Studies on lithium salts to mitigate ASR-induced expansion in new concrete: a critical review

This paper provides a critical review of the research work conducted so far on the suppressive effects of lithium compounds on expansion due to alkali-silica reaction (ASR) in concrete and on the mechanism or mechanisms by which lithium inhibits the expansion. After a thorough examination of the existing literature regarding lithium salts in controlling ASR expansion, a summary of research findings is provided. It shows that all the lithium salts studied, including LiF, LiCl, LiBr, LiOH, LiOH·H₂O, LiNO₃, LiNO₂, Li₂CO₃, Li₂SO₄, Li₂HPO₄, and Li₂SiO₃, are effective in suppressing ASR expansion in new concrete, provided they are used at the appropriate dosages. Among these compounds, LiNO₃ appears to be the most promising one. Although the mechanism(s) for the suppressive effects of lithium are not well understood, several mechanisms have been proposed. A detailed discussion about these existing mechanisms is provided in the paper. Finally, some recommendations for future studies are identified.     

Effects of lithium salts on ASR gel composition and expansion of mortars

Suppression of alkali-silica reaction (ASR) expansion in mortar and concrete by the addition of lithium salts has been confirmed by some workers. It has been revealed that lithium hydroxide tended to reduce the reaction between sodium or potassium hydroxide and reactive silica, and that the ASR gel incorporating lithium was less expansive. However, it has not been reported how the addition of a lithium salt influenced the composition of the ASR gel. The calcium in ASR gel is considered to play an important role in the expansion of the gel. Thus, it is significant to characterize ASR gel composition in mortars containing lithium salts by BSE-EDS analysis. This study aims to discuss the mechanisms of suppression of ASR expansion in mortar by lithium salts from the viewpoint of ASR gel composition. The average CaO/SiO₂ ratio in ASR gels decreased with increasing amount of added lithium salts. It should be noted that the extent of variations in the CaO/SiO₂ ratio in ASR gels significantly decreased with increasing amount of lithium salts. The addition of relatively small amounts of LiOH and Li₂CO₃ resulted in increased expansion. We also obtained an unexpected result that ASR gels became homogeneous with respect to their CaO contents at high dosage levels. However, the reduction in average CaO/SiO₂ ratios and the homogenization in the CaO content of ASR gels due to the addition of lithium salts may not be related to the expansion of mortars.     

Mitigation of ASR by the use of LiNO3—Characterization of the reaction products

The influence of the LiNO₃ on the ASR product was studied both in a model system and in mortars. In the model system, the addition of LiNO₃ decreases the dissolution rate and the solubility of silica. Lithium changes the 2-dimensional cross-linked (Q₃ dominated) network of the ASR product into a less structured, Q₂ dominated product, likely by adopting the role of calcium. In the mortar samples the addition of LiNO₃ decreases expansion and significantly influences the chemical composition and the morphology of the reaction product. Lithium decreases the calcium, sodium and potassium content and changes the relatively porous plate-like reaction product into a dense one without texture. The findings in the mortars indicate that the ASR-suppressing effect of lithium is caused by the lower potential of the reaction product to swell. Furthermore, it forms a protective barrier after an initial reaction slowing down ASR.     

Experimental investigation of the mechanisms by which LiNO3 is effective against ASR

Various series of experiments were carried out on cements pastes, concretes made with a variety of reactive aggregates, composite specimens made of cement paste and reactive aggregate particles, and a variety of reactive natural aggregates and mineral phases immersed in various Li-bearing solutions. The main objective was to determine which mechanisms(s) better explain(s) the effectiveness of LiNO₃ against ASR and variations in this effectiveness as well with the type of reactive aggregate to counteract. The principal conclusions are the following: (1), the pH in the concrete pore solution does not significantly decrease in the presence of LiNO₃; (2), the concentration of silica in the pore solution is always low and not affected by the presence of LiNO₃, which does not support the mechanism relating to higher solubility of silica in the presence of lithium; (3), the only reaction product observed in the LiNO₃-bearing concretes looks like classical ASR gel and its abundance is proportional to concrete expansion, thus is likely expansive while likely containing lithium; this does not support the mechanisms relating to formation of a non or less expansive Si-Li crystalline product or amorphous gel; (4), early-formed reaction products coating the reactive silica grains or aggregate particles, which could act as a physical barrier against further chemical attack of silica, were not observed in the LiNO₃-bearing concretes, but only for a number of reactive materials after immersion in 1 N LiOH at 350 °C in the autoclave (also at 80 °C for obsidian); (5), higher chemical stability of silica due to another reason than pH reduction or early formation of a protective coating over the reactive phases, is the mechanism among those considered in this study that better explains the effectiveness of LiNO₃ against ASR.     

Laboratory study of LiOH in inhibiting alkali-silica reaction at 20 °C: a contribution

At 20 °C, alkali-aggregate reaction (AAR) expansion of mortar incorporated zeolitization perlite could be long-term effectively inhibited by LiOH and the effect increased with the augment of Li/(Na+K) molar ratio. Mortar strength would decrease when LiOH was added. The more LiOH was added, the more the strength would decrease. In addition, there was more effect on 28 days' strength than 3 days', and the influence degree of LiOH to compressive strength was higher than that to flexural one. The initial and final setting times of cement were shortened when LiOH was added, and the more Li/(Na+K) molar ratio of LiOH was added, the more the setting time was cut down. Not only mortar bar expansion, the change in 20 °C, but also, the evidence of reaction and the composition of reaction products after 4-year curing was studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was found that when both Li⁺ and K⁺ (Na⁺) were added, more Li⁺ reacted to form some matter that not as the same as normal alkali-silica reaction (ASR) gel, especially for its nonexpansive property. Such might be the main reason of the phenomenon that ASR expansion could be inhibited by adding lithium compounds.     

Summary of research on the effect of LiNO3 on alkali-silica reaction in new concrete

This paper summarizes findings from a research study conducted at the University of New Brunswick in collaboration with the University of Texas at Austin, and CANMET-MTL, on the effect of LiNO₃ on ASR in new concrete. The studies included expansion testing, silica dissolution measurements and microstructural examinations of cement systems containing glass and two different reactive aggregates (NB and NS). Only a small proportion of the data are presented here for the purpose of highlighting the principal findings of this investigation.Based on these findings, it is proposed that the inhibiting effect of LiNO₃ against ASR in new concrete is attributed to the formation of two reaction products in the presence of lithium, these being a crystalline lithium silicate compound (Li₂SiO₃) crystal and a Li-bearing, low Ca silica gel. These two phases could serve as a diffusion barrier and protective layer to prevent the reactive silica from further attack by alkalis.It was found that the reason the two reactive aggregates selected responded differently to LiNO₃ was due to the difference in their textural features. The NB aggregate contained reactive volcanic glass particles, the surface of which was immediately and equally available to sodium, potassium and lithium, and thus a Li-Si barrier was able to form quickly. The reactive phase in the NS aggregate was microcrystalline and strained quartz, which was embedded in a dense matrix of a non-reactive predominantly alumino-silicate phase and was not easily accessible to lithium.     

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-silica reactivity of some frequently used lightweight aggregates

Lightweight aggregates (LWAs) are frequently used in concrete as well as in thermally insulating mortars and grouts, so that information on their alkali-silica reactivity (ASR) is very important. Four LWAs—expanded vermiculite, expanded clay, expanded glass and perlite—were studied regarding their ASR, using the following test methods: the accelerated mortar bar test (ASTM C 1260), the rapid chemical test (ASTM C 289) and the combined scanning electron microscopy-energy dispersive X-ray technique (SEM-EDX). According to these methods, neither the expanded vermiculite nor the expanded clay exhibited any potential ASR. On the other hand, in the case of the aggregates containing a glassy phase (expanded glass and perlite), the results of SEM-EDX analysis showed serious decomposition of aggregate texture due to ASR, although no deleterious expansion was observed in the accelerated mortar bar test. Therefore, suitable test criteria for ASR need to be defined for LWAs of this type when the AMBT method is used, as has already been suggested for slowly reactive aggregates in Australia.     

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.     

Alteration of alkali reactive aggregates autoclaved in different alkali solutions and application to alkali-aggregate reaction in concrete: (I) Alteration of alkali reactive aggregates in alkali solutions

Surface alteration of typical aggregates with alkali-silica reactivity and alkali-carbonate reactivity, i.e. Spratt limestone (SL) and Pittsburg dolomitic limestone (PL), were studied by XRD and SEM/EDS after autoclaving in KOH, NaOH and LiOH solutions at 150 °C for 150 h. The results indicate that: (1) NaOH shows the strongest attack on both ASR and ACR aggregates, the weakest attack is with LiOH. For both aggregates autoclaved in different alkali media, the crystalline degree, morphology and distribution of products are quite different. More crystalline products are formed on rock surfaces in KOH than that in NaOH solution, while almost no amorphous product is formed in LiOH solution; (2) in addition to dedolomitization of PL in KOH, NaOH and LiOH solutions, cryptocrystalline quartz in PL involves in reaction with alkaline solution and forms typical alkali-silica product in NaOH and KOH solutions, but forms lithium silicate (Li₂SiO₃) in LiOH solution; (3) in addition to massive alkali-silica product formed in SL autoclaved in different alkaline solutions, a small amount of dolomite existing in SL may simultaneously dedolomitize and possibly contribute to expansion; (4) it is promising to use the duplex effect of LiOH on ASR and ACR to distinguish the alkali-silica reactivity and alkali-carbonate reactivity of aggregate when both ASR and ACR might coexist.     

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.     

Formation of ASR gel and the roles of C-S-H and portlandite

Experimental investigations of the reactions between silica, alkali hydroxide solution, and calcium hydroxide show that alkali-silicate-hydrate gel (A-S-H) comparable to that formed by the alkali-silica reaction (ASR) in concrete does not form when portlandite or the Ca-rich, Si-poor C-S-H of ordinary portland cement (OPC) paste is available to react with the silica. Under these conditions, we observe either the formation of additional C-S-H by reaction of Ca(OH)₂ with the dissolving silica or the progressive polymerization of C-S-H. The A-S-H dominated by Q³ polymerization forms only after portlandite has been consumed and the C-S-H polymerized. These conclusions are consistent with previously published results and indicate that the ASR gel of concrete forms only in chemical environments in which the pore solution is much lower in Ca and higher in Si than bulk pore solution of OPC paste. These results highlight the similarity between ASR and the pozzolanic reaction and are supported by data for mortar bar specimens.     

Long-term effectiveness and mechanism of LiOH in inhibiting alkali-silica reaction

A practical alkali reactive aggregate-Beijing aggregate was used to test the long-term effectiveness of LiOH in inhibiting alkali-aggregate reaction (AAR) expansion. In this paper, the most rigorous conditions were so designed that the mortar bars had been cured at 80 °C for 3 years after being autoclaved for 24 h at 150 °C. At this condition, LiOH was able to inhibit long-term alkali-silica reaction (ASR) expansion effectively. Not only was the relationship between molar ratio of n(Li)/n(Na) and the alkali contents in systems established, but also the governing mechanism of such effects was studied by SEM.     

New observations on the mechanism of lithium nitrate against alkali silica reaction (ASR)

In the current study, in order to elucidate the mechanisms for the favorable effects of lithium nitrate in controlling alkali silica reaction (ASR), vycor glass disk immersion specimens and glass disk-cement paste sandwich specimens were prepared and examined by XRD, SEM and Laser Ablation Induction Coupled Plasma Mass Spectrometry (LA-ICP-MS). Results showed that when glass disk was immersed in only NaOH solution, the glass was attacked by hydroxyl ions but no solid reaction product was found, thus the presence of calcium was essential for the formation of ASR gel. In the presence of lithium, the glass surface was covered by a thick layer of Li-Si crystal. With the addition of Ca(OH)₂, the glass surface was completely covered by Li-Si crystal and a lithium-bearing low Ca-Na-(K)-Si gel. These two phases either form a dense matrix with Li-Si crystal serving as the framework, and the gel filling in the void space, or the Li-Si crystal serving as the foundation to completely cover the entire reactive SiO₂ surface, and the gel sitting on top of these crystal particles. Hence, the suppressive effects of LiNO₃ were attributed to the formation of a layer of Li-Si crystals intimately at the reactive SiO₂ particle surface and the formation of Li-bearing low-Ca ASR gel products. The Li-bearing low-Ca ASR gels may have a dense and rigid structure, thus having low capacity to absorb moisture from the surrounding paste, and exhibiting a non-swelling property.     

Expansion of siliceous and dolomitic aggregates in lithium hydroxide solution

Autoclave expansion behaviors of siliceous and dolomite-bearing aggregates in LiOH and KOH solutions were studied. The results show that lithium hydroxide can suppress ASR expansion and induce ACR expansion. It is the duplex effect of lithium hydroxide that could be used for exploring the mechanism responsible for the expansion of dolomite-bearing aggregates in alkali environments. It has been shown that the expansion of argillaceous dolomite limestone with typical texture from Kingston, Ontario, Canada, can be attributed to ACR rather than to ASR. However, some other argillaceous dolomite limestones exhibit both ACR and ASR. Meanwhile, XRD detected that solid products of dedolomitization in LiOH solution were brucite, calcium carbonate and lithium carbonate with a consequent solid volume increase.     

Microstructure of selected aggregate quartz by XRD,and a critical review of the crystallinity index

Reliable assessment of the potential of quartz in aggregate to develop deleterious alkali–silica reaction (ASR) is essential for the construction of durable concrete. The crystallinity index for quartz (QCI) introduced by Murata and Norman [15] has been applied to predict the ASR potential of quartz. Despite a number of technical shortcomings and omissions in the original paper, the method has arguably become the most popular alternative for the ‘petrography + expansion testing’ combo.This paper investigates the ASR potential of twelve Italian concrete aggregates, by petrography, mortar bar expansion testing, and test the quartz potential reactivity by calculating the QCI and by the line profile analysis of the XRD pattern. The results confirm that a relationship between QCI values and aggregate expansion behavior is absent. Contrary, the microstructural analysis is a powerful method for predicting the ASR-reactivity of quartz. Finally, the method introduced by Murata and Norman [15] is critically reviewed.     

Residual mechanical properties of steel fiber reinforced concrete damaged by alkali silica reaction and subsequent sodium chloride exposure

Infrastructures treated with de-icing salts and those which are in direct contact with sea water are subjected to degradation by chloride ingress. Concrete composed of reactive sources of silica and used near such regions can suffer from both, alkali silica reaction (ASR) and chloride ingress subsequently. This research aims at empirically investigating the residual mechanical properties of plain and steel fiber reinforced concrete damaged by alkali silica reaction (ASR) and subsequent chloride ion ingress. Accelerated degradation tests on three concrete mixes such as plain concrete (PC,control), steel fiber reinforced concrete (SFRC) and high strength fiber reinforced concrete (HSFRC) were done. Specimens were initially damaged by ASR, and then submerged in chloride solution at temperature ranges of 5 ^oC, 25 ^oC and 40 ^oC. 1 mol/L NaOH solution and 3% NaCl solution were used for a period of 20 and 40 weeks. Steel fibers were found to be effective in reducing surface crack widths at 5 ^oC and 25 ^oC. Accelerated mortar bar test showed that steel fibers were able to reduce expansion by 31.5% and 65.3% using single and double hooked fibers. By examining the residual compressive and flexure strengths, it was found that exposure to chloride environment aided in hydration reaction which counter-balanced the damage due to ASR. Fiber-matrix bonding developed over time inducing friction which led to higher ductility and less damage in flexure strength in steel fiber reinforced concrete prisms.     

用溶胶-凝胶膨胀法对锂化合物抑制碱-硅酸反应膨胀机理的研究

利用溶胶-凝胶膨胀法对锂化合物在碱硅酸反应中膨胀的抑制机理进行了研究,对加入锂盐后的碱-硅酸反应产物的膨胀量进行了测定,并借助扫描电镜对试样的微观形貌进行了观察,同时还测定了反应后溶液中SiO2的含量,证实了锂化合物的作用在于：抑制骨料中活性SiO2的溶出;改变凝胶产物的性质,使凝胶的吸水能力和膨胀量变小。     

Formulating Portland cement–reactive alumina blend through thermodynamic modeling to prevent the alkali–silica reaction

This study aims to formulate solid and aqueous compositions of hydrated Portland cement incorporated with reactive alumina (RA) via thermodynamic modeling, with the goal to prevent alkali–silica reaction (ASR). Based on SEM/EDS point analysis, the uptake of Al into C–S–H was corrected in the thermodynamic modeling to improve the accuracy. The predicted solid and aqueous compositions were validated by means of XRD, TGA, and ICP. An accelerated mortar bar test was also carried out to confirm the ASR mitigating efficiency, and a preliminary relationship among the ASR expansion, solid assemblage, and aqueous species was established. The critical content for RA is around 5.75% in the studied systems, above which katoite precipitates, resulting in increased Al concentration in the pore solution (∼0.8 mmol/L); hence, ASR can be effectively inhibited.     

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.