Thermal stability of ettringite in alkaline solutions at 80 °C

The thermal stability of synthetic ettringite was examined in NaOH solutions up to 1 M after 12 h of heat treatment at 80 °C, with or without the coexistence of C₃S in the system. Ettringite was found to convert to the U phase, a sodium-substituted AFm phase, over the heat treatment in the absence of C₃S. The presence of C₃S, leading to C-S-H formation, prevents the U phase formation and results in the conversion of ettringite to monosulfate. Sulfate ions generated from ettringite decomposition mostly remain in the solution, but some is incorporated into C-S-H. During subsequent storage at room temperature, the majority of monosulfate slowly converts back to secondary ettringite under moist conditions, using the supply of sulfate ions from the solution and C-S-H. The observations support the current mechanism of delayed ettringite formation (DEF).     

Microstructural and microanalytical studies of sulfate attack. I. Ordinary portland cement paste

Portland cement pastes that had been stored for 6 months in solutions of sodium or magnesium sulfate were examined by scanning electron microscopy using backscattered electron imaging and X-ray microanalysis. For a paste stored in Na₂SO₄ solution, successive changes were observed on passing from the unaltered material in the interior towards the surface. These were (1), replacement of monosulfate by ettringite, which was closely mixed with the C-S-H gel, (2), disappearance of calcium hydroxide, partial decalcification of C-S-H and precipitation of gypsum and (3), further decalcification and leaching. Much of the gypsum occurred in veins sub-parallel to the surface, with which were associated cracks. A paste stored in MgSO₄ solution showed broadly similar effects, as well as a largely continuous surface layer of gypsum and brucite, except at the cube edges, where a gel high in magnesia and silica was formed. This was probably cryptocrystalline serpentine. Neither specimen contained massive deposits of ettringite.     

Effect of ettringite on thaumasite formation

Deterioration of cementitious building materials is often caused by sulphate attack where ettringite and gypsum play the most destructive role at moderate ambient temperatures. In contrast, thaumasite [Ca₃Si(OH)₆·12H₂O](SO₄)(CO₃) is mostly observed at comparatively low temperatures of less then 15 °C. This mineral forms from calcium, sulphate, carbonate and silicon. The latter originates from the decomposition of C-S-H which results in deterioration of the hardened cement paste structure. To investigate the effect of ettringite on thaumasite formation, pastes were mixed using synthetic clinker phases, fly ash and nanosilica. Aqueous suspensions were prepared with the ground-hydrated pastes mixed with calcite and either gypsum or sodium sulphate. Following different storage periods, the solid phase was separated by filtration, dried and analysed by XRD using the Rietveld method as well as ESEM and TEM. The liquid phase was analysed by ICP-OES. The results indicate that thaumasite formation occurs through the heterogeneous nucleation of thaumasite on the surface of ettringite, due to the structural similarities of these minerals. This reaction is followed by further epitaxial growth of thaumasite from its components present in solution.     

Quantitative X-ray diffraction analysis of ettringite,thaumasite and gypsum in concretes and mortars

A method for quantitative Xray diffraction analysis (QXDA) of three sulphate minerals frequently associated with building materials has been devised. Sulphate minerals which form within concretes, mortars and other cementitious-based materials include ettringite, (3CaO.Al₂O₃.3CaSO₄.31H₂O), thaumasite (CaSiO₃.CaCO₃.CaSO₄.15H₂O) and gypsum (CaSO₄.2H₂O). Calibration standards were prepared using pure samples of these minerals and incorporating boehmite as an internal standard. The equations obtained from the standard calibration curves were used to calculate the percentage of ettringite, thaumasite and gypsum in a) samples which contained known percentages of these minerals mixed together and b) laboratory prepared concrete cubes undergoing sulphate attack. The cubes contained 0%, 20% or 40% pulverized fuel ash (pfa) by weigth of cementitious material and had been stored in various sulphate solutions including sea water for one year.Quantitative Xray diffraction analysis of the standard mixtures successfully detected the expected ettringite, thaumasite and gypsum concentrations. The concrete cube results showed that the sulphate mineral concentration within the surface of the cubes decreased when larger amounts of pfa were used in the concrete mixes. This effect was less noticeable in the sea water cubes.     

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.     

Impact of zeta potential of early cement hydration phases on superplasticizer adsorption

The zeta potential of early hydration products of cement was found to be a key factor for superplasticizer adsorption. A highly positive zeta potential results in a strong superplasticizer adsorption whereas a negative zeta potential does not allow adsorption. Synthetic ettringite precipitated from solution shows a highly positive zeta potential, hence it adsorbs great amounts of negatively charged superplasticizer. Monosulfate (AF_m) has a less positive zeta potential. Therefore, it adsorbs smaller amounts of superplasticizers. For syngenite, portlandite and gypsum, the zeta potential is around zero or negative. These phases do not adsorb superplasticizers. Consequently, a hydrating cement grain is best represented by a mosaic structure, with superplasticizer molecules mainly adsorbed on ettringite and some on monosulfate and C-S-H nucleated at surface.     

激发条件对脱硫石膏-矿渣体系水硬强度的影响

利用正交实验研究了硅酸盐水泥和其他两种矿物组分复合激发对脱硫石膏-矿渣体系强度的影响,用SEM、XRD分析了水化样品的微观结构.研究结果表明:硅酸盐水泥等多组分复合激发下,脱硫石膏-矿渣体系在水中标准条件养护,3 d抗压强度达17 MPa以上,28 d抗压强度达58 MPa以上.复合激发剂3种组分的优化组合为6:6:5,复合激发剂的用量为脱硫石膏-矿渣体系质量的17%左右.脱硫石膏-矿渣体系在复合激发条件下的水化产物主要是钙矾石和C-S-H.大量钙矾石、石膏晶体相互交叉连生,未水化石膏、矿渣颗粒所填充其间,在C-S-H凝胶的胶结下,形成了较为致密的晶胶搭配构成的微观结构.     

Effect of solution concentrations and replacement levels of metakaolin on the resistance of mortars exposed to magnesium sulfate solutions

This paper reports an experimental study on the magnesium sulfate resistance of mortar and paste specimens incorporating 0%, 5%, 10% and 15% metakaolin (MK). The resistance of mortar specimens was evaluated using visual examination, reduction in compressive strength and expansion measurements.Results confirmed that mortar specimens with a high replacement level of metakaolin showed lower resistance to a higher sulfate concentration of magnesium solution. However, in a lower concentration, there were no visibly remarkable differences in the deterioration of mortar specimens, even up to 360 days of exposure, regardless of replacement levels of metakaolin.The negative effect of metakaolin on the magnesium sulfate resistance is partially attributed to the formation of gypsum but not ettringite and thaumasite. In addition, the reduction of calcium hydroxide and the increase of secondary C-S-H in the cement matrix due to pozzolanic reaction of metakaolin provided an opportunity to lead to the conversion of primary and secondary C-S-H gel into the M-S-H gel.It is concluded that it is necessary to pay special attention when using metakaolin in concrete exposed to highly concentrated magnesium sulfate solution.     

Effect of temperature on the pore solution, microstructure and hydration products of Portland cement pastes

The effect of temperature on the hydration products and the composition of the pore solution are investigated for two Portland cements from 5 to 50 °C. Increased temperature leads to an initially fast hydration and a high early compressive strength. At 40 and 50 °C, the formation of denser C-S-H, a more heterogeneous distribution of the hydration products, a coarser porosity, a decrease of the amount of ettringite as well as the formation of very short ettringite needles has been observed. At 50 °C, calcium monosulphoaluminate has formed at the expenses of ettringite. In addition, the amount of calcium monocarboaluminate present seems to decrease. The composition of the pore solution mirrors the faster progress of hydration at higher temperatures. After 150 days, however, the composition of the pore solution is similar for most elements at 5, 20 and 50 °C. Exceptions are the increased sulphate concentrations and the slightly lower Al and Fe concentrations at 50 °C.     

Effect of cement C3A content, temperature and storage medium on thaumasite formation in carbonated mortars

The purpose of this study is to determine the effect of cement C₃A content, temperature and composition of the immersion medium (water, gypsum and magnesium sulphate solution) on the rate of thaumasite formation in cement mortars. It also aims to ascertain how the C₃A content influences the composition of the salt formed.The mortar prisms for this study were made with two different cements, one with low and the other with high Al₂O₃ content, with or without gypsum and/or calcium carbonate. After hydration, curing and carbonation, the prisms were partially immersed in distilled water and stored at temperatures ranging from 0 to 5 °C for up to 5 years. Some of the prisms were immersed in a 2% (w/w) gypsum solution or in 1.4% (w/w) magnesium sulphate solution at ambient temperature. Samples were taken at different ages and mineralogical and micro-structurally characterised.Some of the specimens tested were observed to expand, in a process concurring with the formation of thaumasite or a solid solution of thaumasite and ettringite, at both ambient and cooler temperatures. A correlation was found between cement C₃A content and the composition of the deterioration product involved in the expansive process: thaumasite forms in mortars made with low C₃A cement, whereas mixed crystals or solid solutions of thaumasite and ettringite form in mortars made with high C₃A content cement.     

Effect of hydration temperature on the solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pastes

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement and white Portland cement pastes were measured during the first 28 d of curing at temperatures ranging from 5–50 °C. Saturation indices with respect to solid phases known to form in cement paste were calculated from a thermodynamic analysis of the elemental concentrations. Calculated saturation levels in the two types of paste were similar. The solubility behavior of Portlandite and gypsum at all curing temperatures was in agreement with previously reported behavior near room temperature. Saturation levels of both ettringite and monosulfate decreased with increasing curing temperature. The saturation level of ettringite was greater than that of monosulfate at lower curing temperatures, but at higher temperatures there was effectively no difference. The solubility behavior of C-S-H gel was investigated by applying an appropriate ion activity product (IAP) to the data. The IAP_CSH decreased gradually with hydration time, and at a given hydration time the IAP_CSH was lower at higher curing temperatures.     

Formation and stability of gismondine-type zeolite in cementitious systems

Microcrystalline zeolites of the gismondine family are often reported in alkali-activated and blended cement systems. However, little is known about gismondine's compatibility with other cementitious phases to determine stability in long-term phase assemblage. Experimental studies were conducted to investigate the compositional field of gismondine stability in the lime-alumina-silica-hydrate systems, with a particular focus on understanding the compatibility of gismondine with other cement phases such as C-S-H, ettringite, monosulfate, strätlingite, katoite, gypsum, calcite, portlandite, alkali, silica, and aluminosilicate phases. Results show that gismondine-Ca forms readily at ~85°C in high aluminosilicate compositions; and persists in the presence of calcite, gypsum, ettringite, katoite solid solution, low Ca tobermorite-like C-S-H, silica and aluminosilicate phases, at 20-85°C. However, gismondine-Ca reacts with: (a) monosulfate, producing ettringite-thaumasite solid solution; (b) portlandite, forming tobermorite-like C-A-S-H gel and siliceous katoite at >55°C; (c) aqueous NaOH, generating gismondine-(Na,Ca), a garronite-like zeolite P solid solution; and (d) strätlingite leading to the conversion of strätlingite to gismondine indicating the metastability of strätlingite with respect to gismondine at 55°C. The outcomes are discussed to provide insights into the long-term phase assemblage of relevant cement systems such as lime-calcined clay, alkali-activated materials, and potentially ancient Roman concrete.     

掺减水剂钼尾矿胶砂的力学性能和水化产物

将钼尾矿、矿渣、熟料、石膏进行机械力粉磨,制备胶凝材料,研究了减水剂种类和掺量对胶砂力学性能的影响,并对掺钼尾矿胶凝材料的水化产物进行了分析.结果表明,在相同流动度条件下制备胶砂试块,PC减水剂对掺钼尾矿胶砂的强度提高幅度最大,FDN次之,UNF-5最小.当PC高效减水剂掺量为0.4%时,大掺量尾矿胶砂试块28 d的抗压强度可以达到48.8 MPa.粉磨后的钼尾矿表现出一定的火山灰反应活性.掺钼尾矿胶凝材料的水化产物主要是钙矾石和水化硅酸钙凝胶.     

Pessimum effect of externally applied chlorides on expansion due to delayed ettringite formation: Proposed mechanism

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 prior to storage at room temperature in water or limewater, 0.5 M, 2.8 M sodium chloride solutions. It was found that the specimens stored in 0.5 M sodium chloride solution gave a much greater expansion than those stored in limewater or 2.8 M sodium chloride solution. This pessimum influence of chlorides on expansion due to delayed ettringite formation deviates from the commonly held view that chlorides mitigate sulphate attack in concretes. The mechanism of the pessimum effect of chlorides on expansion due to delayed ettringite formation, and the final products of the associated phase transformations have been proposed. X-ray diffraction and differential thermal analysis techniques were used to follow phase transformations.     

Solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pore solutions as a function of hydration time

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement (OPC) and white Portland cement (WPC) pastes were measured during the first 28 days of hydration at room temperature. Saturation indices (SI) with respect to various solid phases known to occur in cement pastes were calculated from a thermodynamic analysis of the elemental concentrations, resulting in good agreement between the two pastes. In agreement with other published work, gypsum was saturated during the first several hours of hydration and then undersaturated thereafter, while portlandite was modestly supersaturated after the first few hours. High levels of supersaturation with respect to ettringite and calcium monosulfoaluminate were calculated, particularly prior to the consumption of gypsum at around 10 h. Results are consistent with published thermodynamic studies that show calcium monosulfoaluminate is metastable with respect to ettringite under normal hydration conditions. Three different ion activity product (IAP) equations for C-S-H were applied to the data. From 10 h onward, each of the IAP values declined gradually over time and the values for the OPC and WPC pastes were in close agreement. The same IAP equations were applied to experimental data from the pure CaO-SiO₂-H₂O system, resulting in good agreement between the cement paste pore solutions and the equilibrium between portlandite and the upper, or metastable, C-S-H solubility curve.     

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.     

利用铁合金渣制备胶凝材料及其微观分析

铁合金渣是具有潜在水硬性和火山灰活性的物质,经物理激发、化学激发后可制备成高强度的胶凝材料.由于其富含 Al2O3,添加化学激发剂后,其生成物不仅有 C-S-H 凝胶,还有大量的钙矾石.本实验,通过一系列的激发手段,将其制备成强度达到了标号为 42.5R 的普通硅酸盐水泥强度要求的胶凝材料,并且采用X衍射、电镜等分析了 C-S-H 凝胶及钙矾石的生成过程.     

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.     

The kinetics of ettringite formation

Conversion of hemihydrate to gypsum, and ettringite formation in the system C₃A---CaSO₄·1/2H₂O---H₂O three hydration temperatures using x-ray diffraction. Analysis of the kinetics of gypsum formation suggests that the rate limiting step changes from an initial dependence on the hemihydrate surface area to a dependence on the gypsum growth rate. The formation of ettringite was found to be controlled by diffusion. In accord with the needle-like morphology that ettringite exhibits, a nucleation and growth model predicted one-dimensional growth. An apparent activation energy calculated for ettringite formation is consistent with a diffusionally controlled process.     

Composite systems fluorgypsum-blastfurnance slag-metakaolin, strength and microstructures

The hydration and properties of composite cementitious pastes with 75% fluorgypsum were investigated; blastfurnace slag and metakaolin were the complementary cementitious materials. The pastes were cured under water at 20 °C for 360 days. All pastes developed and maintained strength under water, except those of commercial gypsum. The addition of metakaolin had a positive effect, after 360 days compressive strengths of 13.4, 13.8 and 14.6 MPa were registered for systems with 0%, 5% and 10% of metakaolin, respectively. The microstructure of the composite pastes was formed of a framework of gypsum crystals, which formed in the initial stages; the matrix was later densified by the formation of C-S-H and ettringite, as a result of the slag and metakaolin reactions. The fluorgypsum reacted rapidly in the first days, however it was still present after one year; the slag reacted in a slower fashion, and the metakaolin was very reactive and contributed with the ettringite since the early ages, which enhanced the strength.