Hydration Characteristics of Portland Cement after Heat Curing: II, Evolution of Crystalline Aluminate-Bearing Hydrates

The development of crystalline aluminate-bearing hydrates in portland cement mortars during water storage at room temperature for periods of up to 1 year after an initial heat cure for 12 h has been observed by quantitative X-ray diffraction analysis and backscattered electron imaging. Ettringite was present in the mortars immediately after a short-term cure at 20° and 60°C, calcium carboaluminate (C₄ĀH₁₁) at 60°C, monosulfate at 85°C, and hydrogarnet at 85°C and above. Ettringite started to form after an induction period ranging from several days to several months after the initial heat cure at 85/100°C, and developed substantially during the period of expansion of the mortar associated with delayed ettringite formation (DEF). Ettringite growth was also observed in the nonexpansive cement mortars. Development of the ettringite bands occurred exclusively in the expansive mortars. Although monosulfate observed in the mortars that had been heat cured at 85°C sometimes increased in amount on initial storage at room temperature, it appeared to vary little in amount for up to 1 year. The amount of hydrogarnet in the heat-cured cement product did not change significantly during storage at room temperature for more than 1 year. DEF expansion of the heat-cured mortars was attributed to ettringite band formation, which started to form at the surface of the cement product and gradually developed inwards.     

Studies on delayed ettringite formation in early-age, heat-cured mortars: I. Expansion measurements, changes in dynamic modulus of elasticity, and weight gains

Mortars were prepared from laboratory cements blended from a set of six representative ground clinkers and Terra Alba gypsum. The addition of gypsum was such that cements containing 1% SO₃ less than the optimum SO₃ content, the optimum SO₃ content, and 1% greater than the optimum SO₃ content were produced. Mortar bars and mortar cubes containing each of these cements were exposed to continuous room temperature (23 °C) curing, or to early-age curing cycles involving maximum temperatures of 55 and 85 °C, followed by long-term exposure at 100% RH over water, but not immersed in water. Measurements of expansion, dynamic elastic modulus, and weight gain were recorded at intervals of up to 900 days. Severe cracking and prominent delayed ettringite formation (DEF)-induced expansions were observed in 85 °C cured mortar bars derived from four of the six “oversulfated” cements. Much smaller expansions were observed in mortar bars from two cements with optimum SO₃ content cements also cured at 85 °C. No expansion or other visible indication of distress was observed for any of the 55 °C or continuously room-temperature-cured mortars. The dynamic elastic modulus increased progressively on prolonged exposure for the unaffected mortar bars, but it decreased precipitously after the onset of expansion in affected mortar bars. Significant weight increases also accompanied the processes of expansion. Mortars that showed severe cracking and deterioration when exposed as mortar bars suffered almost no visible damage when exposed as cubes.     

Delayed ettringite formation symptoms on mortars induced by high temperature due to cement heat of hydration or late thermal cycle

Cases of delayed ettringite formation (DEF) have mainly been detected on mortars or precast concretes steam-cured according to a predefined temperature cycle during hydration. The present study shows that other situations in which the material is submitted to a temperature cycle can induce DEF expansions. Mortar bars were made with three different cements (types 10, 20M, and 30). As a first heat treatment, the mortar bars were steam-cured to reproduce the temperature cycle they would undergo if they were at the center of a large mortar member. The dimensional variations of these specimens were studied for 1 year. After 1 year, half of the specimens were steam-cured for 1 month at 85 °C. The expansions were followed for two more years. The early-age steam-cure-induced expansions for mortar types 10 and 30. Late steam-curing induced expansions for the three cements tested. In one case (cement type 20M), the early-age steam cure has suppressed or delayed the expansion induced by the late steam cure. A scanning electron microscopy (SEM) study showed that typical DEF symptoms are associated with the expansions.     

Effects of gypsum formation on the performance of cement mortars during external sulfate attack

Sodium sulfate attack was studied on C₃S mortars, along with ASTM Type I Portland cement (PC) mortars, in an attempt to independently evaluate the effect of gypsum formation on the performance. The quantity of gypsum and ettringite, as measured by differential scanning calorimetry (DSC), increased with the time of immersion in the sulfate solution. An increase in length of the mortar specimens was also registered along with the increase in the quantity of gypsum. This result suggests that the formation of gypsum could be expansive. Indeed, considerable expansion, although delayed compared to PC mortars, was observed in the C₃S mortars. Thus, it can be concluded that the expansion of the PC mortars occurred due to the combined effect of gypsum and ettringite formation, while the expansion of C₃S mortars occurred as a result of gypsum formation.Thaumasite formation as small inclusions was also detected in both the C₃S and the PC mortars, especially in regions of high gypsum deposition. The formation of thaumasite, despite the absence of carbonate bearing minerals and low temperatures, could be because of the carbonation of the surface zones of the mortars. However, it would be speculative to attribute any expansion to the formation of thaumasite, since it was detected only in minute amounts in the microstructural investigation.     

Dual effectiveness of lithium salt in controlling both delayed ettringite formation and ASR in concretes

The influence of lithium nitrate on expansions due to delayed ettringite formation (DEF) and alkali-silica reaction (ASR) has been investigated. Effects of the lithium salt were examined in heat-cured mortars and concretes containing one or both damage mechanisms. The mortars and concretes made using reactive and/or non-reactive aggregates were subjected to heat treatment consisting of a hydration delay period of 4 h at 23 °C followed by steam-curing at 95 °C and then stored in limewater. Results showed that the lithium salt admixture was able to reduce the occurrence of deleterious expansion due to delayed ettringite formation in addition to controlling alkali-silica reaction in cementitious systems containing one or both mechanisms. In concretes made using non-reactive limestone aggregates, incorporation of lithium nitrate in a proportion of 0.74 M ratio of Li to (Na + K) was found to control delayed ettringite formation during the one-year period of this study.By analyzing the leaching properties of lithium and other alkalis from mortars during storage, it was found that a substantial amount of lithium was retained in the cementitious system in a slightly soluble form, and is expected to be responsible for reducing DEF.     

Implications of pre-formed microcracking in relation to the theories of DEF mechanism

Microcracks were induced in cementitious systems by freeze-thaw action and by alkali-silica reaction. These mechanisms often co-exist with delayed ettringite formation in concretes. Mortars and concretes were subjected to a heat treatment cycle consisting of a pre-set period of 4 h at 23 °C followed by accelerated curing at 95 °C. To isolate the mechanical effects of induced microcracking, heat-cured specimens were subjected to varied prescribed damage induced by freeze-thaw or alkali-silica reaction prior to the onset of delayed ettringite formation. It was found that inducement of pre-formed microcracks led to an earlier onset of expansion due to delayed ettringite formation. Initially, microcracks enhanced ultimate expansion until a certain relatively high extent of microcracking was reached. Thereafter, ultimate expansion decreased with any further increase in microcracking. This report gives support to the paste expansion theory.     

The behavior of silicocarbonatite aggregates from the Montreal area

In a previous paper, it was concluded that silicocarbonatite aggregates from the Francon quarry, Montreal contributed to durability problems in Portland cement concrete. Results show that, at 2 days after casting, concrete made with silicocarbonatite aggregates contained over 1.5% more Na₂O than similar bars made with Exshaw limestone aggregates. A reaction involving the rare mineral dawsonite in the silicocarbonatite is thought responsible for the higher Na₂O content. In turn, this caused increased expansion of concrete bars made with alkali expansive aggregates. Also, concrete made with alkali-carbonate reactive Pittsburg aggregate showed more expansion when cured at 80 °C than bars cured at 23 °C. Concrete bars made with Exshaw limestone aggregates cured for 4 h at 85 °C showed late-stage expansion, which is attributed to delayed ettringite formation (DEF). However, no expansion was shown by heat-cured concrete prisms or mortar bars made with silicocarbonatite aggregates. Release of alkalis, aluminates and carbonates by the dawsonite reaction may have inhibited DEF. Concrete bars made with nonreactive Nelson dolostone and 10% silicocarbonatite cured at 80 °C for 4 h showed up to 0.15% expansion after several years at 23 °C and 100% relative humidity (R.H.), indicating that a deleterious reaction did occur.     

Influence of the Ca/Si ratio of the C–S–H phase on the interaction with sulfate ions and its impact on the ettringite crystallization pressure

The effect of the Ca/Si ratio of the calcium–silicate–hydrate (C–S–H) phase on the interaction with sulfate ions is investigated for C–S–H phases (Ca/Si = 0.83, 1.25, 1.50) and mortar samples of blended Portland cements. It is shown that leaching of calcium from C–S–H and portlandite affects the composition of the pore solution and contributes to the developing crystallization pressure of ettringite. Sulfate profiles show that sulfate binding before cracking is similar for different Ca/Si ratios whereas the highest expansion rates are observed for the mortars with the highest Ca/Si ratios. After leaching in sulfate solutions, the C–S–H samples have been characterized by ²⁹Si MAS NMR, thermogravimetric analysis, and elemental solution analysis. Generally, the exposure to sulfate solutions results in decalcification of the C–S–H, which increases with decreasing Ca/Si ratio. The data are in good agreement with thermodynamic modeling, indicating that equilibrium is almost achieved in the leached systems. Finally, the expansion of mortar samples exposed to sulfate solutions was much less at lower Ca/Si ratios of the cement blends. This reduced expansion can be related to the decrease of the supersaturation of the pore solution with respect to ettringite at lower Ca/Si ratios of the C–S–H and in the absence of portlandite.     

Studies on delayed ettringite formation in heat-cured mortars: II. Characteristics of cement that may be susceptible to DEF

Expansions of mortar bars, stored over (but not in) water after simulated steam curing to 85 °C, were related to certain cement compositional parameters. The relationship is expressed in the form of a “delayed ettringite formation (DEF) index.” The DEF index is computed as the joint product of the SO₃/Al₂O₃ molar ratio of the cement, the sum of its SO₃ and Bogue C₃A percentages divided by 10 and the square root of the alkali content expressed as equivalent % Na₂O. The mortars studied were made with 18 different cements, prepared from a set of six representative clinkers by incorporating Terra Alba gypsum to total SO₃ contents that were 1% below optimum, at optimum and 1% above optimum (as defined in ASTM C 563). Measurements of expansion were recorded at intervals for up to 1400 days. Severe cracking and prominent DEF-induced expansions were observed in mortar bars derived from four of the six ‘oversulfated’ cements and lesser expansions from three of the six cements prepared at optimum SO₃ contents. No expansion was found for cements of DEF index below a threshold value; above this value expansions were approximately proportional to the difference between DEF index and its threshold value. The relationship confirms the significance of all three compositional parameters making up the index, e.g., the SO₃/Al₂O₃ molar ratio, the joint contents of SO₃ and C₃A, and the alkali content, in influencing the extent of DEF-induced expansion. In these measurements, the apparent pessimum effect for SO₃ content previously reported by others was not found, although SO₃ contents examined spanned the supposed pessimum value of 4%. Rather, expansion increased with increasing SO₃ content for mortars made with all clinkers exhibiting expansion.     

大体积补偿收缩混凝土中延迟钙矾石生成产生危害的条件

研究了大体积补偿收缩混凝土中延迟钙矾石生成可能对混凝土产生危害的条件,用步进扫描半定量XRD法确定水化产物的物相组成,同时测定胶砂试件的限制膨胀率,遭受过早期高温的补偿收缩混凝土对于长期使用过程中环境的湿度变化情况比较敏感,只有在适当的湿度条件下养护,才能获得适量的膨胀,以补救早期膨胀损失所造成的不利影响,养护环境过于干燥或过于潮湿,有可能导致遭受过早期高温的补偿收缩混凝土结构收缩开裂或膨胀破坏。     

The relevance of laboratory studies on delayed ettringite formation to DEF in field concretes

Many laboratory studies of delayed ettringite formation (DEF) have been conducted on thin mortar bar specimens, heat treated, and then immersed in water. Under these conditions, rapid diffusion of alkali hydroxide into the surrounding water occurs and necessarily reduces the alkali hydroxide concentration of the mortar pore solution. Results reported recently by Famy indicate that the DEF process is triggered as a consequence of such leaching. When it is prevented by immersion into alkali hydroxide solution instead of water DEF expansion is delayed or prevented entirely. Results reported by Zhang indicate that 51-mm mortar cubes behave differently than more leaching-susceptible mortar bars when exposed to the same wet environment. Mortars that show severe DEF as mortar bars remain almost free of DEF symptoms if they are stored as cubes, even after 900 days. Attention is called to the fact that DEF in concrete is found commonly in thick concrete members where the possibility of leaching is remote. For such concrete, the reduction in internal alkali hydroxide concentration that occurs with ASR can substitute for the effect of leaching. It is postulated that without effective reduction of alkali hydroxide concentration by one or the other process, DEF remains latent.     

A concrete performance test for delayed ettringite formation: Part I optimisation

Delayed ettringite formation (DEF) is a rare internal swelling reaction of concrete that, in a wet environment, may considerably reduce the durability of a structure or a member that has been prefabricated in a factory. As a result of a large number of studies, the main causes of this problem have been established and this paper proposes a test method to predict the susceptibility of a concrete to DEF and provide long-term protection against this hazard. After an examination of the feasibility of a test based on wetting and drying cycles, the results of an optimisation study based on a factorial experimental plan are presented. The test is in four stages: concrete manufacture, a heat treatment that simulates steam curing in a factory or the heating that occurs in mass cast in-situ concrete, wetting and drying cycles and the monitoring of the longitudinal expansion of specimens that are immersed in water. Optimisation essentially concerns the last two stages and resulted in the decision to apply two drying cycles (38 °C at a relative humidity of approximately 30%) and wetting cycles (in water at 20 °C) followed by permanent immersion in water (at 20 °C). This test is applied to concrete that has undergone early age heating. It provides a means of evaluating the potential reactivity of “mix design–heating cycle” pairs as realistically as possible within a reasonable period of time.     

Microstructural study of sulfate attack on ordinary and limestone Portland cements at ambient temperature

This paper presents an investigation on the mechanism of sulfate attack on Portland cements (PCs) containing limestone filler. It is based on the analysis of microstructure and composition of mortar specimens (ASTM C 1012) stored for 2 years in sodium sulfate solution (0.352 M). Microstructure was studied using quantitative X-ray diffraction (XRD) on samples taken from the surface to the core of the specimens. The profile of compounds formed by sulfate attack was determined millimeter by millimeter at 1 and 2 years. Results show that sulfate attack in mortars containing limestone filler is characterized by an inward movement of the reaction front leading first to the formation of ettringite, later to gypsum deposition, and finally to thaumasite formation when the decalcification of mortar leads to the breakdown of C-S-H.     

Role of delayed release of sulphates from clinker in DEF

Two clinkers rich in sulphate burned in the pilot plant rotary kiln and cements prepared from them were investigated. Clinker richer in sulphate (SO₃=3.6%) contained independent anhydrite grains as well as inclusions of anhydrite in belite. The mortar from it expanded after heat treatment at 90 °C and the addition of Na₂SO₄ or NaOH accelerated and increased this expansion. The expansion occurred irrespective of the fact that the clinker contained only 3% of C₃A, although the C₄AF content was 13%. The second clinker with 2.6% SO₃ contained mainly calcium langbeinite and expanded only when 2% of Na₂SO₄ was added. The SEM examination of the mortars revealed the presence of numerous bands of massive ettringite around sand grains. Agglomerates of cracked ettringite in cement gel were also present. In addition, microcracks were seen inside the darker C-S-H gel. The conclusion is that anhydrite forming inclusions in belite gives an expanding mortar after heat treatment at 90 °C independently of the tricalcium aluminate content. Such clinkers are not typical of industrial conditions. The expansion is caused by the bands of massive ettringite as well as its agglomerates present in the cement gel and nanometric ettringite in the C-S-H phase.     

Mechanism of sulfate attack: A fresh look: Part 1: Summary of experimental results

This paper reports the results of an investigation on the effects of sodium and magnesium sulfate solutions on expansion and microstructure of different types of Portland cement mortars. The effects of using various sulfate concentrations and of using different temperatures are also reported. The results suggest that the expansion of mortars in sodium sulfate solution follows a two-stage process. In the initial stage, Stage 1, there is little expansion. This is followed by a sudden and rapid increase in the expansion in Stage 2. Microstructural studies suggest that the onset of expansion in Stage 2 corresponds to the appearance of cracks in the chemically unaltered interior of the mortar. Beyond this point, the expansion proceeds at an almost constant rate until the complete deterioration of the mortar specimen. In the case of magnesium sulfate attack, expansion occurs at a continually increasing rate. Microstructural studies suggest that a layer of brucite (magnesium hydroxide) on the surface forms almost immediately after the introduction of the specimens into the solution. The attack is then governed by the steady diffusion of sulfate ions across the brucite surface barrier. The ultimate failure of the specimen occurs as a result of the decalcification of the calcium silicate hydrate (C-S-H), and its conversion to magnesium silicate hydrate (M-S-H), after prolonged exposure to the solution. The effects of using various admixtures, and of changing the experimental variables such as the temperature and concentration of the solution, are also summarized in this paper. Models for the mechanism of the attack resulting from sodium and magnesium sulfate solutions will be presented in Part 2.     

Influence of bicarbonate ions on the deterioration of mortar bars in sulfate solutions

This work investigates the influence of bicarbonate ions on the deterioration of cementitious material exposed to sulfate ions. Mortars based on a CEM I and on a CEM III/B cement were investigated. Experimental investigations were compared to thermodynamic modeling and phase characterization to understand the differences in deterioration.The presence of bicarbonate ions significantly reduced the expansion of the CEM I mortars. Thermodynamic modeling showed that at high concentrations of bicarbonate ettringite and gypsum become unstable. Microstructural characterization combined with information from thermodynamic modeling suggests that conditions of high supersaturation with respect to ettringite are unlikely in the samples exposed in solutions containing bicarbonate. Consequently, expansive forces are not generated by the crystallization pressure of ettringite.There was little expansion of the CEM III/B sample even in the sodium sulfate solution. In the bicarbonate solution this mortar showed a highly leached zone at the surface in which calcite was observed.     

硫酸盐浸泡环境下CaO-Na2CO3激发矿渣砂浆性能退化及机理研究

以Na₂SO₄和MgSO₄溶液为侵蚀介质,研究了在浸泡环境下CaO-Na₂CO₃激发矿渣(CNS)砂浆和普通硅酸盐水泥(OPC)砂浆经硫酸盐侵蚀前后的抗折强度、抗压强度及不同深度处的SO^2-₄浓度,结合X射线衍射(XRD)、扫描电子显微镜(SEM)、压汞法(MIP)等测试方法分析了CNS砂浆和OPC砂浆的侵蚀产物及孔结构,对比讨论了Na₂SO₄和MgSO₄对CNS砂浆和OPC砂浆的侵蚀机理。结果表明:CNS砂浆的水化产物主要是低Ca/Si比的水化硅铝酸钙(C-A-S-H),不存在氢氧化钙,碳酸钙的填充作用使其孔结构优于OPC砂浆,并且在相同侵蚀环境下,CNS砂浆的抗硫酸盐侵蚀能力大于OPC砂浆;MgSO₄侵蚀环境下CNS砂浆的侵蚀产物主要是水镁石(腐蚀后期会带动试件表面的砂浆一起剥落)和无黏聚力的水化硅铝酸镁(M-A-S-H);与Na₂SO₄相比,MgSO₄对CNS砂浆的腐蚀性更强。     

Expansion and shrinkage of oversulphated portland cements

Expansion and shrinkage tests on mortars made from a range of cements with SO₃ contents in excess of the limits specified in BS 12 are described. The mortar specimens were cured under water from 24 hours to 7 days and then dried at 65% RH and 20°C. It is shown that there is a small range of SO₃ contents within which sharp increases in 7 day expansion occur ranging from 80 to 170 micro-strain/SO₃% and that within this small range of SO₃ contents the subsequent drying shrinkage is not adversely affected.     

石灰石粉对水泥基材料抗硫酸盐侵蚀性的影响及其机理

用天然石灰石粉等质量取代水泥20%和30%,将制备的水泥净浆和砂浆试件常温浸泡在0.35 mol/L Na2SO4溶液中,测量试件的线长度和抗折强度随浸泡时间的变化.结果表明:石灰石粉对水泥基材料的抗硫酸盐性有严重的影响,它们使水泥基材料在硫酸盐环境中的强度急剧下降并导致水泥基材料产生较大体积膨胀,引起开裂.掺石灰石粉的水泥基材料主要因形成大量较大尺寸的石膏晶体而膨胀开裂.石膏的形成导致硫酸盐侵蚀水泥基材料产生膨胀开裂.因此,在硫酸盐侵蚀环境下,不宜采用含石灰石粉的复合水泥或将石灰石粉作为矿物掺合料制备的混凝土.     

Influence of shrinkage and water transport mechanisms on microstructure and crack formation of tile adhesive mortars

Shrinkage and expansion of cementitious materials like tile adhesive mortars depend on the presence of water as well as on the drying and rewetting history. Particularly for large-sized tiles such volumetric changes induce stress concentrations, which may result in cracking. This study focuses on the interplay between water infiltration and cracking, starting at the early curing of the mortar during the first days after application until water transport in the hardened system. Based on laboratory experiments, different events concerning the effect of water transport were induced by variation of the experimental setups in order to provoke cracking. Cracks at the tile–mortar interfaces suggest these domains to reflect zones of mechanical weakness. Along these cracks, water can enter the system inducing precipitation of secondary minerals in cracks and pores already after one wetting cycle. These processes reveal increasing importance during repeated cycles of drying and wetting, i.e. under outdoor conditions and may lead to failures.