method due to their self-constraint.

Multiphysical damage characteristics of concrete exposed to external sulfate attack: Elucidating effect of drying–wetting cycles

Drying–wetting cycles (DWC) are suspected of influencing the performance of concrete under sulfate attack, but the nature and extent of this influence are still unclear. Therefore, this research concerns understanding the effects of drying–wetting regimes. To this end, pastes, mortars, and concretes were subjected to cyclic drying–wetting with an interval of 12, 24, or 48 h prior to various physical and microstructural analyses. Additionally, drastic drying–wetting conditions with forced drying at 60 °C to reduce relative humidity from 100% to 55% and field-like drying–wetting conditions with gentle stepwise drying in the natural environment were adopted to examine the relationship between the laboratory and field testing. The results indicate that the conditioning regime with a dry–wet ratio of 3:1 and a dry–wet interval of 48 h can be considered a means to accelerate external sulfate attack (ESA) in a short duration and take both reliability and reproducibility into account without overly sacrificing the anticipated filed conditions. The results confirm that the drying–wetting cycle length and the drying mode affect the transport and reaction mechanisms of sulfate ions and the degradation kinetics of sulfate attack. The implications of these findings inform the design of drying–wetting regime for aggressive aqueous environments.

Zero-Cement Concrete Resistance to External Sulfate Attack: A Critical Review and Future Needs

The durability of zero-cement concrete or alkali-activated materials (AAMs) is a subject of controversy, especially when exposed to sulfate attacks. This can be due to the different elemental and microstructural compositions of the broad alkali-activated systems that exhibit different degradation mechanisms in sulfate-rich environments. Various parameters, such as properties of raw source materials (nature, fineness, and mineralogy), activators (type and concentration), mixture design, curing regime, types of sulfate salt and sulfate ion concentrations, and weathering conditions, are considered to have a significant impact on zero-cement concrete sulfate attack resistance. Furthermore, the adequacy of the standard sulfate immersion tests raises more concerns about the reported behavior. This paper presents a critical review of the current aging protocol associated with ordinary cement resistance and zero-cement concrete or AAMs to external sulfate attack.

Assessment of Reactive powder concrete subjected to three different sodium sulfate Concentrations: Compressive Strength, Absorption, Porosity, Microstructure, and durability

Reactive Powder Concrete (RPC) is a relatively new type of concrete material characterized by very low water-to-binder ratios (w/b), the addition of supplementary cementitious materials like silica fume (SF), and the presence of very fine aggregates instead of conventional coarse aggregates. The durability of RPC with various w/b (0.18, 0.22, and 0.26) and SF content (15%, 20%, and 25%) against external sulfate attack (ESA) at various sodium sulfate (Na2SO4) concentrations (0.35 M, 0.7 M and 1.4 M) was investigated in this study through examinations of compressive strength, expansion and mass change, microstructural characteristics, mineralogy, porosity, and sorptivity. The results indicated that: 1) owing to its improved microstructure, RPC could accommodate expansive products of ESA reactions, which can provide additional strength; 2) RPC showed very high resistance to Na2SO4 attack under the ASTM C1012 criteria; 3) considerable sulfate ingress did not result in significant expansion and strength reduction for RPC mixtures at early stages of exposure, especially for the mixtures with high initial sorptivity and high interparticle connectivity; 4) considering the coupled effect of w/b and SF content on the number of RPC characteristics, mixture with w/b = 0.22 and 20% SF content showed the best performance upon exposure to ESA.