硫酸盐侵蚀下石膏的形成及破坏机制研究现状

硫酸盐侵蚀是影响水泥基材料耐久性的重要因素,它不仅会缩短材料的服役寿命,甚至可能危及结构安全。在硫酸盐侵蚀过程中,钙矾石、石膏和碳硫硅钙石等侵蚀产物不断形成,从而导致材料出现膨胀、开裂、软化和剥落等不同形式的破坏。由于不同侵蚀产物的形成条件和对水泥基材料的侵蚀机理存在明显差异,而侵蚀机理是工程实践中指导预防硫酸盐侵蚀的重要依据,因此探明不同侵蚀产物的形成及稳定条件以及各侵蚀产物对材料的作用机理成为该研究课题的主要内容。 从现有研究来看,钙矾石型硫酸盐侵蚀是目前研究最为成熟的一种硫酸盐侵蚀。钙矾石是在高碱性硫酸盐溶液条件下形成的主要侵蚀产物,并且当其在狭小封闭的孔洞中生长时会导致材料发生膨胀、开裂破坏,相应的膨胀机理有吸水肿胀理论、结晶压理论和固相反应理论等。另外,在钙矾石型硫酸盐侵蚀的预防方面,发现通过控制水泥中铝酸三钙含量可有效减小因钙矾石形成而造成的膨胀危害。近年来,世界各地的研究者竞相报道了碳硫硅钙石的形成对混凝土结构造成严重破坏的工程实例,这使得碳硫硅钙石型硫酸盐侵蚀也逐渐受到重视。目前普遍认为碳硫硅钙石的形成主要导致材料出现泥化和分解现象,但其形成条件较为复杂,只在一些特殊环境下才有可能发生。 石膏是水泥基材料在硫酸盐侵蚀下形成的另一种较为常见的腐蚀产物,它的形成同样影响着水泥基材料的耐久性能。研究发现,硫酸盐溶液浓度越高,越利于形成石膏,但后来发现溶液pH值对石膏的形成及稳定影响更为显著,同时溶液温度、离子种类以及腐蚀制度等对石膏的形成也有一定影响。由于石膏的化学组成相对简单且不含铝相,因此采用普通抗硫酸盐侵蚀方法并不能有效抑制石膏的形成及破坏。石膏的形成往往伴随着水化产物的溶解脱钙,从而导致材料出现软化和剥落现象,但在石膏的膨胀问题上仍存在较大争议。 本文综述了硫酸盐侵蚀下水泥混凝土中石膏形成的影响因素,总结了石膏的生长位置及其引起的脱钙作用,最后对石膏的膨胀作用进行了相关探讨。     

不同环境中矿物掺合料混凝土抗硫酸盐侵蚀性能的研究进展

硫酸盐侵蚀是混凝土耐久性研究的热点之一。矿物掺合料的掺入改变混凝土内部的组成,细化了混凝土的孔结构,对混凝土抗硫酸盐侵蚀起着重要作用。掺合料的化学组成、细度、掺量等对混凝土抗硫酸盐侵蚀均有很大的影响。外界腐蚀环境的不同,矿物掺合料混凝土抗硫酸盐侵蚀的性能也有显著的差别。大量的研究表明,在连续浸泡的硫酸盐溶液中,矿物掺合料只要掺量适当能够提高混凝土抗硫酸盐侵蚀的能力。在干湿循环与硫酸盐溶液共同作用下,矿物掺合料混凝土抗硫酸盐侵蚀的能力有所争议,有些研究表明矿物掺合料能够提高干湿循环条件下混凝土抗硫酸盐侵蚀的能力;然而有些研究结果却呈现相反的结论,这需要进一步探索。     

水泥基材料硫酸盐侵蚀机理的研究进展

硫酸盐侵蚀是影响水泥基材料结构耐久性的重要因素。本文在分析硫酸盐侵蚀对水泥基胶凝材料铝相水化产物和C-S-H凝胶影响的基础上,归纳了现有水泥基胶凝材料硫酸盐侵蚀的作用机理,提出了硫酸盐侵蚀作用下水泥基材料微结构研究中存在的不足和进一步的研究方向。     

混凝土抗硫酸盐侵蚀性能试验研究

研究了不同水胶比混凝土试件在(20±2)℃全浸泡作用下的抗硫酸盐侵蚀性能。试验制备0. 32与0. 36两种水胶比的普通硅酸盐水泥、高抗硫水泥以及复掺矿粉和硅灰的混凝土试件,检测了试件标养28 d后的抗压强度、动弹性模量及各试件在(20±2)℃、质量分数为3%Na_2SO_4溶液中全浸泡侵蚀的强度、动弹性模量变化情况,对混凝土在(20±2)℃全浸泡侵蚀下的抗硫酸盐侵蚀性能进行了评价。结果表明,在(20±2)℃全浸泡作用下,0. 36水胶比混凝土抗硫酸盐侵蚀性能低于0. 32水胶比混凝土,抗硫酸盐侵蚀性能随着水胶比的降低而提高;混凝土复掺矿粉和硅灰后抗硫酸盐侵蚀性能较好;高抗硫水泥通过限制C_3A的含量,进而改善混凝土抗硫酸盐侵蚀性能,不一定在任何环境下都适用;混凝土连通孔隙率从侧面证实了低水胶比混凝土和复掺矿粉和硅灰混凝土抗压侵蚀系数和相对动弹性模量较高,说明低水胶比混凝土复掺矿粉和硅灰后,可以显著提高混凝土抗硫酸盐侵蚀性能。     

水泥混凝土硫酸盐侵蚀综述

从化学反应的角度详细介绍了水泥混凝土的硫酸盐侵蚀机理及其相应的侵蚀产物,分别从内部和外部两方面阐述了硫酸盐侵蚀的因素,论述了目前抗硫酸盐侵蚀的一些方法以及这些方法的优劣性,并展望了水泥混凝土抗硫酸盐侵蚀的前景。     

混凝土抗硫酸盐侵蚀性能及其离子扩散影响因素研究

为了进一步解释混凝土受硫酸盐侵蚀劣化机理,研究了不同水胶比以及养护时间对全浸泡下混凝土抗硫酸盐侵蚀性能的影响,分析了硫酸盐在混凝土中的扩散过程.试验制备了0.32、0.35、0.38这3种水胶比的普通硅酸盐水泥混凝土试件,对其质量、 动弹性模量、 抗压强度、SO42-浓度等指标进行分析.结果表明:全浸泡条件下,混凝土抗...     

水泥基胶凝材料氧化物含量与氯离子结合量的定量关系

为揭示氯盐溶液及存在硫酸盐复合盐溶液中氯离子对水泥基材料的侵蚀特性,合理选用氯盐环境下水泥基材料组分,减小海洋环境和盐湖卤水对混凝土造成的氯离子侵蚀问题,本工作通过添加矿物掺合料(矿渣和粉煤灰)改变主要氧化物含量,研究水泥基材料氧化物含量与氯离子结合量的关系.采用等温吸附法测试样品的氯离子结合量,采用X射线衍射仪(XRD)和同步热分析仪(TG-DSC)表征侵蚀产物.实验结果表明,在氯盐溶液侵蚀下,水泥基材料的氯离子结合量与其Al2 O3、SiO2含量成正比,与CaO含量成反比.在硫酸盐存在的复合盐溶液侵蚀下,硫酸盐浓度较低(3.5％NaCl+0.5％Na2 SO4)时,氯离子结合量略有下降,测试结果表明Friedel盐的生成量不会因硫酸根离子的引入而降低,此时两种离子没有明显的竞争关系;当硫酸盐浓度提高到5％Na2 SO4时,氯离子结合量下降显著,实验结果表明Friedel盐的生成量降低,两种离子间竞争关系显著,但不影响氧化物含量与氯离子结合量之间的相关性.     

硫酸盐侵蚀玄武岩纤维轻骨料混凝土力学性能研究

为增强轻骨料混凝土抗硫酸盐侵蚀性能,对不同掺量的玄武岩纤维轻骨料混凝土(BF-LAC)进行不同龄期的硫酸盐侵蚀试验,研究BF-LAC在硫酸盐侵蚀环境下的力学性能和劣化程度.采用浓度为5％的硫酸盐溶液侵蚀玄武岩纤维掺量为0％、0.1％、0.2％、0.3％的BF-LAC并每隔60 d测试抗压强度、劈裂抗拉强度、有效孔隙率和...     

氯盐和硫酸盐交互作用下水泥基材料的破坏机理综述

西部盐湖和海洋环境中存在大量的氯盐和硫酸盐,当水泥基材料处于氯盐-硫酸盐环境中,其破坏规律以及破坏机理与单一侵蚀因素作用明显不同,因此有必要探究两种盐在侵蚀过程中所呈现的交互作用。已有文献对氯盐和硫酸盐侵蚀下水泥基材料破坏规律的研究可以分为两部分:硫酸盐存在下氯盐对水泥基的侵蚀、氯盐存在下硫酸盐对水泥基的侵蚀。硫酸盐可以明显降低水化产物的氯离子结合量,因为硫酸根不仅能够分解Friedel盐,而且能先与C₃A或AFm反应,抑制Friedel盐的产生。但是,氯离子扩散速率高,可以先于硫酸根离子进入水泥基材料内部与水泥水化产物反应,延缓钙矾石的产生,抑制硫酸盐的侵蚀。两种离子在向材料内部侵蚀过程中相互影响,相互牵制。最后,根据目前的研究,提出几个有意义的思考方向。     

粉煤灰地聚物混凝土性能与环境影响的综合评价

目前对粉煤灰地聚物混凝土环境影响评价大多数只考虑环境效益单方面的因素，而力学性能和耐久性能是材料在工程应用中的重要评价指标，材料选取需要同时考虑其力学性能、耐久性能与环境影响。因此，本工作采用生命周期评价(LCA)方法，选取抗压强度、抗硫酸盐侵蚀性能和环境影响为评价指标，设计九组配合比的粉煤灰地聚物混凝土进行环境影响和抗硫酸盐侵蚀的量化分析，利用灰色聚类评价方法建立综合评价模型。结果表明：(1)粉煤灰地聚物混凝土的环境影响主要来自原材料生产阶段，占比高达73%以上；(2)粉煤灰地聚物混凝土的环境效益随碱激发剂用量、粉煤灰用量和硅酸钠与氢氧化钠比值的增大而降低，碱激发剂用量对环境效益影响最大；抗压耐蚀系数越大其抗硫酸盐侵蚀性能越好；(3)在本试验中，配合比1的环境效益最优，配合比5抗硫酸盐侵蚀性能最优，配合比3综合性能最优。     

Thaumasite sulfate attack in field and laboratory concretes: implications for specifications

The reported studies made on field elements affected by thaumasite sulfate attack are discussed, together with the reported laboratory performance of concrete and mortars immersed in sulfate solutions maintained at 5 °C. It is concluded that magnesium ions ingressing from groundwater or resulting from dedolomitization of an aggregate containing dolomite play a major role in increasing the risk of thaumasite attack and that attack can also arise from exposure to sulfuric acid bearing groundwaters, or sulfuric acid resulting from oxidation of pyrites within aggregates. Guidance is given on concrete qualities and materials to resist sulfate attack and thaumasite sulfate attack resulting from salts present in groundwater.     

水泥混凝土TSA侵蚀研究进展

TSA侵蚀是水泥混凝土在低温、高湿、硫酸盐与碳酸盐长期作用下发生的一种特殊的硫酸盐侵蚀形式.它直接导致水泥石中CSH凝胶体的分解,生成无任何胶结性的硅灰石膏(thaumasite)晶体,使混凝土最终变为一种白色的烂泥状混合物.综述了国内外对此类侵蚀的研究现状和进展,包括破坏机理、影响因素和防治措施等,并指出应进一步的研究方向.     

Theoretical analysis of the effect of weak sodium sulfate solutions on the durability of concrete

A theoretical analysis of the detrimental influence of weak sodium sulfate solutions (Na₂SO₄) on the durability of concrete is presented. It was conducted using a numerical model that takes into account the coupled transport of ions and liquid and the chemical equilibrium of solid phases within the (partially) saturated system. Numerous simulations were performed to investigate the influence of various parameters such as water/cement (w/c) ratio (0.45, 0.65 and 0.75), type of cement (CSA Type 10 and Type 50), sulfate concentration (0–30 mmol/l of SO₄) and the gradient in relative humidity across the material. All input data related to the properties of concrete were obtained by testing well-cured laboratory mixtures. Numerical results indicate that exposure to weak sulfate solutions can result in a significant reorganization of the microstructure of concrete. The penetration of sulfate ions into the material is not only at the origin of the precipitation of sulfate-bearing phases (such as ettringite and eventually gypsum) but also results in calcium hydroxide dissolution and C–S–H decalcification. Data also clearly emphasize the fact that w/c ratio remains the key parameter that controls the durability of concrete to sulfate attack.     

混凝土在碳化和干湿循环作用下的抗硫酸盐腐蚀性能

采用自然浸泡和干湿循环的试验方法, 研究了碳化后的粉煤灰混凝土(FAC)、 大掺量矿物掺合料混凝土(HVMAC)及高性能混杂纤维增强膨胀混凝土(HPHFREC)在5%硫酸镁溶液中的损伤过程。结果表明: 碳化一定程度上密实了混凝土表层, 但改变了混凝土表层的化学组成, 降低混凝土的抗硫酸镁腐蚀性能。干湿循环加速硫酸镁的扩散作用, 扩展混凝土内部原有的微裂缝。在碳化+硫酸镁双重破坏因素作用下, HVMAC具有优异的抗腐蚀性能, 适合应用于硫酸镁腐蚀的严酷环境; 在碳化+干湿循环+硫酸镁多重破坏因素作用下, HPHFREC2的三元纤维混杂起到明显的增强增韧效果, 抗腐蚀性能较好。     

Sulfate resistance of plain and blended cements exposed to varying concentrations of sodium sulfate

Concrete deterioration due to sulfate attack is the second major durability problem, after reinforcement corrosion. This type of deterioration is noted in the structures exposed to sulfate-bearing soils and groundwater. Though concrete deterioration due to sulfate attack is reported from many countries, the mechanisms of sulfate attack have not been thoroughly investigated, particularly the effect of sulfate concentration and the cation type associated with the sulfate ions on concrete deterioration. This study was conducted to evaluate the performance of plain and blended cements exposed to varying concentrations of sodium sulfate for up to 24 months. Four types of cements, namely Type I, Type V, Type I plus silica fume and Type I plus fly ash, were exposed to five sodium sulfate solutions with sulfate concentrations of 1%, 1.5%, 2%, 2.5% and 4%. These concentrations are representative of the sulfate concentration in highly saline soils. The sulfate resistance was evaluated by visual examination and measuring the and reduction in compressive strength. The maximum deterioration, due to sulfate attack, was noted in Type I cement followed by silica fume and Type V cements. The performance of Type V, Type I plus silica fume and Type I plus fly ash was not significantly different from each other. The enhanced sulfate resistance noted in the Type I cement blended with either silica fume or fly ash indicates the usefulness of these cements in both sulfate and sulfate plus chloride environments.     

The distribution and evaluation of sulfur species in geological materials and manmade fills

Groundwaters containing acidity and/or dissolved sulfate species are a primary factor in the attack on concrete and other construction materials. In the case of concrete, attack may be prevented by the use of suitably resistant concrete, eliminating sources of acidity and/or sulfate ions or by protecting the concrete structure from the offending solutions. Official guidelines, which have recently been modified to take account of the possible formation of sulfate and acidity after construction, are provided for the design of concrete structures to be placed in potentially aggressive situations. Limiting values for sulfates and other chemical species are also given for highway, road and bridge structures, but recent problems involving buried galvanised steel structures have highlighted the need for revised procedures to be put in place. Potential sources of aggressive solutions include the ground surrounding the structure and the construction materials, including fills, situated proximally to the structure.

Unfortunately the current British Standard testing procedures are unsuitable for the purpose of assessing the potential aggressive nature of the ground and construction materials, hence risk assessments prove inadequate. This paper considers potential sources of ground acidity and sulfate rich solutions. Attention is given to suitable testing procedures for the evaluation of sulfur species in geological and construction materials. Particular attention is given to limiting values appropriate to the use of buried steel that forms part of highway, road and bridge structures.     

Sulfate attack and role of silica fume in resisting strength loss

This paper presents a detailed experimental study on the sulfate attack of Portland cement mortars, and the effectiveness of silica fume in controlling the damage arising from such attack. The test solutions used to supply the sulfate ions and cations were 5% sodium sulfate solution and 5% magnesium sulfate solution. Tap water was used as the reference solution. The main variables investigated in the study were the water/cementitious materials ratio, and the level of cement replacement. Compressive strength measured on 50 mm cubes was used to assess the changes in the mechanical properties of mortar specimens exposed to sulfate attack for 510 days. X-ray diffraction and differential scanning calorimetry were used to evaluate the microstructural nature of the sulfate attack. The test results showed that the presence of silica fume had a beneficial effect on the strength loss due to sodium sulfate attack. The best resistance to sodium sulfate attack was obtained with a SF replacement of 5–10%, but even then, a strength loss of 15–20% can be expected. On the other hand, mortars with silica fume were severely damaged in the magnesium sulfate environment. Further, the compressive strength loss actually increased with increasing SF content. The test results thus showed clearly that the use of SF in concrete exposed to magnesium sulfate solution is not recommended. The test results also showed that the w/cm ratio is the most critical parameter influencing the resistance of concrete to sulfate attack. All the tests reported in the study were carried out at 20 ± 1 °C.     

An experimental study of combined acid and sulfate attack of concrete

There is disagreement about the role of sulfuric acid in the thaumasite form of sulfate attack (TSA) of concrete. Some researchers suggest that thaumasite is formed only at pH above 10.5, whereas others report that the primary cause of deterioration in the affected M5 bridge foundations was sulfuric acid attack followed by neutral TSA. The aim of this work is to reconcile these conflicting views by undertaking parallel studies of concrete exposed to aggressive acid and sulfate solutions and concrete/clay interface work using weathered Lower Lias clay.

Concrete specimens have been exposed to BRE Digest 363 sulfate class solutions and acidic and acidic-sulfate solutions at 4.5 ± 0.5 °C. Selected samples are being characterised at intervals up to 5 years. At this stage, results are reported for 5-month samples. Various binders including Portland cement, Portland–limestone cement, blastfurnace slag cement, pulverized-fuel ash cement and sulfate-resisting Portland cement at water/binder ratios (w/b) from 0.35 to 0.5 have been studied.

Initial visual observations and X-ray diffraction analyses have identified thaumasite in some of the systems after 5 months immersion in solution.

An overview of the ongoing parallel concrete/clay interaction work is also presented to contextualise the concrete work.     

The resistance of metakaolin (MK)–Portland cement (PC) concrete to the thaumasite-type of sulfate attack (TSA)––Programme of research and preliminary results

The thaumasite form of sulfate attack (TSA), is a deleterious physico-chemical attack of the calcium silicate hydrate (C–S–H), binding phase of concrete. Water:binder ratios (w:b), are known to control ingress of potentially deleterious ions by pore structure refinement at low (0.40) values. Equally, a physical–chemical barrier exists at about w:b ratio of 0.45 regardless of the binder type. The inclusion of ultra-fine pozzalans (e.g. metakaolin) in the binder has been shown to impart such properties through densification of the matrix and removal of calcium hydroxide.

A small-scale experimental design programme to establish the potential resistance of metakaolin–Portland cement (MK–PC), blended concrete to the thaumasite-type of attack is ongoing. Results are presented for concrete incorporating dolomitic limestone aggregate and with 0% and 7% metakaolin replacement of a high-C₃A PC binder at w:b ratios to 0.40 and 0.46. Exposure to three environments containing a sulfatic clay, sulfate solution and water has produced evidence of deleterious reactions within these samples. Visual data are quantified by a wear rating and supported by compressive strength and expansion values for up to 280 days of exposure. These results confirm the importance of low water binder ratios in the resistance of ion ingress and give an early indication of the desirable durability-enhancing properties of MK replacement of PC. Finally, the results also show that the two sulfatic environments of exposure produce marked differences in the degradation modes, which may be important in the methodologies used to determine TSA in the laboratory with reference to field observations.     

Thaumasite field trial at Shipston on Stour: three-year preliminary assessment of buried concretes

A combined field and laboratory trial to investigate the thaumasite form of sulfate attack (TSA) in buried concrete was instigated by BRE in 1998 following the discovery of several cases of TSA in UK below-ground construction. Two identical sets of 98 concrete specimens were buried in sulfate-bearing Lower Lias Clay at a site in Central England. The first set of specimens was excavated in June 2001, after three years. The other set will be excavated after 10 years. A further similar assemblage has been subjected in the laboratory at 5 °C to a sulfate solution which simulates the site groundwater. Each set contains specimens with a range of quality and composition, mixes including combinations of eight binder types and four aggregate types, including limestone and siliceous aggregate.

After three years, identical concretes have generally behaved similarly in the field and laboratory studies, with a range of sulfate attack from none to moderate surface attack, dependent on concrete quality and composition. XRD analysis has shown the dominant form of sulfate attack to be TSA.