Studies of alkali-silica reaction. Part 4. Effect of different alkali salt solutions on expansion

It has been shown that the extent of expansion due to alkali-silica reaction depends on the type of alkali compound and its concentration in the sorrounding liquid phase. It is of particular interest to note that a 3N alkali hydroxide solution causes less expansion than a 3N alkali salt solution. The result also showed that the expansion due to alkali-silica reaction is not directly proportional to the degree of chemical reaction. These results, though at variance to a generally held belief, are consistent with a recently proposed mechanism of alkali-silica reaction. The results indicate that in an accelerated alkali-silica test the use of an alkali salt is preferable to an alkali hydroxide.     

Influence of mineral admixtures on the alkali-aggregate reaction

Different mineral admixtures were used as cement replacements to study the alkali-aggregate reaction between quartzite and cement paste. The experimental program included expansion measurement of the mortar-bar specimens, observation of the microstructure, and analysis of the gel composition. Two Portland cements with different alkali contents were used to prepare mortar bars containing different amounts of natural pozzolan, fly-ash, and slag. Three natural pozzolans with different alkali contents were also utilized to assess the effect of their alkalies on the mortar expansion. The results indicate that not all gels are equally expansive and that mineral admixtures change the composition of the gel. The double layer theory is used to explain the expansion results of the mortar bars containing different levels and types of mineral admixtures. As this theory predicts, the experimental results show a strong negative correlation between expansion and the charge fraction of bivalent cations in the gel.     

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.     

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.     

Use of perlite powder to suppress the alkali-silica reaction

Reported below are the results from a study aimed at mitigating the deleterious alkali-silica reaction by using perlite powder as an admixture. The expansion of mortar bars containing various amounts of silica fume (SF), expanded perlite, and natural perlite was studied. Two kinds of reactive aggregates were used in the study: highly reactive river aggregate containing opal and marginally reactive monzo-diorite aggregate. Expanded perlite and silica fume were tested with both aggregate, separately; on the other hand, natural perlite was tested only with monzo-diorite aggregate. The bars were cast in accordance with ASTM C1260, accelerated mortar bar method, and were stored in NaOH solution for 30 days. Length changes were measured and reported. The results showed that both expanded and natural perlite powder (NPP) have potential to suppress the deleterious alkali-silica expansion.     

天然沸石对碱-硅酸反应的抑制及其机理

用高碱水泥和天然沸石制备了含沸石水泥，测定了其砂浆棒的膨胀率，可溶性碱量和有效碱量。用能谱分析（EDXA）了C－S－H凝胶的化学组成。讨论了总碱量，可溶性碱量和有效碱量与膨胀率的关系。结果显示，由碱－硅酸反应（ASR）引起的膨胀与可溶性碱量和有效碱量之间有很好的关系。沸石主要通过离子交换和提高C－S－H凝胶对Na^ 和K^ 的吸收，使可溶性碱量和有效碱量降低，从而起到抑制ASR的作用。     

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。     

A rapid method for identification of alkali reactivity of aggregate

A rapid method for the identification of alkali reactivity of aggregate within two days has beeb achieved. 1×1×4 cm mortar bars with cement:aggregate = 10:1, w/c = 0.3, size of aggregate = 0.15?0.75mm were demolded after one-day curing and subjected subsequently to 100°C steam curing for 4 hours, after which they were immersed in 10% KOH solution and autoclave-treated at 150°C for 6 hours. After each stage of curing expansion measurements were carried out. From the data of more than thirty species of rocks, the authors arrived at the conclusion that the rapid method could be used to distinguish reactive and non-reactive aggregate. The results of microscopic observation made clear that the expansion of mortar bars was caused by alkali-silica reaction. This method cannot only be used to identify the alkali reactivity of aggregate, but when combined with the use of optic and electron microscope, can be also used to study the mechanism of alkali-aggregate reaction.     

Reactivity of reclaimed concrete aggregate produced from concrete affected by alkali-silica reaction

This paper presents results from a research program that focused on studying the reactivity of reclaimed concrete aggregate (RCA) produced from concrete affected by alkali-silica reaction (ASR). The results showed that RCA produced from ASR-affected concrete causes significant expansion when used in new concrete. The expansion was similar to that produced in concrete containing the reactive aggregate used originally in the old concrete. It is believed that crushing the old concrete exposed fresh faces of the reactive aggregate which causes renewed reaction and expansion in the new concrete. The alkalis contributed from the RCA are also believed to contribute to the expansion. The amount of supplementary cementing materials required to mitigate the expansion in new concrete containing ASR-affected RCA was higher than those normally needed in concrete containing the virgin reactive aggregate. The results showed a good agreement between the 14-day expansion of accelerated mortar bars and the expansion of concrete prisms.     

Effect of the cement chemistry and the sample size on ASR expansion of concrete exposed to salt

Mortar bars and concrete prisms made with a very alkali-silica reactive limestone were stored at 38 °C in 1 M NaOH and NaCl solutions. A high-alkali (HA) cement and a low-alkali (LA) cement were used in order to evaluate the cement chemical composition on the expansion and on the chemistry of the pore water. The mortar bars immersed in 1 M NaOH presented much more expansion than mortar bars stored at 100% RH or in 1 M NaCl. The behaviour of the concrete prisms was completely different. Low expansion was obtained for concrete prisms made with the LA cement immersed for more than 5 years in 1 M NaCl solution, while the expansion was over 0.45% for concrete prisms made with the HA cement. Chemical equilibrium between the pore waters and the immersion solution was much longer to obtain for the concrete prisms (near 3 years) than for the mortar bars (less than 3 months). The results obtained in this study show that the type of sample used (mortar bars or concrete prisms) and the cement composition strongly influence the harmful effects of ASR in concrete exposed to salt.     

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.     

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.     

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.     

Prevention of alkali-silica expansion by using slag-Portland cement

A previously postulated hypothesis that concrete or mortar prisms devoid of free Ca(OH)₂ will not suffer from alkali-silica expansion has been tested. In this investigation high slag-Portland cements were used to make mortar prisms with a reactive sand. Storage of these prisms in a saturated NaCl bath at 50°C caused no expansion. At the end of the storage period in NaCl bath, the prisms were found to be free of Ca(OH)₂. The results are consistent with the proposed hypothesis.     

硅质集料碱活性快速砂浆棒试验方法 1.试验温度与水泥碱含量对砂浆膨胀的影响

研究了养护温度和水泥碱含量（质量分数，下同）与硅质集料砂浆试样膨胀的关系。温度对砂浆膨胀影响较大，采用８０℃碱溶液养护，试验更迅速，结果更敏感。水泥碱含量对砂浆膨胀的规律影响较小，但对膨胀值有明显影响。采用加碱方法调整水泥碱含量可以有效地控制试验条件，使试验更迅速、准确     

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 reactivity of mortars containing chert and incorporating moderate-calcium fly ash

This paper reports the results of an experimental program, which aimed to investigate the alkali reactivity of chert and the effect of a moderate-calcium fly ash on the alkali–silica reaction. To determine the expansions, mortar bars were cast and tested in accordance with ASTM C1260. Mortar aggregate was replaced by chert, in controlled amounts, to find out the pessimum limit, if any. To evaluate the degree of cracking, sonic pulse velocity measurements and petrographic analysis were carried out on the cracked bars and on the thin sections taken from these bars, respectively. In the next series of tests, limestone and chert were blended together as mortar aggregate and cement was replaced by different dosages of fly ash to examine the changes in the mortar bar expansion as well as in the chemistry of reaction products. Microstructural observations were done on polished sections using a scanning electron microscope, equipped with energy dispersive X-ray analysis. The results showed that the chert used in this investigation had a pessimum proportion in the range of 5–15%. Sufficient fly-ash additions suppressed the expansion caused by chert. The study also revealed out that as the CaO/Na₂O_eq of alkali–silica gel increased, the expansivity of the gel decreased.     

Alkali-silica reactivity of large silica fume-derived particles

Pozzolanic materials, including silica fume, are commonly added to concrete to reduce expansion due to alkali-silica reaction (ASR). It has been noted, however, that commercial silica fume is not always adequately dispersed, and large agglomerates may be present. These large particles have been hypothesized to act as amorphous silica aggregates, thereby participating in an expansive reaction with the alkalis present in cement paste pore solution. If such were the case, some silica fume particles would actually aggravate expansion due to ASR rather than suppress it. The present investigation characterizes the microstructure and morphology of agglomerated and sintered silica fume particles and compares their effects on alkali-silica-related expansion. While a 5% replacement of moderately reactive sand with sintered silica fume aggregates caused significant expansion under accelerated testing conditions (modified ASTM C1260), the replacement with large agglomerates of densified silica fume decreased expansion compared with control mortar bars containing only sand. Both the sintered aggregates and the agglomerates reacted with the pore solution; one reaction was expansive, while the other was not.     

胶砂比和集料级配对ASR膨胀的影响

为建立快速鉴定硅质集料碱活性的方法 ,以沸石化珍珠岩和硅质砾石为集料研究了胶砂比和集料级配对砂浆试体ASR膨胀的影响。结果表明 :对检测集料ASR活性而言 ,快速砂浆棒法 (ASTMC12 6 0和CSAA2 3 2 - 2 5A)所建立的集料级配和胶砂比不是最敏感条件。采用单级配集料和多胶砂比砂浆试体可以更快速、可靠地鉴定集料ASR活性     

Electrochemical chloride extraction: efficiency and side effects

Some specimens of reinforced concrete cast with an alkali-resistant aggregate, previously maintained in a solution of NaCl, were subjected to an electrochemical chloride extraction (ECE). The chloride profiles before and after treatment were determined. Likewise, alkali ions profiles before and after treatment were determined. After treatment, some specimens were stored in a controlled atmosphere (60 °C and 100% RH) in order to accelerate the alkali-silica reaction, if any.Results of chloride content after treatment show that about 40% of the initial chloride is removed within 7 weeks. About one-half of the chloride close to steel was removed, but at the same time, significant amounts of alkali ions were observed around the steel.Microstructural observations by scanning electron microscopy (SEM) showed that after treatment, new cementitious phases containing higher concentrations of sodium, aluminum and potassium were formed. Moreover, alkali-silica gel was observed in the specimens stored at 60 °C and 100% RH. It may be possible that the ECE accumulates locally high amounts of alkali ions that stimulate the alkali-silica reaction even though the concrete contained nominally inert siliceous aggregates. The specimen expansions were not recorded, but no cracks were observed.