Evaluation of laboratory test method for determining the potential alkali contribution from aggregate and the ASR safety of the Three-Gorges dam concrete

The releasable alkali from granite, which was used in the Three-Gorges concrete dam project in China, and from gneiss and feldspar was estimated by extraction in distilled water and super-saturated Ca(OH)₂ solution. Results show that: i) the finer the particles and the higher the temperature, the greater and faster the release of alkali; ii) compared with extraction by distilled water, super-saturated Ca(OH)₂ solution had a stronger activation on feldspar than on granite and gneiss; iii) for the three rocks tested, thermal activation had the largest effect on gneiss and a lower and similar effect on granite and feldspar. For very fine particles, temperature had a similar effect on the release of alkali by all three rocks.Because the aggregate used in the Three-Gorges dam concrete is non-reactive and a low calcium fly ash was used in the concrete, ASR would not be an issue for the dam, despite the release of alkali from the aggregate into the concrete.     

Modified model of alkali-silica reaction

Experimental studies have been carried out for understanding why soft and fluid hydrated alkali silicate generated by the alkali-silica reaction (ASR) of aggregate with alkaline pore solution accumulates the expansive pressure for cracking the aggregate and the surrounding concrete. The elemental analysis of aggregate (andesite) embedded in a cement paste has revealed that the alkali silicate has no ability of generating expansive pressure unless the aggregate is tightly packed with a reaction rim. The reaction rim is slowly generated from the alkali silicate that covers the ASR-affected aggregate. Consumption of alkali hydroxide by the ASR induces the dissolution of Ca²⁺ ions into the pore solution. The alkali silicate then reacts with Ca⁺ ions to convert to an insoluble tight and rigid reaction rim. The reaction rim allows the penetration of alkaline solution but prevents the leakage of viscous alkali silicate, so that the alkali silicate generated afterward by the ASR is accumulated in the aggregate to give an expansive pressure enough for cracking the aggregate and the surrounding concrete. The ASR of very tiny aggregate such as fly ash and municipal waste incinerator bottom ash may not cause the deterioration of concrete, since the ASR is completed before the formation of reaction rims.     

Alkali-silica reaction, pessimum effects and pozzolanic effect

The pessimum proportion and pessimum size effects for alkali-silica reaction-induced deterioration of concrete (ASR) and the pozzolanic effect of fine siliceous admixtures in concrete have been explained based on the proposed ASR model [T. Ichikawa, M. Miura, Modified model of alkali-silica reaction, Cem. Concr. Res. 37 (2007) 1291-1297.]. The attack of alkali hydroxide to aggregate particles composed of ASR-reactive minerals generates the layer of hydrated mature alkali silicate and the layer of less hydrated immature alkali silicate under the mature layer. The mature alkali silicate preferentially reacts with Ca²⁺ ions to convert to fragmental calcium alkali silicate, because the reaction accompanies a significant volume contraction. The immature alkali silicate gradually reacts with Ca²⁺ ions to cover the surface of the reactive minerals with tight layers of calcium alkali silicate called reaction rims. The reaction rims allow the penetration of alkaline solution but prevents the leakage of viscous alkali silicate generated afterward, so that the alkali silicate is accumulated inside the rims to give an expansive pressure enough for cracking the aggregate and the surrounding concrete. Due to the absorption of Ca²⁺ ions by mature alkali silicate, too much increase of the proportion of reactive aggregate causes the deficiency of Ca²⁺ ions for the formation of reaction rims, so that the ASR expansion decreases after passing the pessimum proportion. Very fine reactive aggregate and admixtures with the grain size less than ~ 50 µm preferentially react with alkali hydroxide to convert to mature alkali silicate without leaving any reactive minerals. Homogeneous mixing of the sufficient amount of very fine siliceous admixtures in concrete therefore inhibits the ASR by absorbing Ca²⁺ ions for the rim formation. The resultant fragmental calcium silicate fills the pores in concrete to increase the strength and the durability of the concrete. The admixtures thus act as pozzolanic materials.     

含碱矿物碱性环境下分解的热力学分析

应用热力学方法探讨了298K时钾长石、钠长石、霞石等含碱矿物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-硅酸反应的影响进行了讨论.结果表明:在pH值大于13的碱性环境下,各含碱矿物最大析碱能力顺序依次为:片铝钠石＞白榴石＞霞石＞脱水方沸石＞钠云母＞白云母＞硬玉＞钠长石＞钾长石;随着溶液pH值增大,片钠铝石、霞石分解能力略有降低,而集料中常见的钠长石、钾长石矿物的分解析碱能力则显著提高.此外,根据热力学分析结果,提出降低孔溶液碱度、采用低的水灰比以及控制外部水分进入孔溶液是抑制集料碱析出的有效措施,研究结果为相应的碱-集料反应抑制措施的提出提供了理论依据.     

长石在压蒸条件下的碱溶出

测试了压蒸条件下两种天然长石在水及饱和Ca(OH) 2 溶液中碱溶出情况。结果表明 :长石在水中碱溶出量比较少 ,溶出的碱主要是钠碱。长石在饱和Ca(OH) 2 溶液中碱溶出量比较多 ,溶出的碱包括钠碱和钾碱。集料碱溶出对混凝土碱含量影响比较小     

Influence of acetate and formate-based deicers on ASR in airfield concrete pavements

In the past few years, the issue of external alkalis and their influence on ASR in concrete has become more important since several concrete airfield pavements have shown ASR-distress related to the use of alkali-containing airfield deicers based on acetates and formates.Experiments with model pore solutions and cement pastes as well as speciation calculations and ASR performance tests were conducted to investigate possible mechanisms. The obtained results indicate that the solubility of portlandite is increased in the presence of acetate-based and formate-based deicers due to the formation of Ca-acetate and Ca-formate complexes. The additional release of OH⁻ ions from portlandite and the supply of alkalis can initiate and highly accelerate ASR in concretes with reactive aggregates. There is also evidence for a reaction of ettringite with such airfield deicers.     

Alkali–silica reaction: Kinetics of chemistry of pore solution and calcium hydroxide content in cementitious system

This paper presents the results of the investigations on the chemistry of pore solutions, the contents of calcium hydroxide, and the expansions in mortars containing both reactive and non-reactive aggregates. In order to examine the effect of the temperature, experiments were performed at three different temperatures (23 °C, 38 °C and 55 °C).The compositions of the pore solution were measured at short time intervals for a period of up to 130 days in order to capture the kinetics of the chemistry of pore solution. The results showed that the changes in the concentrations of alkali ions can be best explained by the first order reaction. In addition, the proposed rate equation could reasonably simulate the changes in the actual concentrations of alkalis. Finally, the results in this paper suggest that the rate of the alkali–silica reaction in cementitious system containing highly reactive aggregate can be also expressed as the first order reaction.     

The relevance of laboratory studies on delayed ettringite formation to DEF in field concretes

Many laboratory studies of delayed ettringite formation (DEF) have been conducted on thin mortar bar specimens, heat treated, and then immersed in water. Under these conditions, rapid diffusion of alkali hydroxide into the surrounding water occurs and necessarily reduces the alkali hydroxide concentration of the mortar pore solution. Results reported recently by Famy indicate that the DEF process is triggered as a consequence of such leaching. When it is prevented by immersion into alkali hydroxide solution instead of water DEF expansion is delayed or prevented entirely. Results reported by Zhang indicate that 51-mm mortar cubes behave differently than more leaching-susceptible mortar bars when exposed to the same wet environment. Mortars that show severe DEF as mortar bars remain almost free of DEF symptoms if they are stored as cubes, even after 900 days. Attention is called to the fact that DEF in concrete is found commonly in thick concrete members where the possibility of leaching is remote. For such concrete, the reduction in internal alkali hydroxide concentration that occurs with ASR can substitute for the effect of leaching. It is postulated that without effective reduction of alkali hydroxide concentration by one or the other process, DEF remains latent.     

硅质集料碱活性快速砂浆棒试验方法2.养护碱溶液和砂浆胶砂比与试样膨胀的关系

研究了养护碱溶液性质和胶砂比与硅质集料砂浆试样膨胀的关系。养护碱溶液质量分数愈高、碱离子活度愈高 ,试样膨胀愈大 ;胶砂比的影响比较复杂 ,集料潜在碱活性不同 ,试样对应的敏感胶砂比不一样。     

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 effect of supplementary cementing materials on alkali-silica reaction: A review

This paper reviews studies on the effect of supplementary cementing materials (SCM) on alkali-silica reaction (ASR). SCMs control expansion due to ASR by binding alkalis and limiting their availability for reaction with alkali-silica reactive aggregate. The efficacy of the SCM is dependent on the composition of the SCM. Increased amounts of SCM are required to control ASR as its calcium and alkali content increase, as its silica content decreases, as the alkali contributed by the Portland cement increases and as the reactivity of the aggregate increases. There is evidence that the alumina content of the SCM also affects its alkali-binding capacity, however, the precise role and contribution of the alumina is not clear.     

Chemical Sequence and Kinetics of Alkali‐Silica Reaction Part I. Experiments

The deterioration induced by alkali‐silica reaction (ASR) is initiated by complicated heterogeneous chemical reactions. This study describes the experimental results obtained from the model reactant experiments focused on the kinetics of physical and chemical changes in the reactive aggregate‐simulated pore solution system undergoing ASR. Specifically, the study investigated the products formed by exposing reactive silica mineral (α‐cristobalite) to two alkali solutions in the presence of solid calcium hydroxide [Ca(OH)₂]. The experimental results showed that, as long as the Ca(OH)₂ remains in the system, the dissolution of the silica mineral proceeds at a constant rate and the only reaction product formed is the tobermorite‐type C–S–H. However, once the supply of Ca(OH)₂ in the system is exhausted, the level of dissolved silica ions starts to increase. At the same time, the previously formed C–S–H changes in composition by incorporating silicon and alkali ions from the solution. Continuous increase in the concentration of silica leads to formation of the ASR gel as a result of interaction between silica and alkali ions.     

Effects of externally supplied lithium on the suppression of ASR expansion in mortars

Lithium salts are being externally supplied for mitigating the progress of deterioration of ASR-affected concrete structures. However, it is not clear whether the sodium or potassium in the ASR gel in concrete is replaced by the lithium supplied from the outside. In this article, we examine changes in the composition of the ASR gel, previously formed in mortar specimens, after they are immersed in LiOH solution, using backscattered electron (BSE) imaging and energy-dispersive X-ray (EDX) analysis, associated with length change measurement of the mortar prisms. The intrusion of lithium ions into mortar specimens containing a reactive aggregate could arrest their further expansion within a relatively short time after immersion in 0.50 N LiOH solution. The alkali ions incorporated in most ASR gels, located not far away from interfaces between the cement paste and reactive aggregate particles, appear to be replaced by the lithium ions supplied from the solution. However, the ASR gel within the reacted aggregate particles did not appear to have been affected by the lithium ions.     

Alkali mass balance during the accelerated concrete prism test for alkali-aggregate reactivity

The alkali mass balance was calculated in concrete specimens submitted to the storage conditions of the Canadian standard CSA A23.2-14A concrete prism test for expansion due to alkali-aggregate reaction (AAR). The alkali concentration of both the concrete pore solution expressed under high pressure and the water below specimens in storage pails (bottom water) was measured. Measurements were conducted over a 1-year period, which corresponds to the length of the above test. Two reactive aggregates were tested [Potsdam sandstone (PO) and Spratt limestone (SP)]. Each aggregate was incorporated in two concrete mixtures (mass concrete and structural concrete), for a total of four batches. Significant alkali leaching occurred at 38 °C while performing tests in high moisture storage conditions even though prisms were covered with plastic sleeves. After 52 weeks, the alkali loss ranged from 12% to 25% of the original Na₂O_e content of the concrete, depending on the mixture proportioning and the aggregate type. After estimation of the proportion of alkalis fixed in cement hydrates, it appears that about 23% to 39% of the original alkalis released by the cement are quickly sorbed on aggregate surfaces or have rapidly migrated inside aggregate particles, which may have been incorporated with time in the AAR product. After 52 weeks at 38 °C, the pore solution alkalinity expressed from mass concrete made with PO was 250 mmol/l, whereas the alkalinity was 270 mmol/l in mass concrete incorporating SP. Since prisms of both mixtures were still expanding at 1 year, these alkalinity values are above the thresholds required for sustaining AAR in these concrete mixtures.     

Lithological influence of aggregate in the alkali-carbonate reaction

The reactivity of carbonate rock with the alkali content of cement, commonly called alkali-carbonate reaction (ACR), has been investigated. Alkali-silica reaction (ASR) can also contribute in the alkali-aggregate reaction (AAR) in carbonate rock, mainly due to micro- and crypto-crystalline quartz or clay content in carbonate aggregate. Both ACR and ASR can occur in the same system, as has been also evidenced on this paper.Carbonate aggregate samples were selected using lithological reactivity criteria, taking into account the presence of dedolomitization, partial dolomitization, micro- and crypto-crystalline quartz. Selected rocks include calcitic dolostone with chert (CDX), calcitic dolostone with dedolomitization (CDD), limestone with chert (LX), marly calcitic dolostone with partial dolomitization (CD), high-porosity ferric dolostone with clays (FD). To evaluate the reactivity, aggregates were studied using expansion tests following RILEM AAR-2, AAR-5, a modification using LiOH AAR-5Li was also tested. A complementary study was done using petrographic monitoring with polarised light microscopy on aggregates immersed in NaOH and LiOH solutions after different ages. SEM-EDAX has been used to identify the presence of brucite as a product of dedolomitization. An ACR reaction showed shrinkage of the mortar bars in alkaline solutions explained by induced dedolomitization, while an ASR process typically displayed expansion. Neither shrinkage nor expansion was observed when mortar bars were immersed in solutions of lithium hydroxide.Carbonate aggregate classification with AAR pathology risk has been elaborated based on mechanical behaviours by expansion and shrinkage. It is proposed to be used as a petrographic method for AAR diagnosis to complement the RILEM AAR1 specifically for carbonate aggregate. Aggregate materials can be classified as I (non-reactive), II (potentially reactive), and III (probably reactive), considering induced dedolomitization ACR (dedolomitization degree) and ASR.     

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.     

Alkali binding in cement pastes: Part I. The C-S-H phase

The binding of sodium and potassium into cement paste influences the performance of concrete: for example, alkali balances between solid and paste constituents and pore fluid affect the potential for reaction with alkali-susceptible aggregates. However, quantification of the binding potential into paste solids has proven to be difficult, although much empirical data are available from pore fluid analyses. In this study, single-phase homogeneous C-S-H phases have been prepared at Ca:Si molar ratios of 1.8, 1.5, 1.2, and 0.85 and reacted with six alkali hydroxide concentrations, both NaOH and KOH, between 1 and 300 mM, giving a grid of 48 alkali concentrations and Ca:Si ratios. A steady-state alkali partition is attained in less than 48 h. A distribution coefficient, R_d, was calculated to express the partition of alkali between solid and aqueous phases at 20°C. The numerical value of R_d is independent of alkali hydroxide concentration and depends only on Ca:Si ratio. Approximate reversibility is demonstrated, so the R_d values are constants of a C-S-H over wide ranges of alkali concentration. The trend of R_d values indicates that alkali binding into the solid improves as its Ca:Si ratio decreases.     

Coupled effects of aggregate size and alkali content on ASR expansion

This work is a part of an overall project aimed at developing models to predict the potential expansion of concrete containing alkali-reactive aggregates. First, this paper reports experimental results concerning the effect of particle size of an alkali-reactive siliceous limestone on mortar expansion. Special attention is paid to the proportions of alkali (Na₂O_eq) in the mixtures and reactive silica in the aggregate. Results show that ASR expansion is seven times larger for coarse particles (1.25-3.15 mm) than for smaller ones (80-160 μm). In mortars for which the two size fractions were used, ASR expansion increased in almost linear proportion to the amount of coarse reactive particles, for two different alkali contents. Then, an empirical model is proposed to study correlations between the measured expansions and parameters such as the size of aggregates and the alkali and reactive silica contents. Starting with the procedure for calibrating the empirical model using the experimental program combined with results from the literature, it is shown that the expansion of a mortar containing different sizes of reactive aggregate can be assessed with acceptable accuracy.     

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.