Studies of alkali-silica reaction — part II effect of air-entrainment on expansion

From an extensive petrographic investigation of concrete samples suffering from alkali-silica reaction, it has been hypothesized that a deliberately introduced air-bubble system will reduce expansion due to alkali-silica reaction. The above hypothesis has been tested using mortar bars made from 35 sand types of differing degrees of alkali-silica reactivity. The results show that on the average the introduction of 4% air decreased the expansion by about 40%. A petrographic examination of mortar bars has shown that in the case of reactive sand the air-bubbles tend to get filled up by gel, but the air-bubbles remain empty in the case of unreactive sand. It has also been noted that this filling up of the air-bubbles will decrease their effectiveness in a freezethaw environment.     

A review of alkali-silica reaction and expansion mechanisms 1. Alkalies in cements and in concrete pore solutions

Current concern with alkali silica reactions is due to rising alkali contents of cements, changed concrete technology, and necessity of employing marginal aggregates in many areas, as well as to new reports of field damage coming to light. A brief overall view of the physicochemical basis for alkali-silica reactions is given. The reaction is primarily one of hydroxide ions rather than of alkali cations; nevertheless the latter are of critical importance. Alkalies in cement are found primarily as alkali sulfates or as solid solution substituents in calcium aluminate or in belite. The rates at which these reach the pore solution phase are discussed, and data indicating that concentrations as high as 0.7 molar may be attained in a month or so and maintained indefinitely are discussed. The relationship between alkali cation concentration and hydroxide ion concentrations are explored, and after a few days of hydration published data indicate that the two are substantially equivalent. Thus reacting pore solutions may have hydroxyl ion concentrations of the order of 0.7 molar, more than 15 times that of pure saturated calcium hydroxide solutions.     

A review of alkali-silica reaction and expansion mechanisms 2. Reactive aggregates

Recent literature on the properties of aggregates that take part in alkali-silica or alkali-silicate reactions is reviewed, with special emphasis on those features which are thought to influence the mechanisms and kinetics of the alkali attack.     

Studies of alkali-silica reaction. Part 3. Mechanisms by which NaCl and Ca(OH)2 affect the reaction

Recently it has been observed that a concentrated solution of NaCl accelerates alkali-silica reaction and that the presence of free Ca(OH)₂ is a prerequisite for expansion to occur. This paper reports work done to understand the chemical processes involved.From the results of this investigation following mechanism has been proposed to explain the above observations. In the presence of free Ca(OH)₂, Na⁺ ions from alkali salts and OH⁻ ions from Ca(OH)₂ together with water molecules penetrate reactive grains. SiOSi bonds in reactive grains are broken by penetrating Na⁺ and OH⁻ ions thereby opening the grains for further penetration of materials. At the same time silica ions tend to diffuse out of the reactive grains. Expansion occurs when more materials enter the reactive grains than diffuse out.     

Alteration of alkali reactive aggregates autoclaved in different alkali solutions and application to alkali-aggregate reaction in concrete (II) expansion and microstructure of concrete microbar

The effect of the type of alkalis on the expansion behavior of concrete microbars containing typical aggregate with alkali-silica reactivity and alkali-carbonate reactivity was studied. The results verified that: (1) at the same molar concentration, sodium has the strongest contribution to expansion due to both ASR and ACR, followed by potassium and lithium; (2) sufficient LiOH can completely suppress expansion due to ASR whereas it can induce expansion due to ACR. It is possible to use the duplex effect of LiOH on ASR and ACR to clarify the ACR contribution when ASR and ACR may coexist. It has been shown that a small amount of dolomite in the fine-grained siliceous Spratt limestone, which has always been used as a reference aggregate for high alkali-silica reactivity, might dedolomitize in alkaline environment and contribute to the expansion. That is to say, Spratt limestone may exhibit both alkali-silica and alkali-carbonate reactivity, although alkali-silica reactivity is predominant. Microstructural study suggested that the mechanism in which lithium controls ASR expansion is mainly due to the favorable formation of lithium-containing less-expansive product around aggregate particles and the protection of the reactive aggregate from further attack by alkalis by the lithium-containing product layer.     

Influence of lithium hydroxide on alkali-silica reaction

Several papers show that the use of lithium limits the development of alkali-silica reaction (ASR) in concrete. The aim of this study is to improve the understanding of lithium's role on the alteration mechanism of ASR.The approach used is a chemical method which allowed a quantitative measurement of the specific degree of reaction of ASR. The chemical concrete sub-system used, called model reactor, is composed of the main ASR reagents: reactive aggregate, portlandite and alkaline solution. Different reaction degrees are measured and compared for different alkaline solutions: NaOH, KOH and LiOH.Alteration by ASR is observed with the same reaction degrees in the presence of NaOH and KOH, accompanied by the consumption of hydroxyl concentration. On the other hand with LiOH, ASR is very limited. Reaction degree values evolve little and the hydroxyl concentration remains about stable.These observations demonstrate that lithium ions have an inhibitor role on ASR.     

Preliminary study of effect of LiNO2 on expansion of mortars subjected to alkali-silica reaction

The effect of LiNO₂on the expansion of mortars made completely of reactive aggregate with five particle size fractions was investigated by means of an autoclave test. The results show that the effect of LiNO₂ varies with its content and Na₂O level, or more specifically, with the molar ratio of Li/Na in mortars. When the ratio reaches 0.8 at higher Na₂O level (greater than 2% in this study), ASR expansion is substantially suppressed and the ratio below or beyond this value will give different results. For the other cases of lower Na₂O levels the results are distinguished from this and the best Li/Na molar ratios of 0.1, 0.3 and 0.5 corresponding to the Na₂O levels of 0.5, 1.0 and 1.5%, respectively, are recommended.     

Effects of uniaxial stress on alkali–silica reaction induced expansion of concrete

asr affected concrete in real structures is usually subject to loads which affect the macroscopic expansion of the material. An experimental study was undertaken using sensors embedded in reactive and non-reactive samples loaded on modified creep frames. Numerical analysis was used to link micro-structural damage under the load to expansion. We show that load influences the micro-crack propagation, which changes the shape of the expansion curve.     

Influence of equivalent reactive quartz content on expansion due to alkali silica reaction

In this research, the effect on the reactivity of aggregates containing different quartz crystal sizes (0–10 μm; 10–60 μm, 60–130 μm) has been studied, proposing a unique limit for all of them applied to their weighted sum (Equivalent Reactive Quartz). For this aim, the expansion in mortar bars has been correlated with the content of reactive components, and values of the individual mathematic weights for different crystal sizes have been obtained: 8.3 vol.% of quartz 10–60 μm; and 2.6 vol.% of quartz < 10 μm (corresponding to 0.1% expansion at 14 days). The results show that 60–130 μm crystal size of quartz has a negligible effect on the expansion.These individual limits can be reduced to a unique one equal to 2.6 vol.% applied to the Equivalent Reactive Quartz. The use of this concept allows to evaluate the reactivity of aggregates in which there are simultaneously different reactive quartz crystal sizes.     

Effects of aggregate size and angularity on alkali-silica reaction

The effects of reactive aggregate size and aggregate angularity on alkali-silica reaction (ASR) were studied. An all-in natural reactive aggregate was used. The coarse aggregate particles were crushed to obtain crushed fine particles. The angularity of the aggregate was determined using ASTM C1252 and EN 933-6 methods. ASTM C1260 accelerated mortar bar test was conducted to compare the ASR expansion caused by various aggregate size fractions. The effect of the size of the particles on ASR expansion was studied by replacing each size fraction of the non-reactive aggregate with the reactive aggregate of the same size. In spite of similarity of the chemical and mineralogical compositions, the crushed aggregate caused higher ASR expansion than the natural aggregate in all size fractions. The summation of the expansions of individual reactive size fractions of both aggregates was found to be higher than that of corresponding control mixtures.     

Studies of alkali-silica reaction — part I a comparison of two accelerated test methods

The alkali-silica reactivity of 35 different sand types have been evaluated by both the “German” and NaCl bath methods. The results show that although there is a general correlation between the results of these two methods, the evaluations of sand by the two methods are not identical. Analyses of the procedures show the reasons and indicate that the salt bath method is preferable.The results also indicate that the use of a low alkali cement in conjunction with a reactive aggregate may not protect a concrete sample if sosium salts, from an outside source, can migrate and concentrate in it.     

Role of fibers in controlling unrestrained expansion and arresting cracking in Portland cement concrete undergoing alkali-silica reaction

An experimental study was undertaken to investigate the role of polypropylene or brass-coated steel fibers in controlling unrestrained expansions and delaying and arresting cracking in Portland cement concrete due to alkali-silica reaction. Portland cement concrete and fiber-reinforced concrete (FRC) mixtures were prepared at a w/c ratio of 0.40 using modified Type I cement, reactive fine particles, and coarse limestone aggregates. Prism (5×5×30 cm) and plate (13.5×13.5×3 cm) specimens were prepared and cured for 7 or 28 days before exposure to a special treatment to accelerate ASR. Expansion, time of cracking, and ultrasonic pulse velocity were determined over a treatment period of 65 days using prism specimens. Ultimate cracking pattern and extent were determined after a treatment period of 85 days using plate specimens. The results showed that while fibers did not contribute significantly to controlling pre-cracking and post-cracking expansions, they played a significant role in delaying cracks formation and limiting their extent. Considering its lower cost and content, the performance of polypropylene fibers was superior to that of brass-coated steel ones. The potential of brass-coated fibers in arresting ASR cracking was significantly affected by age of concrete when subjected to treatment.     

Chemo-mechanical modeling for prediction of alkali silica reaction (ASR) expansion

The effect of the size of the aggregate on ASR expansion has already been well illustrated. This paper presents a microscopic model to analyze the development of ASR expansion of mortars containing reactive aggregate of different sizes. The attack of the reactive silica by alkali was determined through the mass balance equation, which controls the diffusion mechanism in the aggregate and the fixation of the alkali in the ASR gels. The mechanical part of the model is based on the damage theory in order to assess the decrease of stiffness of the mortar due to cracking caused by ASR and to calculate the expansion of a Representative Elementary Volume (REV) of concrete. Parameters of the model were estimated by curve fitting the expansions of four experimental mortars. The paper shows that the decrease of expansion with the size of the aggregate and the increase of the expansion with the alkali content are reproduced by the model, which is able to predict the expansions of six other mortars containing two sizes of reactive aggregate and cast with two alkali contents.     

The effects of lithium ions on chemical sequence of alkali-silica reaction

This paper presents the results of the investigation on the effects of Li⁺ ions on the chemical and physical changes in the cementitious system undergoing alkali-silica reaction (ASR). Specifically, this paper focuses on determining which chemical steps of ASR processes are affected by the presence of Li⁺ ions in the pore solution in order to provide better understanding of the role of Li⁺ ions in the mitigation process of ASR.The results presented in this paper strongly support the hypothesis that the Li⁺ ions facilitate the formation of a physical barrier on the surface of reactive silica, and thus prevent further attack on the reactive sites by hydroxyl ions.     

氯化钠废盐制铵碱的气液反应过程强化研究

为了解决氯化钠废盐对环境的危害,将侯德榜制碱法应用于氯化钠废盐资源化处理过程,形成了氯化钠废盐制备碳酸氢钠、氯化铵的工艺。设计了用于氯化钠废盐资源化处理的气升式环流反应器,对氯化钠废盐资源化处理过程中的铵化、碳化等气-液反应过程强化进行了研究。利用计算流体力学（CFD）软件研究了环流反应器内部的流动及传质情况,通过比较实验值与模拟值确认了CFD模拟的可靠性。对氯化钠废盐资源化处理过程中的铵化过程、碳化过程以及后处理过程进行了实验研究。优化实验条件：在表观气速为0.044 4 m/s条件下,铵化过程通氨气的时间为70 min、碳化过程通二氧化碳的时间为105 min、第二次铵化过程通氨气的时间为40 min。在此条件下所得碳酸氢钠的纯度为89.68%、氯化铵的纯度为85.11%,氯化铵经重结晶精制所得产品纯度可达到99.9%,碳酸氢钠和氯化铵产品纯度均符合市场需求。     

The effects of potassium and rubidium hydroxide on the alkali-silica reaction

Expansion of mortar specimens prepared with an aggregate of mylonite from the Santa Rosa mylonite zone in southern California was studied to investigate the effect of different alkali ions on the alkali-silica reaction in concrete. The expansion tests indicate that mortar has a greater expansion when subjected to a sodium hydroxide bath than in a sodium-potassium-rubidium hydroxide bath. Electron probe microanalysis (EPMA) of mortar bars at early ages show that rubidium ions, used as tracer, were present throughout the sample by the third day of exposure. The analysis also shows a high concentration of rubidium in silica gel from mortar bars exposed to bath solutions containing rubidium. The results suggest that expansion of mortar bars using ASTM C 1260 does not depend on the diffusion of alkali ions. The results indicate that the expansion of alkali-silica gel depends on the type of alkali ions present. Alkali-silica gel containing rubidium shows a lower concentration of calcium, suggesting competition for the same sites.     

Use of ternary blends containing silica fume and fly ash to suppress expansion due to alkali-silica reaction in concrete

This paper investigates the effects of cementitious systems containing Portland cement (PC), silica fume (SF) and fly ash (FA) on the expansion due to alkali-silica reaction (ASR). Concrete prisms were prepared and tested in accordance with the Canadian Standards Association (CSA A23.2-14A). Paste samples were cast using the same or similar cementitious materials and proportions that were used in the concrete prism test. Pore solution chemistry and portlandite content of the paste samples are reported. It was found that practical levels of SF with low-, moderate- or high-calcium FA are effective in maintaining the expansion below 0.04% after 2 years. Pore solution chemistry shows that while pastes containing SF yield pore solutions of increasing alkalinity at ages beyond 28 days, pastes containing ternary blends maintain the low alkalinity of the pore solution throughout the testing period (3 years).     

Cathodic reduction of different metal salt solutions Part I: synthesis of metal hydroxides by electrogeneration of base

The yield of metal hydroxides obtained by electrolysis of chloride baths is greater than the yield obtained from nitrate baths showing that the hydrogen evolution reaction (HER) contributes as much to electrogeneration of base as the nitrate reduction reaction. This observation has implications for the fabrication of nickel hydroxide positive electrodes for alkaline secondary cells, and widens the scope of electrosynthesis to include chloride, sulphate and other anion containing -nickel hydroxides and other layered hydroxides. Electrosynthesis of Mg(OH)2 has been investigated as a model synthesis.     

Effect of applied stresses on alkali-silica reaction-induced expansions

The mechanical effects of alkali-silica reaction (ASR) have to be modeled in order to assess the deterioration level and the stability of ASR-damaged concrete structures. Several experimental programs have shown the effects of compressive stresses on ASR-induced strains. The effect is so significant that assessment models have to take into account the modification of ASR expansions due to applied stresses and the consequences on the mechanical response of damaged structures. This paper presents and analyzes measurements performed on concrete specimens subjected to several states of stresses along the three directions (due to applied stresses and to passive restraint). Mechanical calculations show that the volumetric expansion imposed by ASR is constant whatever the stresses conditions. They point out the “expansion transfer” occurring along the directions which are less compressed; thus, the effect of stresses on ASR expansions anisotropy can be precisely quantified.