高温环境下碳硫硅钙石-钙矾石固溶体的研究

由碳硫硅钙石引起的硫酸盐侵蚀破坏在许多国家都有相关工程案例.铝相可以加速碳硫硅钙石的生成,并能够与钙矾石形成固溶体,但铝相的作用机理目前尚无定论.在不同温度下合成了一系列固溶体,并利用XRD,SEM-EDS及红外对固溶体进行了测试分析,结果表明:40℃温度下亦可以生成碳硫硅钙石/固溶体;20℃与40℃均能够形成碳硫硅钙石型与钙矾石型两种形态固溶体;a轴固溶体间隙随温度升高而增大,20℃环境下固溶体间隙为1.1076～1.1182nm,40℃时固溶体间隙为1.1069～1.1189nm;随温度升高碳硫硅钙石-钙矾石固溶体晶体形貌更加细长.     

碳硫硅钙石和钙矾石的稳定性

采用化学方法合成出纯的碳硫硅钙石和钙矾石,对比研究了二者的稳定性。结果显示:钙矾石比碳硫硅钙石更易受到碳酸钠破坏;碳硫硅钙石与氯化钡几乎不发生反应,钙矾石则较易被氯化钡破坏;在碳化条件下,钙矾石较易分解,碳硫硅钙石则几乎没变化。碳硫硅钙石在碳酸钠或氯化钡溶液中的稳定性、热分解稳定性和碳化稳定性均明显高于钙矾石,可能是由于碳硫硅钙石结构中离子键和氢键强度高于钙矾石。     

内掺硫酸镁加速水泥基材料TSA破坏

为满足实验室研究水泥基材料的碳硫硅钙石型硫酸盐侵蚀(thaumasite form of sulfate attack,TSA)破坏的需要,研究了一种快速生成碳硫硅钙石的途径。将内掺10%硫酸镁的水泥-石灰石粉净浆样品浸泡在(5±2)℃水中,进行硫酸盐侵蚀加速试验,观察侵蚀后样品外观,并用X射线衍射、红外光谱分析腐蚀产物。结果表明:浸泡6个月后,样品失去强度,变为一种白色、柔软且无黏结力的烂泥状物质,腐蚀产物以碳硫硅钙石、石膏为主,并含有少量钙矾石,具有典型的TSA破坏特征。与硫酸盐溶液外部侵蚀相比,内掺法显著加速水泥基材料TSA破坏。X射线衍射结合红外光谱分析可以有效鉴定碳硫硅钙石存在。     

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。     

碳硫硅钙石鉴别方法研究

碳硫硅钙石和钙矾石的晶体结构和形态极为相似,对其准确鉴别难度较大.本文采用化学方法合成出纯的钙矾石和碳硫硅钙石,并用XRD、EDS能谱、IR光谱和Raman光谱等微观测试手段对它们的结构进行表征,以探求碳硫硅钙石鉴定方法.研究结果表明,碳硫硅钙石和钙矾石的XRD图谱极为相似;碳硫硅钙石EDS能谱显示其主要含有S、Si和Ca等元素,而钙矾石则主要为S、Al和Ca等元素;碳硫硅钙石的IR光谱和Raman光谱分别在500cm-1处和658 cm-1处有特征吸收峰.研究发现,拉曼光谱是鉴别碳硫硅钙石存在的最有效方法.     

氧化镁质和硫铝酸钙膨胀剂对工程水泥基复合材料性能的影响

本文研究了氧化镁质和硫铝酸钙膨胀剂对工程水泥基复合材料水化产物及微观结构的影响。结果表明:两种膨胀剂均会引起工程水泥基复合材料流动性能、力学性能的下降;掺入氧化镁质膨胀剂使得材料体系中大量生成富镁硅钙石,而掺入硫铝酸钙膨胀剂使得材料体系中钙矾石含量增大,两种水化产物均可以细化材料的孔结构,改善试样的抗氯离子渗透性能,5%(质量分数)掺量下硫铝酸钙膨胀剂试样的孔径比氧化镁质膨胀剂试样的更小,结构更加稳定。     

以山砂为骨料的电解锰渣蒸压砖工况使用强度失效问题研究

对以山砂为骨料的锰渣蒸压砖在贵州地区自然条件下使用1年后性能的变化进行了分析,利用XRD、XRF和SEM-EDS等方法分析了其强度变化机理.结果显示,失效锰渣蒸压砖出现二水石膏、少量碳硫硅钙石和钙矾石物相,原始锰渣蒸压砖存在的钙铝硅酸盐消失.锰渣蒸压砖性能失效是二水石膏相、钙矾石、碳硫硅钙石综合作用的结果,锰渣高含量的硫酸盐和山砂中大量的碳酸盐是这些物相形成的主要诱因.     

矿粉对水泥基材料抗碳硫硅钙石侵蚀破坏的定量分析

研究了矿粉对水泥基材料抗碳硫硅钙石侵蚀性的影响,将制得试件在温度5℃、质量浓度10%MgSO4溶液中养护,对不同龄期试件进行力学性能测试,并对侵蚀产物进行X射线衍射、拉曼光谱定性分析及Rietveld方法定量分析。结果表明：未掺矿粉试件受侵蚀后腐蚀产物中碳硫硅钙石占12.68%,而掺20%和40%矿粉试件中碳硫硅钙石分别只占2.79%和2.19%。矿粉的掺入可以使水泥基材料的抗碳硫硅钙石侵蚀破坏性能提高,但不能完全抑制碳硫硅钙石侵蚀破坏的发生。     

合成条件对钙矾石形貌的影响

钙矾石是硫铝酸盐水泥水化的主要产物,决定着硫铝酸盐水泥的性能,而在硅酸盐水泥混凝土中影响也非常重要。钙矾石的形成条件决定了其形貌,其形成时间影响着水泥和混凝土的性能。采用化学合成方法合成钙矾石,研究了不同合成条件对钙矾石形成和形貌的影响。结果表明:不同原料合成的钙矾石形貌显著不同且长宽比差异显著;适当提高温度能够促进钙矾石的形成,80℃条件下反应可以形成钙矾石且形貌多样,100℃条件下反应钙矾石不能稳定存在;随着液相p H值的增加,钙矾石的长宽比逐渐减小;硼酸抑制钙矾石在(001)方向的生长。     

内掺白云石水泥基材料抗硫酸盐侵蚀性能

为考察白云石对水泥基材料抗硫酸盐侵蚀性能的影响，本文采用10%、20%、30%(质量分数，下同)白云石掺入水泥净浆与水泥砂浆试件中，在低温条件下浸泡于5%硫酸镁和5%硫酸钠溶液中，并进行硫酸盐侵蚀试验。定期观察试件的宏观形貌变化，并定量分析其侵蚀产物。测定水泥砂浆试件抗折强度与抗压强度并进行宏观分析，以此得出不同种类硫酸盐对试件生成碳硫硅钙石的影响。采用热力学模拟探究白云石对水泥胶凝体系产物的影响。结果表明：当白云石掺量为10%～20%时，能抑制水泥基材料中碳硫硅钙石的生成，水泥基材料的抗硫酸盐侵蚀性能有较大提高，水泥砂浆试件抗折强度有明显改善，这与热力学模拟结果基本一致。     

Understanding the atomic and electronic origin of mechanical property in thaumasite and ettringite mineral crystals

Ettringite and thaumasite are 2 of the relevant secondary products in portland cement minerals. They are rare crystals with high water content. Two density functional theory based ab initio packages VASP and the OLCAO were used for exploring the geometry structure, electronic, and mechanical properties of ettringite and thaumasite. The calculations focus on the comparison of their structures and properties to gain insights that can reveal the minute difference in properties due to differences in their structure, composition and water content. Detailed analysis of interatomic bonding shows the similarities and subtle difference between them even though thaumasite has the Si tetrahedron as the backbone unit in contrast to ettringite with Al‐columns and no Si atoms. Moreover, thaumasite has strong C–O bonds that are absent in ettringite, and ettringite has far more water molecules with a substantial contribution from the hydrogen bonding. It is shown that the role played by relatively strong S–O bonds in both crystals has a large impact on the hydration in these crystals. On the basis of calculated total bond order density, it is concluded that thaumasite is slightly more cohesive than ettringite consistent with the calculated mechanical properties. The detailed comparative results for these 2 important minerals we obtained in this paper, many of them for the first time, will pave the way to understanding the structure and electronic structure origin of other more complex cement crystals.     

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.     

XRD, EDX and IR analysis of solid solutions between thaumasite and ettringite

Solid solutions between thaumasite and ettringite were prepared by methods analogous to those well established for the preparation of thaumasite and ettringite. The extent of immiscibility in this system is investigated by varying the Al:Si and SO₄²⁻:CO₃²⁻ ratios in reactant mixtures. The solids produced were analysed by quantitative X-ray diffraction, with Rietveld refinement also providing accurate unit cell dimensions, energy-dispersive X-ray analysis and infrared spectroscopy. The compositional and unit cell variations in the solid solution are discussed. A wide variety of solid solution compositions were produced with both the thaumasite and ettringite structures, but all preparations were considerably diluted by secondary amorphous products.     

A thermodynamic and experimental study of the conditions of thaumasite formation

The formation of thaumasite was investigated with the progressive equilibrium approach (PEA). This approach experimentally simulates the conditions of various levels of sulfate addition in hardened cement pastes. The influence of limestone, time, C₃A content, temperature and leaching on thaumasite formation was investigated. The results show that thaumasite formation is favoured at lower temperatures (8 °C) independently of the type of cement clinker (high or low C₃A content) used. Thaumasite was found to form only in systems where limestone was present and where sufficient sulfate had been added. Thaumasite precipitated only in systems where the Al present has already been consumed to form ettringite and the molar SO₃/Al₂O₃ ratio exceeded 3. In leached samples (reduction of portlandite and alkalis) slightly less thaumasite was formed whereas gypsum and ettringite are favoured under these conditions. The PEA, used to investigate the chemical aspects of sulfate attack was found to be a good tool for simulating external sulfate attack. Generally, thaumasite was detected were it was modelled to be stable in significant amounts. However, in this study equilibrium conditions were not reached after 9 months.     

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.     

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.     

Thausamine in failed cement mortars and renders from exposed brickwork

The mineral thaumasite occurs within certain cement-based building materials as a direct result of sulphate attack. It readily forms within certain types of brickwork and not only contributes to expansive cracking of the brickwork but is also accompanied by severe softening of the cement matrix. Samples were taken of both brickwork mortars and renders, some of which were still relatively sound whereas others had deteriorated into a paste. The mortar samples were investigated using a quantitative Xray diffraction technique in order to determine the amounts of thaumasite, ettringite and gypsum present. Results show that if conditions are favourable then thaumasite formation can proceed rapidly and can even result in the complete breakdown of very strong renders.     

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).     

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.