Predicting the durability of Portland cement systems in aggressive environments—Laboratory validation

Portland cement systems are often exposed to severe environments, and their long-term performance is of concern. The main results of a comprehensive investigation of deterioration processes that may affect the behavior of Portland cement systems exposed to chemically aggressive environments is presented. As part of this investigation, well-cured cement paste discs were fully characterized and exposed to deionized water and sodium sulfate solutions. Degradation experiments were conducted under saturated and unsaturated conditions. At the end of the exposure period, microstructural alterations were investigated by microprobe analyses, scanning electron microscope observations and energy-dispersive X-ray analyses. Test results provide information on the basic aspects of various degradation phenomena, such as decalcification and external sulfate attack. Experimental results were also compared with results obtained by a numerical model. The analysis reveals that the intricate microstructural features of the degraded samples could be accurately reproduced by the model.     

Effect of limestone filler on the deterioration of mortars and pastes exposed to sulfate solutions at ambient temperature

This paper presents data on engineering properties such as compressive strength, visual change and expansion of mortar specimens incorporating limestone filler subjected to severe sulfate attack at ambient temperature. Specimens with four replacement levels of limestone filler (0, 10, 20 and 30% of cement by mass) were immersed in sodium and magnesium sulfate solutions with 33,800 ppm of SO₄²⁻ concentration. In order to identify the products formed by sulfate attack, microstructural analyses such as XRD and SEM were also performed on the paste samples with similar replacement levels of limestone filler.The test results demonstrated that mortar and paste samples incorporating higher replacement levels of limestone filler were more susceptible to sulfate attack irrespective of types of attacking sources. However, the deterioration modes were significantly dependent on the types of sulfate solutions. Additionally, although the samples were exposed to sulfate solutions at 20 ± 1 °C, the deterioration was strongly associated with thaumasite formation in both sulfate solutions.The deterioration mechanism and resistance to sulfate attack of cement matrix incorporating limestone filler at ambient temperature is discussed in the light of the test results obtained.     

SO42-/C3A对单矿C3S水泥浆体中碳硫硅钙石形成的影响

碳硫硅钙石型硫酸盐侵蚀是一种以碳硫硅钙石为生成产物的硫酸盐侵蚀,碳硫硅钙石的形成过程与水泥基材料中铝源及外界环境中硫酸盐密切相关。以铝酸三钙(C₃A)为铝源,Na₂SO₄溶液为侵蚀介质,采用单矿硅酸三钙(C₃S)制备水泥浆体,通过XRD、FTIR、SEM/EDS等测试技术表征不同SO^2-₄与C₃A摩尔比(S/Al比)对单矿C₃S水泥浆体中碳硫硅钙石形成的影响,旨在揭示碳硫硅钙石形成机理并探讨抑制其形成的方法。结果表明:当S/Al比为3时,侵蚀14个月后在侵蚀产物中依然没有检测到碳硫硅钙石;当S/Al比为6和9时,侵蚀3个月即可检测到碳硫硅钙石,这表明外部硫酸根离子浓度越高越有利于碳硫硅钙石的形成。     

Sulfate attack monitored by microCT and EDXRD: Influence of cement type, water-to-cement ratio, and aggregate

X-ray microtomography (microCT) and spatially resolved energy dispersive X-ray diffraction (EDXRD) were used in combination to non-destructively monitor the physical and chemical manifestations of damage in Portland cement paste samples subjected to severe sodium sulfate attack. Additional measurements of expansion and compressive strength were made on complementary mortar and cement paste specimens. Specifically, the influences of cement type (ASTM Types I and V), water-to-cement ratio (0.485 and 0.435), and the presence of aggregate on the rate and forms of damage were examined. As expected, Type V cement samples exhibited less cracking and expansion than the Type I cement samples. EDXRD indicated an anticorrelation between ettringite and gypsum in the near-surface region for Type V samples, which may be associated with crack formation. An unanticipated result for Type I cement pastes was that cracking was apparent at earlier exposure times and progressed more rapidly for samples with w / c of 0.435, than for those with w / c of 0.485. Possible mechanisms for this behavior are proposed. The presence of aggregate particles resulted in a more rapid rate of cracking, as compared to the corresponding cement paste sample.     

硫酸盐侵蚀下硬化水泥浆体微结构演变及膨胀过程的数值模拟

根据硫酸盐侵蚀机理,利用改进的CEMHYD3D水化模型和随机概率方法,建立了硫酸盐侵蚀下硬化水泥浆体的微结构演变模型。在微观层次上,模拟了浆体孔溶液中硫酸根离子的自由扩散、随机碰撞和转化反应,分析了膨胀性侵蚀产物生长导致的微结构损伤和体积膨胀,计算了侵蚀过程中石膏和钙矾石的生成量及浆体的膨胀应变,并与已有试验结果对比分析验证了模型的合理性。在此基础上,数值模拟了硫酸盐侵蚀下不同水灰比水泥浆体的微结构演变及膨胀过程。结果表明：同一硫酸盐浓度下,硬化水泥浆体中氢氧化钙和含铝物相与孔隙的接触面积越小,浆体的膨胀应变越低;水灰比为0.25、0.30和0.35的硬化水泥浆体的孔隙填充程度分别达到9.09%、9.27%和9.41%时,浆体膨胀应变开始快速增大;硫酸盐侵蚀溶液浓度增大,浆体体积快速膨胀的时间提前。     

X-ray microtomography (microCT) of the progression of sulfate attack of cement paste

High-resolution X-ray computed tomography (i.e., microCT or microtomography) was used to study the sulfate attack of cylinders of Type I cement paste cast with water-cement (w/c) ratios of 0.45, 0.50 and 0.60. Damage levels in samples exposed to a Na₂SO₄ solution with 10,000 ppm sulfate ion concentration were qualitatively rated from 0 (no damage) to 4 (extreme damage) based upon visual examination of the samples' exteriors and microtomography of the samples' interiors. The greater the w/c ratio, the more rapid the onset of sulfate damage. The corners of the cylinders appeared to be particularly susceptible to spalling, and damage may have continued into the cement paste by formation of subsurface cracks.     

Sulfate attack of concrete building foundations induced by sewage waters

A case history of a severe degradation of concrete foundation plinths and piers of an about-35-year-old building located in Northern Italy is described. Significant amounts of gypsum, near ettringite and/or thaumasite were detected by X-ray diffraction analyses performed on ground concrete samples. Large gypsum crystals were mainly located at the interface between the cement paste and aggregates, as observed by scanning electron microscopy coupled to energy-dispersive spectroscopy microanalysis. The degradation effects increased with decreasing the distance of concrete structures from an absorbing well located in the courtyard of the building. The well was recognized as the sulfate source due to the microorganism metabolism of sulfur compounds present in the sewage. Consequences of this attack were a very poor bond strength between cement paste and aggregates and a severe cracking of the concrete cover of the steel reinforcement.     

Modeling of frost salt scaling

This paper discusses the numerical modeling of deterioration in cement-based materials due to frost salt scaling (FSS). Several aspects of FSS are investigated such as carbonation, microstructure, mechanical properties and testing conditions. Mainly blast-furnace slag cement (henceforth slag cement) systems are of interest in this paper since several reports have been indicated that cementitious materials bearing slag-rich cement are critically vulnerable under combined attack of frost and de-icing salts.In the first part, the paper deals with the effect of carbonation on the micromechanical properties and FSS resistance of 1-year-old slag cement and ordinary Portland cement pastes with W/C 0.45. The micromechanical properties were evaluated by the nano-indentation technique and the results are used to evaluate the behavior of these pastes under frost salt attack. FSS damage on the paste samples is modeled according to the glue-spall theory with the aid of Delft Lattice Model. Additionally, the carbonated cement paste microstructures are characterized by ESEM/BSE.In the second part, parameters that are varied in the investigation are the salt concentration in the external water layer and ice-layer thickness on the surface. Again the lattice type model is used to simulate the mechanism in which the material structure is implemented using digital images of the real material. Both experiments and the simulation with the model show that the amount of scaling increases with increasing thickness of the ice layer on the surface. Furthermore it is shown that with the model the well known pessimum effect for salt concentration in the water (which causes maximum damage at 3% salt) can be reproduced.The outcome of the model indicates that glue-spall theory can successfully explain FSS.     

"混凝土硫酸盐结晶破坏"微观分析(Ⅰ)——水泥净浆

在多孔材料内部检测发现硫酸盐晶体是证明盐结晶破坏最直接的证据。运用环境扫描电子显微镜、扫描电子显微镜、能谱仪和X射线衍射等微观分析手段,研究了稳定环境和变化环境中,硫酸钠溶液对半浸泡纯水泥净浆和水泥粉煤灰净浆的破坏作用,结果表明:在破坏的净浆试件中并没有发现硫酸钠晶体,反而发现大量的混凝土硫酸盐化学侵蚀产物(钙矾石和石膏等)晶体,硫酸盐化学侵蚀依然是引起净浆试件破坏的主要原因。     

Sulfate attack on cementitious materials containing limestone filler — A review

This review summarizes the results of sulfate performance in laboratory and field tests where limestone is used as a constituent of cement (PLC) or as a sand replacement where it is particularly beneficial to the properties of self compacting concretes (SCC).Laboratory studies on paste, mortar or concrete specimens exposed to Na₂SO₄ and MgSO₄ solutions in a wide range of concentrations at different temperatures as well as mixtures with different compositions, cement compositions and limestone proportions are considered in a conceptual analysis as for the resistance to external sulfate attack and, especially, thaumasite sulfate attack.A detailed analysis of environmental aggressiveness (concentration, temperature and pH), mixture composition and cement composition used in each study are presented for PLC and SCC. Reported field studies are also shown, only a few cases have used limestone filler in their composition. A conceptual graphical analysis is then proposed to relate the degree of surface deterioration and mineralogical composition of attacked surface to the main variables of external sulfate attack: water/cementitious material ratio, limestone content and C₃A content of the cement. Observation of graphical analysis clearly shows that deterioration by ESA is mainly governed by effective w/c ratio and C₃A content of the cement. Surface damage is controlled when low effective w/c ratio and low C₃A are used. In MgSO₄ solution, low temperatures increase the degree of deterioration. Thaumasite is the last attack stage in the different sulfate environments.     

Physical and microstructural aspects of sulfate attack on ordinary and limestone blended Portland cements

The consequences of external sulfate attack were investigated by traditional test methods, i.e. length and mass change, as well as by a newly developed, surface sensitive ultrasonic method, using Leaky Rayleigh waves (1 MHz). The macroscopic changes are discussed and compared with thermodynamic calculations and microstructural findings (SEM/EDS). The results show that the main impact of limestone additions on resistance to sulfate degradation are physical — i.e. addition of a few percent in Portland cement reduces the porosity and increases the resistance of Portland cement systems to sulfate; but higher addition of 25% increase porosity and lower resistance to sulfate. The kinetics of degradation were dramatically affected by the solution concentration (4 or 44 g Na₂SO₄/l) and the higher concentration also resulted in the formation of gypsum, which did not occur at the low concentration. However the pattern of cracking was similar in both cases and it appears that gypsum precipitates opportunistically in pre-formed cracks so it is not considered as making a significant contribution to the degradation. At 8 °C limited formation of thaumasite occurred in the surface region of the samples made from cement with limestone additions. This thaumasite formation led to loss of cohesion of the paste and loss of material from the surface of the samples. However thaumasite formation was always preceded by expansion and cracking of the samples due to ettringite formation and given the very slow kinetics of thaumasite formation it was probably facilitated by the opening up of the structure due to ettringite induced cracking.The expansion of the samples showed a steady stage, followed by a rapidly accelerating stage, with destruction of the samples. The onset of the rapidly accelerating stage occurred when the thickness of the cracked surface layer reached about 1–1.5 mm–10–15% of the total specimen thickness (10 mm).     

Effect of water and other dielectrics on subcritical crack growth in portland cement paste

In studying the effect of water and a series of aliphatic alcohols on subcritical crack growth in cement paste, log crack velocity-stress intensity factor curves were obtained by means of a double-torsion technique. The relative position of the curves was found to be dependent on the dielectric constant of the test media. Flexural strength of cement paste saturated in organic fluids is also dependent on the dielectric constant. Stress corrosion processes involving chemical attack of Si---O bonds in cement paste appear to be operative.     

Effect of magnesium and sulfate ions on durability of silica fume blended mixes exposed to the seawater tidal zone

The effect of silica fume on deterioration resistance to sulfate attack in seawater within tidal zone and simulated wetting-drying condition has been studied in Portland cement concretes and pastes containing silica fume (SF) with/without ground granulated blast furnace slag (GGBS). Changes in the compressive strength and capillary water absorption of specimens as a function of SF content have been investigated combined with phases determination by means of scanning electron microscopy and X-ray energy dispersion analysis. The strength change factors (SCFs) of specimens with SF (the more SF content, the higher strength loss) were greater than that of the mixes without SF or cured under tap water. Mg²⁺ ion originated attack found to be the dominating deterioration mechanism as confirmed by X-ray and chemical analyses.Further, the incorporation of GGBS with SF mixes in different exposure conditions led to the worst performance in all of the test environments. Lower cement content and hydration rate accompanied with particular chemical composition of GGBS made concrete and paste specimens to be more susceptible to deleterious seawater environment.     

水泥硬化体在(NH4)2SO4溶液中的溶解动力学

水泥基材料在服役过程中会受到来自外界的腐蚀,其中(NH4)2 SO4腐蚀是一种较为严重的硫酸盐腐蚀.通过测定水溶液中的pH值的变化分析水泥硬化体在(NH4)2 SO4溶液中的溶解过程.结果表明,水泥硬化体粉末在(NH4)2 SO4溶液中的pH值的变化趋势符合Avrami溶解模型,即pH值随时间的增加而增加,从而得出水泥硬化体粉末的溶解速率与(NH4)2 SO4溶液的浓度之间的关系.     

Decalcification shrinkage of cement paste

Decalcification of cement paste in concrete is associated with several modes of chemical degradation including leaching, carbonation and sulfate attack. The primary aim of the current study was to investigate the effects of decalcification under saturated conditions on the dimensional stability of cement paste. Thin (0.8 mm) specimens of tricalcium silicate (C₃S) paste, white portland cement (WPC) paste, and WPC paste blended with 30% silica fume (WPC/30% SF) were decalcified by leaching in concentrated solutions of ammonium nitrate, a method that efficiently removes calcium from the solid while largely preserving silicate and other ions. All pastes were found to shrink significantly and irreversibly as a result of decalcification, particularly when the Ca/Si ratio of the C-S-H gel was reduced below ∼ 1.2. Since this composition coincides with the onset of structural changes in C-S-H such as an increase in silicate polymerization and a local densification into sheet-like morphologies, it is proposed that the observed shrinkage, here called decalcification shrinkage, is due initially to these structural changes in C-S-H at Ca/Si ∼ 1.2 and eventually to the decomposition of C-S-H into silica gel. In agreement with this reasoning, the blended cement paste exhibited greater decalcification shrinkage than the pure cement pastes due to its lower initial Ca/Si ratio for C-S-H gel. The similarities in the mechanisms of decalcification shrinkage and carbonation shrinkage are also discussed.     

Combined experimental and numerical study of fracture behaviour of cement paste at the microlevel

The aim of this paper is to investigate the fracture performance of cement paste at the microlevel. An experimental procedure for micromechanical testing of consecutive layers in cement paste is developed. Every layer is tested by nanoindentation and subsequently imaged in ESEM to link the phase distribution with the measured micromechanical properties. The obtained micromechanical properties were used directly as input for numerical simulation of fracture behaviour of cement paste. A comparison is made between simulated 2D fracture response from individual slices and simulated fracture response of 3D structure, which is reconstructed from the 2D slices. In addition, the influence of heterogeneity on fracture response was studied. 3D microstructure enables more stable crack propagation, branching and crack trapping, which is attributed to more restrain and phase connectivity. Neglecting heterogeneity and original phase distribution in numerical simulations of cement paste leads to overestimation of both tensile strength and fracture energy.     

Slow Crack Growth in Cement Paste

The dependence of crack velocity on stress intensity was measured in hardened cement paste. Fracture toughness, modulus of rupture, and dynamic elastic modulus were also measured to permit calculation of the time-to-failure (life expectancy) of stressed cement paste. The double torsion method was used for slow crack growth studies and fracture toughness measurements. Fracture toughness was also measured by a notched-beam method; the agreement between the two methods was excellent. The crack-velocity-stress-intensity curve for cement paste resembles that of glass in water. The slope of the log V -log K _I curve is ∼35.     

5℃环境下水泥砂浆抗硫酸盐侵蚀性能试验研究

研究了不同水泥品种、矿物掺合料对水泥基材料在5℃下抗硫酸盐侵蚀的性能的影响,分别采用普通硅酸盐水泥、中抗硫水泥以及加入矿粉与硅灰的水泥砂浆试件,测试各试样在(5±1)℃的3%Na2 SO4溶液浸泡后的强度变化情况,综合考虑砂浆强度与抗蚀系数对砂浆抗硫酸盐侵蚀性能进行评价,并运用SEM、EDS、XRD分析方法对腐蚀机理进行了分析.结果表明:在5℃环境下,砂浆试样的强度普遍低于常温环境下,砂浆抗硫酸盐侵蚀能力15%矿粉+3%硅灰>中抗硫水泥>15%矿粉+1%硅灰>普通硅酸盐水泥;加入矿物掺合料明显改善了水泥砂浆抗硫酸盐侵蚀性能,并且硅灰的含量越高效果越明显;低温下腐蚀产物不仅有石膏,还有碳硫硅钙石的生成.     

熟料硫碱比对水泥浆流变性的影响

模拟预分解窑煅烧制度，在电炉中烧制不同硫碱比的高碱熟料，分别对掺与不掺石膏的浆体，用旋转黏度计、微型坍落度仪以及维卡仪（测净浆针入度）等方法测其流变性能；并结合岩相分析、扫描电镜（SEM）对熟料硫碱比影响浆体流变性的机理进行分析探讨。结果表明：对于高碱熟料，硫碱比过低会导致水泥浆体的流变性变差，其主要原因是煅烧熟料时液相黏度高将导致矿物晶体发育不完善、晶粒细小，水化加快。适当提高熟料硫碱比，有利于改善水泥浆体的流变性。     

Mineralogical and chemical assessment of concrete damaged by the oxidation of sulfide-bearing aggregates: Importance of thaumasite formation on reaction mechanisms

Damages in concrete containing sulfide-bearing aggregates were recently observed in the Trois-Rivières area (Quebec, Canada), characterized by rapid deterioration within 3 to 5 years after construction. A petrographic examination of concrete core samples was carried out using a combination of tools including: stereomicroscopic evaluation, polarized light microscopy, scanning electron microscopy, X-ray diffraction and electron microprobe analysis.The aggregate used to produce concrete was an intrusive igneous rock with different metamorphism degrees and various proportions of sulfide minerals. In the rock, sulfide minerals were often surrounded by a thin layer of carbonate minerals (siderite). Secondary reaction products observed in the damaged concrete include “rust” mineral forms (e.g. ferric oxyhydroxides such as goethite, limonite (FeO (OH) nH₂O) and ferrihydrite), gypsum, ettringite and thaumasite. In the presence of water and oxygen, pyrrhotite oxidizes to form iron oxyhydroxides and sulphuric acid. The acid then reacts with the phases of the cement paste/aggregate and provokes the formation of sulfate minerals. Understanding both mechanisms, oxidation and internal sulfate attack, is important to be able to duplicate the damaging reaction in laboratory conditions, thus allowing the development of a performance test for evaluating the potential for deleterious expansion in concrete associated with sulfide-bearing aggregates.