粉煤灰抑制ASR的机制

本文研究粉煤灰对ASR的抑制作用.测定了不同粉煤灰掺量砂浆棒膨胀率、水泥石中Ca(OH)2含量以及主要水化产物C-S-H凝胶的微观结构和化学组成,并对粉煤灰抑制ASR的机制进行探讨.结果表明:随粉煤灰掺量增加,其对ASR具有显著的抑制效果.水泥石中Ca(OH)2含量不断减少,二次反应生成大量低n(Ca)/n(Si)的C-S-H凝胶.随n(Ca)/n(Si)降低,C-S-H凝胶中固溶了大量的Al,凝胶的固碱能力增强,从而使混凝土孔溶液中的有效碱大大减少,减轻了ASR程度.凝胶的固碱能力增强除了因n(Ca)/n(Si)降低,使C-S-H凝胶的固碱能力增强之外.还可能因为Al3 代替Si4 形成[AlO4]5-四面体,由于Al3 比Si4 少一价,引起电价不平衡,系统带负电,因此碱离子更容易进入C-S-H凝胶中,减少了碱与活性集料反应的机会.     

碱硅酸反应与碱碳酸盐反应

碱集料反应（AAR）可分为两类,即碱硅酸反应（ASR）与碱碳酸盐反应（ACR）。二者的共同点是与碱发生的化学反应可导致混凝土中集料的体积增大,从而可能使混凝土甚至整个建筑物或构筑物发生膨胀开裂。文章着重从膨胀过程和机理以及岩石的结构特征探讨二者的特性与差异。ASR类型岩石具有碱活性的前提条件是较低的二氧化硅结晶完整度。只有隐晶质、微晶质、玻璃质或发生过应变的二氧化硅才会具有较高的化学活性,导致混凝土破坏。通过系统研究证实,对碱碳酸盐反应,虽然结晶的完整程度以及白云石（CaCO3·MgCO3）分子式中Ca/Mg比也将影响其与碱反应的速率,但起决定作用的是白云石晶体的尺寸及其在岩石中的分布状态和被基质包围的紧密程度。从微观结构得出的这些特征将有助于加深对碱集料反应膨胀机理的认识。文中还介绍了形成活性白云石的地质环境和碱硅酸反应与碱碳酸盐反应的区分方法。     

LiNO_3对碱硅酸反应及其膨胀的影响

本文研究了38℃下LiNO_3对石英玻璃在NaOH溶液中碱硅酸反应(ASR)的影响,采用等离子发射光谱仪(ICP)、酸化处理、X射线衍射仪(XRD)和扫描电镜(SEM)对溶液中的离子浓度、SiO_2分布、固相产物的组成和微观形貌进行分析和表征。测定了长龄期在38℃湿气养护下LiNO_3对沸石化珍珠岩和防城港砂岩混凝土微柱中ASR膨胀的作用效果。结果表明:掺LiNO_3的碱溶液中,Li~+先于Na~+与溶出的SiO_4~(4-)反应,生成低溶解性的含锂产物,并附着在石英玻璃表面,降低了溶液中OH~-对石英玻璃的溶蚀,进而减缓了ASR的反应速率。混凝土试件中[Li]/[Na+K]摩尔比越高,LiNO_3抑制ASR膨胀效果越好;随着龄期的延长,后期掺锂试件仍存在ASR膨胀。短龄期下LiNO_3抑制活性集料的ASR膨胀效果良好,长龄期作用效果减弱。     

无碱速凝剂在溪洛渡水电站的运用实践

通过溪洛渡水电站地下工程喷射混凝土运用无碱速凝剂的实践,并对无碱速凝剂与碱性速凝剂进行了大量的室内和现场喷射的对比试验,结果表明：无碱速凝剂比碱性速凝剂有更明显的技术优势和保证良好的施工环境,应大力推广使用。     

GRC制品抗泛碱性能的提升及机理

玻璃纤维增强水泥(GRC)制品因其自重轻、安装易、强度高、可预制等诸多优点,成为装饰水泥基材料中应用最为广泛的产品之一.制约其使用的一个重要因素是在降水过后水分蒸发等干湿循环服役环境下,GRC制品表面会析出白霜,即发生泛碱现象.目前解决水泥基材料泛碱问题的主要方法包括掺入超细集料、表面疏水处理等,没有从材料根本上解决泛碱问题.本工作使用基于微生物矿化技术的微生物抗泛碱剂提升GRC制品的抗泛碱性能,将GRC制品内部的钙离子固定,避免其随水分迁移至表面并生成泛碱物质.试验结果表明,掺加微生物抗泛碱剂时,GRC制品的泛碱面积率可显著降低至3％左右.同时探究了耐碱玻璃纤维对GRC制品抗泛碱性能的影响.X-CT和MIP测试结果表明,二者对GRC制品抗泛碱性能的影响机理均与孔结构有关.此外,本工作改进了水泥基材料抗泛碱性能的测试方法并提出了新的定量评价指标,即泛碱均匀度.     

不同环境因素作用下玻纤/环氧乙烯基酯复合材料的冲蚀行为

为探究环境因素对玻璃纤维增强环氧乙烯基酯树脂基(GF/EVE)复合材料性能的影响,对其进行了湿热和碱腐蚀老化试验,通过对不同老化时间下GF/EVE复合材料吸湿率、微观形貌、表面元素含量、表面化学结构及冲蚀失重率变化的分析,探讨了GF/EVE复合材料在湿热环境和碱腐蚀介质中的老化机理以及不同老化时间下的抗冲蚀性能变化.结果表明:随着老化时间延长,吸湿率增大,且碱性介质中的吸湿率较湿热环境大;在水分子及腐蚀介质的扩散作用下,树脂基体发生塑化、水解,纤维/基体界面出现脱黏,纤维表面腐蚀降解;同时老化造成树脂分子链断裂,交联密度降低,导致树脂初始分解温度下降;湿热和碱腐蚀老化初期冲蚀失重率分别下降了3. 2%和1. 8% ,老化结束后分别增加了17. 6%和20. 8% .     

固态碱组分碱矿渣水泥的研制及其水化机理和性能的研究

本研究针对液态碱组分碱矿渣水泥的缺陷,研制成功了强度标号达425R～625R的固态碱组分碱矿渣水泥,并对这种新型胶凝材料的生产工艺、水化机理、物理力学性能和耐久性进行了深入的研究。 运用现代测试手段详细研究了矿渣玻璃体的微观结构,发现矿渣玻璃体具有主要由富钙相和富硅相组成的微观分相结构,并且各相的存在状态和性质有很大的差异。在这一发现的基础上,本研究很好地解释了矿渣水硬活性的潜在性以及固态碱组分碱矿渣水泥的水化机理。研究表明,固态碱组分碱矿渣水泥的水化过程中没有诱导期或诱导期相当短,该水化过程主要经历矿渣的解体、新相的形成和长大、水化产物的缩聚和“混凝”三个阶段,其水化产物主要是沸石类矿物,另外还有少量的低碱性水化硅酸钙C-S-H(Ⅰ)。 研究表明,固态碱组分碱矿渣水泥属于低热水泥;其水泥石的孔隙率较小,且孔径大于10~3A的有害孔所占的比例较小;其抗渗性、抗冻性、抗碳化性能及抗化学腐蚀性等耐久性能良好。通过试验证实了用这种水泥配制的混凝土不会发生因碱集料反应而引起的膨胀破坏。 本研究研制成功的固态碱组分碱矿渣水泥不仅性能优良,而且充分利用工业废渣、成本低廉、生产工艺简单、使用方便,具有重要的实用意义。     

碱激发材料氯离子传输性能测试方法及影响因素研究进展

水泥基材料中氯离子的传输是一个非常复杂的过程。在介绍水泥基材料氯离子传输机理及常用试验方法的基础上,综述了碱激发材料氯离子传输性能测试方法及影响因素。碱激发材料氯离子传输性能受激发剂种类的影响,改变矿渣掺量、碱掺量和水玻璃模数能不同程度地改变体系的氯离子传输性能。快速氯离子渗透试验结果受孔溶液化学组成影响,碱激发材料孔溶液碱性高、化学组成更复杂,孔溶液影响更显著,所以该方法不适用于评价碱激发材料氯离子传输性能。自然扩散试验因时间长而不常用。非稳态电迁移试验是目前快速测试水泥基材料中氯离子传输性能最好的方法,但由于碱激发材料与普通水泥基材料的碱度不同,其变色边界氯离子浓度也会不同,将该方法用于评价碱激发材料时,还需进一步研究测试样品的准备和硝酸银变色边界氯离子浓度。     

碱激发锰渣胶凝材料的探索研究

以锰渣为主要研究对象,采用X射线衍射分析、差热分析等测定方法对原材料进行了物性分析,锰渣的主要矿物组成有SiO_2和CaO,属于碱性废渣,当温度低于550℃时其热稳定性较好.通过对复合碱激发剂的探索可知,当水玻璃模数为1.6时,25%水玻璃、2.5%NaOH和1%K_2CO_3复合激发锰渣后,其碱胶凝材料的凝结时间满足浆体的一般工作要求.在该复合激发剂作用下,以10%硅酸盐水泥熟料等量替代锰渣后,制成的碱激发锰渣胶凝材料的力学强度发展符合胶凝材料的一般规律;其水化过程分析表明,随水化龄期的延长,SiO_2被剥蚀解体量增多,生成较多的C-S-H凝胶及少量沸石类结构复杂的物质,强度逐渐提高.     

碱对粉煤灰的活化和微观结构的影响

用X-射线、SEM研究了粉煤灰在不同碱度环境下的活化机制、水化产物和微观结构。研究表明:粉煤灰硅酸盐水泥在常温下养护,粉煤灰的反应能力较低,这是因为在Ca(OH)2存在条件下的活化很慢,只有提高养护温度或在复合碱和硫酸盐存在条件下才有利于结构的解体和水化产物的稳定。     

Alkali release from aggregate and the effect on AAR expansion

In this study, two types of alkali-bearing aggregate, nepheline syenite and alaskite are investigated on their effects on AAR expansion using a new accelerate test method. Crushed nepheline syenite and alaskite with size between 0.15 mm and 0.65 mm are immersed in different alkaline solutions and cured at 150°C, the amounts of released alkalis from aggregates into solutions are determined. Results indicate that nepheline syenite and alaskite could increase AAR expansion when low alkali cements are used, but the enhance effect is weaken largely when high alkali cements are used. Nepheline mineral is stable in saturation Ca(OH)₂ solution, but decomposes when alkali is added into the solution and form new product. Na-feldspar and K-feldspar are stable in alkaline solution, only little dissolution might be found in the crystal interface. It is concluded that alkalis release from these alkali-bearing minerals mainly depend on ion change between surface layer of minerals and Ca²⁺ ion in solution.     

Pozzolanic reaction of glass powder and its role in controlling alkali–silica reaction

It is well recognized that finely ground soda-lime glass exhibits high pozzolanic reactivity. Fine glass grains will not undergo an Alkali-silica reaction (ASR) in the presence of alkali, and can even mitigate the ASR between alkali and reactive aggregates. Influences of the pozzolanic reaction of glass powder on solid phases, pore solution in cement paste, and the ASR mitigating effect are investigated in the study. The pozzolanic reaction of glass not only consumes portlandite to form in-situ C-S-H, which appears as reaction rim around glass grains, and precipitated C-S-H, but also reduces monosulfate level. The impacts of the pozzolanic reaction on species in pore solution are characterized by increased aluminum, sulfate, sodium, and silicon concentrations and decreased calcium concentration. The increase in aluminum and sulfate concentrations results from the decrease in solid monosulfate. Glass powder controls ASR by increasing aluminum concentration in pore solution to reduce the dissolution of amorphous silica from reactive aggregates.     

How does fly ash mitigate alkali–silica reaction (ASR) in accelerated mortar bar test (ASTM C1567)?

ASTM C1567 [1] is a commonly used accelerated test method to determine the required dosage of supplementary cementitious materials (SCMs) to mitigate alkali–silica reaction (ASR) in mixtures containing reactive siliceous aggregates. Past research suggested that fly ash and other SCMs inhibit ASR, primarily through alkali dilution and binding. In ASTM C1567, however, the alkalinity of the pore solution is largely influenced by the penetration of NaOH from the external soak solution; and this could erase the beneficial effects of alkali dilution and binding. To better understand why fly ash inhibits ASR in this test, the present study performs a quantitative evaluation of six potential ASR mitigation mechanisms: (1) alkali dilution, (2) alkali binding, (3) mass transport reduction, (4) increasing tensile strength, (5) altering ASR gel, and (6) reducing aggregate dissolution rate. The results suggest that (2), (3), (4), and (6) are the primary mitigation mechanisms, while (1) and (5) show a negligible impact.     

A method for predicting the alkali concentrations in pore solution of hydrated slag cement paste

The alkalinity of the pore liquid in hardened cement paste or concrete is important for the long-term evaluation of alkali-silica reaction (ASR) expansion and corrosion prevention of steel bar in steel reinforced structures among others. It influences the reactivity of supplementary cementitious materials as well. This paper focuses on the alkali binding in hydrated slag cement paste and a method for predicting the alkali concentrations in the pore solution is developed. The hydration of slag cement is simulated with a computer-based model CEMHYD3D. The amount of alkalis released by the cement hydration, quantities of hydration products, and volume of the pore solution are calculated from the model outputs. A large set of experimental results reported in different literatures are used to derive the alkali-binding capacities of the hydration products and practical models are proposed based on the computation results. It was found that the hydrotalcite-like phase is a major binder of alkalis in hydrated slag cement paste, and the C?CS?CH has weaker alkali-binding capacity than the C?CS?CH in hydrated Portland cement paste. The method for predicting the alkali concentrations in the pore solution of hydrated slag cement paste is used to investigate the effects of different factors on the alkalinity of pore solution in hydrated slag cement paste.     

Influence of aggregate mineralogy on alkali–silica reaction studied by X-ray powder diffraction and imaging techniques

Reliable assessment of the potential alkali reactivity of aggregate to develop deleterious alkali–silica reaction is essential for construction of durable concrete structures. The potential alkali reactivity of silicified limestone and two limestones has been investigated. Preliminary characterisation of aggregate was performed by optical and environmental scanning electron microscopy. X-ray powder diffraction peak profile analysis was used to predict the aggregates’ potential alkali reactivity. Samples were aged in accordance to the RILEM AAR-2 procedure and further characterised by means of optical and environmental scanning electron microscopy as well as by synchrotron X-ray microtomography, where quantitative analysis relative to damage due to the alkali–silica reaction (ASR) was performed by morphometric analysis of volume data. Results highlight that (1) the microstructural domain size and microstrain values extracted form XRPD line profile analysis seem to be good parameters for predicting the potential alkali reactivity of quartz in aggregate, and (2) the mineralogy of the aggregate influences the weathering products (i.e. aggregate dissolution, ASR gel growth and microcracking) due to ASR in cement-based materials.     

The capacity of ternary blends containing slag and high-calcium fly ash to mitigate alkali silica reaction

The efficiency of ternary blends containing high-calcium fly ash and slag in mitigating alkali-silica reaction (ASR) was evaluated. The concrete prism expansions showed that the ternary blends did not offer significant advantage over binary blends of portland cement and either of the individual material at the same total SCM content. The ability of a particular blend to mitigate ASR was related to its capacity to retain alkalis in its hydration products, as evaluated by an alkali leaching test. For the slag and fly ash used in this study, the capacity to retain alkalis increased with the ability of the blend to consume Ca(OH)₂ during its pozzolanic reaction. For the blends investigated here, the alkali leaching test was more realistic than the accelerated mortar bar test in predicting the 2-year expansion of concrete prisms. The adopted alkali leaching test is proposed to be used as a tool to compare the efficacy of different cementing blends to mitigate ASR.     

Chemical evolution of alkali–silicate reaction (ASR) products: a Raman spectroscopic investigation

The objective of this study was to use Raman spectroscopy to study the morphology and chemical changes of alkali–silicate reaction (ASR) products over time. The reaction products induced by ASR on soda-lime glass slides in a high temperature alkaline environment (1 N NaOH at 80 °C) enriched with calcium hydroxide were studied at 0, 1, 4, 7, 14 and 28 days. The results show that the morphology of the granular-like and fan-like fascicles structure that formed at early ages was more ordered in terms of polymerization and dominated by Q³ and Q² units. This information implies that the ASR products were probably of mainly alkali silica composition with low content of calcium in the structure. As the reaction proceeded the products depolymerized, forming a cloud-like morphology of decreased structural order which surrounded the initial product particles. It can be hypothesized that more calcium ions in the soak solution entered into the particle structure, promoting a depolymerization and possible formation of a C–S–H phase which was indicated by the dominant presence of Q¹ units after 7 days of exposure. To corroborate the interpretation of Raman spectra, 28-day ASR products were verified by Fourier transform infrared.     

An investigation of mortars affected by alkali-silica reaction by X-ray synchrotron microtomography: a preliminary study

A first attempt to investigate samples affected by alkali-silica reaction (ASR) by synchrotron X-ray microtomography has been made. The setup available at the SYRMEP beamline, at the third generation synchrotron Elettra (Trieste, Italy), allowed collecting phase-contrast enhanced images, with a detectability approaching that of optical microscopy (a few microns). In this study, mortar cylinders were prepared and immersed in a 1-M NaOH solution at 80 °C for 14 days to enhance the ASR. The weathered samples were studied using the traditional 2D techniques such as optical microscopy and scanning electron microscopy as well as using the 3D micro-CT. Over the aged samples, the 3D imaging allows the ASR weathering to be studied, showing the reactive aggregate progressive dissolution with subsequent deposition of gel and microcracks development. This technique has proven to be a valuable, non-destructive, method which allows the rendering of the microstructural features in specimen affected by ASR.     

Performance of ground clay brick in ASR-affected concrete: Effects on expansion, mechanical properties and ASR gel chemistry

The effects of ground clay brick (GCB) on alkali-silica reaction (ASR) expansion as well as on mechanical properties of ASR-affected concrete are investigated. Crushed red clay brick originated from demolished masonry was ground in a laboratory ball mill and replaced for portland cement at levels of 15% and 25% by weight in concrete mixes produced with alkali reactive sand. ASR expansion, compressive strength, flexural strength, and modulus of elasticity of the concrete mixes were evaluated. Effect of GCB on ASR gel chemistry was also studied on Pyrex glass-paste specimens using SEM/EDS (scanning electron microscope equipped with energy dispersive X-ray spectroscopy). The results indicate that GCB effectively reduces ASR expansion in concrete: associated cracking and loss on mechanical properties are also significantly reduced. SEM study suggests that GCB alters alkali silica gel chemistry thus resulting in a less expansive product.