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。     

不同掺量碱集料的活性交流阻抗谱

本文以交流阻抗谱、膨胀率和MIP为研究方法,对不同掺量碱集料活性进行研究。结果表明:交流阻抗谱电化学参数与材料孔隙微观结构有着密切的联系。电阻Rs以及相角θ能表示碱集料活性的大小,可作为碱集料活性的评定方法;当混凝土中活性SiO2含量较高时,孔溶液中OH-离子会被活性SiO2颗粒大量吸附,导致参与碱硅酸反应的碱离子不足,碱硅酸反应生成的碱硅酸凝胶体数量减少。在碱硅酸反应中,活性碱集料与非活性集料之间的比值存在最劣点。     

碱集料反应危害及其预防措施

本文阐述了水泥混凝土碱集料反应（AAR）对水泥混凝土产生的危害,并从碱的来源、破坏原因的诊断、预防措施及如何判断混凝土发生碱集料破坏的可能性几方面进行分析,从而提出预防碱集料反应发生的主技术措施。     

长石类矿物析碱对碱-硅酸反应的影响

应用热力学方法探讨了298K时钾长石、钠长石、霞石在碱性环境下的碱析出能力,并对这些含碱矿物分解析出的碱对碱-硅酸反应(ASR)的影响进行了讨论.结果表明,由于矿物类型不同,碱性环境下的分解难易程度不同,其中霞石较易分解,钠长石次之,钾长石较难分解;随着溶液pH值增大,除霞石分解能力略有降低外,钠长石、钾长石的分解能力均显著提高.此外,通过加速实验方法证实含有霞石、钠长石、钾长石矿物的集料在使用低碱水泥条件下能显著促进ASR膨胀.     

混凝土碱骨料反应及力学性能细观模拟

混凝土碱骨料反应是降低混凝土耐久性的主要因素之一,充分认识其对混凝土力学性能的劣化机理,有助于防治碱骨料反应对混凝土结构的危害。该文用颗粒元(PFC2D)方法建立了由骨料、砂浆和界面三相共同组成的混凝土细观模型,模拟了碱骨料反应引起的混凝土膨胀变形与微裂缝扩展过程,并对碱骨料反应劣化后的混凝土试件进行劈裂抗拉试验,研究碱骨料反应对混凝土力学性能的影响。数值模拟结果表明,该模型可有效预测混凝土碱骨料反应膨胀趋势,碱骨料反应显著降低了混凝土的劈裂抗拉强度。     

高性能混凝土中碱骨料反应的抑制措施研究

本文介绍了碱骨料反应的定义以及碱骨料反应发生的条件、对比了国内外抑制碱骨料反应的主要措施以及用于高性能混凝土中的抑制措施研究状况，通过比较可以得出抑制碱骨料反应的主要措施为：（1）使用非活性集料；（2）控制水泥和混凝土碱含量（Na2O，Na2O＋0．658K2O）；（3）控制湿度；（4）使用混合材或化学外加剂。     

高性能混凝土中碱骨料反应的抑制措施研究

本文介绍了碱骨料反应的定义以及碱骨料反应发生的条件、对比了国内外抑制碱骨料反应的主要措施以及用于高性能混凝土中的抑制措施研究状况，通过比较可以得出抑制碱骨料反应的主要措旌为：（1）使用非活性集料：（2）控制水泥和混凝土碱含（Na2O，Na2O＋0．658K2O）;（3）控制湿度;（4）使用混合材或化学外加剂。     

压蒸条件下热碱溶液中白云石的稳定性1．反应的热力学基础

通过计算热碱溶液中白云石与岩石中的杂质成份及水泥水化产物间可能发生的化学反应的平衡条件来评估压蒸条件下热碱溶液中白云石的稳定性。热碱溶液中白云石与石英、微斜长石、滑石、硬石膏、氢氧化钙或ＯＨ－有可能发生化学反应，白云石与白云母、绿泥石或水硅钙石之间的反应在热力学上是不可能的。     

人工神经网络在碱—集料反应中的应用

碱－集料反应使混凝土体积膨胀，严重时会导致混凝土的破坏，影响碱－集料反应的因素很多，混凝土的蒸压又加速了一反应。本文应用人工神经元网络这样一种以样本集体系为基础的因果关系手段，预测诸因素的影响和膨胀率之间的关系，其结果和实测值比较具有较高的精度。     

混凝土材料损伤机理(英文)

本文分别在现场以及试验室内研究了某机场跑道混凝土出现严重的开裂和剥落现象的原因。碱集料反应以及冻融循环均会导致开裂和剥落,通过XRD、岩相分析以及集料的碱活性检测排除了发生碱集料反应的可能性。采用快冻法测试了机场跑道现场钻取的芯样混凝土的抗冻性。同时借助于显微镜、SEM、MIP分析了损伤混凝土的微观结构,揭示出跑道混凝土的损伤与抗冻性较差有关,并讨论了改善严酷地区混凝土耐久性的措施。     

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.     

Investigation of structural properties associated with alkali–silica reaction by means of macro- and micro-structural analysis

Structural properties associated with alkali–silica reaction were systematically investigated by means of macro-structural accelerated mortar prism expansion levels testing, combined with micro-structural analysis. One part of this study is to determine the reactivity of the aggregate by means of accelerated mortar bar tests, and also to evaluate perlite aggregate constituents, especially the presence of deleterious components and find main causes of the alkali–silica reaction, which was based on the petrographic studies by optical microscope and the implication of X-ray diffraction on the aggregate. Results implied that the aggregate was highly alkali–silica reactive and the main micro-crystalline quartz-intermediate character and matrix that is mainly composed of chalcedony are potentially suitable for alkali–silica reaction. The other part is to study the long-term effect of lithium salts against alkali–silica reaction by testing accelerated mortar prism expansion levels. The macro-structural results were also consistent with the micro-structural mechanisms of alkali–silica reaction of mortar prisms containing this aggregate and the effect of chemical admixtures by means of the methods of scanning electron microscope–X-ray energy-dispersive spectroscopy and X-ray diffraction. It was indicated by these techniques that lithium salts, which were introduced into concrete containing reactive aggregate at the mixing stage, suppressed the alkali–silica reaction by producing non-expansive crystalline materials.     

The effect of meta-halloysite on alkali–aggregate reaction in concrete

Damage to concrete structures may occur as a result of internal effects. Alkali silica reaction (ASR) is a long term reaction between alkalis and reactive aggregate present in the concrete. The reaction product is sodium–potasium–calcium silica gel, able to absorb water, resulting in the expansion and cracking of concrete. The key problem is to find the right method for mitigating the internal damage. This paper presents the results of an investigation into the effectiveness of calcined halloysite (meta-halloysite) in improving the resistance to alkali-silica reaction (ASR). The pozzolanic reactivity of meta-halloysite was also evaluated using Thermo-Gravimetric Analysis. Microstructures of mortar bars were observed by Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (EDS) to investigate the location and chemical composition of ASR gel. The results from this study showed satisfactory level of pozzolanic reactivity when cement was partially replaced by meta-halloysite. It was demonstrated that a 20% addition of meta-halloysite are able to mitigate ASR and lower expansion of mortar bars with reactive aggregate to a safe level of not more than 0.1% at 14 days. Microstructural observations of the specimens containing meta-halloysite indicated the presence of a calcium–alkali–silicate–hydrate gel. But fewer reaction products and with different composition than those forming in the pastes without mineral additives are present.     

Effect of environmental conditions on expansion in concrete due to alkali–silica reaction (ASR)

The environmental conditions to which a concrete element incorporating alkali–silica reactive aggregates is exposed play a major role in dictating the progression and manifestation of the reaction. This paper reports and analyses the results of research programs investigating the comparative evaluation of the effect of environmental conditions on the development of alkali–silica reaction (ASR) in concrete specimens stored in outdoor exposure at the authors' respective laboratories. This data is compared to samples subjected to laboratory testing (controlled environmental conditions). Concrete mixtures that incorporate reactive aggregates with varying reactivity levels are compared. The focus of this paper is on control concretes e.g. 100% opc + reactive aggregate (coarse or fine).     

Mineralogy,geochemistry and expansion testing of an alkali-reactive basalt from western Anatolia,Turkey

In this paper, the alkali–silica reaction performance of a basalt rock from western Anatolia, Turkey is reported. It is observed that the rock causes severe gel formation in the concrete microbar test. It appears that the main source of expansion is the reactive glassy phase of the basalt matrix having approximately 70% of SiO₂. The study presents the microstructural characteristics of unreacted and reacted basalt aggregate by optical and electron microscopy and discusses the possible reaction mechanism. Microstructural analysis revealed that the dissolution of silica is overwhelming in the matrix of the basalt and it eventually generates four consequences: (1) Formation of alkali–silica reaction gel at the aggregate perimeter, (2) increased porosity and permeability of the basalt matrix, (3) reduction of mechanical properties of the aggregate and (4) additional gel formation within the aggregate. It is concluded that the basalt rock is highly prone to alkali–silica reaction. As an aggregate, this rock is not suitable for concrete production.     

Development of a universal accelerated test for alkali-silica and alkali-carbonate reactivity of concrete aggregates

A universal accelerated test for both alkali-silica and alkali-carbonate reactivity was proposed based on extensive comparative studies on existed Accelerated Mortar Bar Test (AMBT), (e.g., ASTM C1260, CSA A23. 2–25A, RILEM TC191-ARP-AAR02) and Chinese accelerated procedures. A single size fraction of 2.5–5.0 mm aggregate particles is used in the test instead of five-graded requirements in the AMBT, and 0.15–0.80 mm fine particles for ASR, 5–10 mm particles for ACR in existed Chinese accelerated tests. Three short-fat bars, 40 × 40 × 160 mm, made at fixed cement-aggregate ratio of 1:1, and water-cement ratio of 0.33 are used and the length change of the bars is monitored till 28 days in 1 M NaOH solution at 80°C after being soaked in 80°C water for 24 h. Over 40 kinds of aggregates from various origins, which include both ASR and ACR aggregates and show a broad range of reactivity levels in the concrete prism test (CPT), were used to evaluate the reliability of the new test in this study. Experimental results indicate that, for ASR aggregates, the new test gives a better indication than the AMBT does of both the reactive/nonreactive characteristic and reactive levels of almost all tested aggregates based on an acceptance criteria of 0.093% at 14 days, although some very highly reactive aggregates show low expansions relative to the CPT. The “abnormal” low expansion of some highly reactive aggregate in the test is mainly due to the rapid formation and loss of fair amount of low viscosity ASR product into the soaking alkali solution. The results on some typical ACR aggregates usually undetected by the AMBT show that the new test gives the same outcome as using 5–10 mm particles in the Chinese Accelerated Concrete Microbar Test for ACR aggregates and is in agreement with the CPT, which suggests that it has good potentials to be used for ACR aggregate when an expansion criteria of 0.1% after 28 days is used.     

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.     

Chemistry of alkali–silica reaction and testing of aggregates

Engineers and real state owners are demanding that concrete aggregates should be tested for their alkali–silica reactivity, before their use, by some accelerated test method. An analysis of the consumer’s requirements shows that actually two, contradictory, demands are made on the test methods. These are: (a) to ascertain alkali–silica reactivity of an aggregate in a reasonably short time and (b) to evaluate and set an acceptance limit of long-term expansion of a reactive aggregate–cement combination. This contradiction is often ignored in national standards and only one test method is specified. However, it is known that concrete expansion, due to alkali–silica reaction, could be suppressed by using sufficient quantity of a pozzolan even when the concrete is exposed to a strong alkaline solution.Most of the available test methods for the evaluation of alkali–silica reactivity of aggregates are empirical. In this paper, I propose to examine different test methods in the light of recent fundamental understanding of the mechanism of alkali–silica reaction and expansion. The emphasis is on the alkali–silica reactivity of aggregates themselves and not on the acceptance limit. The mechanism of alkali–silica reaction and expansive pressure generation suggest that the most appropriate condition of testing is to expose the test specimens to a solution of moderate hydroxyl ion concentration, preferably Ca(OH)₂ solution, and high ionic strength. The high ionic strength should be achieved by adding sufficient quantities of an “indifferent” electrolyte like a neutral alkali salt to the alkaline solution.The fundamental understanding of the mechanism of alkali–silica reaction suggests a simple chemical test method for the evaluation of aggregates themselves. This method has been tested with Danish aggregates and an acceptance criterion has been suggested. Preliminary experiments show that the reactivity of samples of Japanese andesite, British Greywacke, Swedish porphyritic rhyolite, Norwegian mixed aggregate of rhyolite and quartzite and a silicified limestone aggregate from Belgium could be detected within 24 h. This is one of the quickest methods for the evaluation of alkali–silica reactivity of aggregates.     

Alkali-aggregate behaviour of alkali-activated slag mortars: Effect of aggregate type

The alkali–silica reaction in waterglass-alkali-activated slag (waterglass-AAS) and ordinary Portland cement (OPC) mortars was evaluated using three types of (siliceous and calcareous) aggregates. The tests were conducted to the ASTM C1260-94 standard test method. The mortars were studied by volume stability, mechanical strength and Hg intrusion porosity. The ASR products were studied with XRD, FTIR and SEM/EDX techniques.According to the results obtained, under the test conditions applied in this study, waterglass-AAS mortars are stronger and more resistant to alkali-aggregate reactions than OPC mortars. When the mortars were made with a reactive siliceous aggregate, expansion was four times greater in the OPC than in the AAS material. When a reactive calcareous (dolomite) aggregate was used, no expansion was detected in any of the mortars after 14 days, although the characterization results showed that the dolomite had reacted and calcareous-alkali products (brucite) had in fact formed in both mortars. These reactive processes were more intense in OPC than in AAS mortars, probably due to the absence of portlandite in the latter. When the calcareous aggregate was non-reactive, no expansions were observed in any of the mortars, although a substantial rise was recorded in the mechanical strength of AAS mortars exposed to the most aggressive conditions (1 M NaOH and 80 °C).     

Modelling of expansion induced by ASR — New approaches

The mechanism of ASR expansion has been approached by thermodynamics and kinetics of the chemical reaction considering the diffusion law and by probabilistics of random gradients of alkalis and silica reactive sites and local formation of gel. Induced mechanical effects are elucidated through: (1) the capillary pressure relating the volume of expansive gels dissipated in connected porous zones to the initiation of cracks the linear fracture mechanics applied to the propagation and orientation of initiated cracks; and (2) a law of proportionality between confinement, temperature, relative humidity and reactivity describing the non-uniform anisotropic three-dimensional (3-D) distribution of the AAR expansion. Numerical results are very close to experimental data for the pessimun content of reactive silica, the evolution of linear expansions vs the consumed alkalis and the benefit of an isotropic confinement on expansion.