Chapter 3 Lithium Compounds for Controlling Asr

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3.1 INTRODUCTION

This chapter reviews research to date on using lithium compounds to control ASR. Included are discussions on the basic chemistry and production of lithium compounds, and the mechanisms by which lithium compounds control ASR. It also provides a critical review of various laboratory evaluations of lithium, including the effects of lithium on ASR and on other concrete properties. Significant emphasis is placed on combining lithium technology with other more traditional materials, including fly ash and slag.

The information and data presented in this chapter are limited to laboratory evaluations using lithium compounds. CHAPTER 4 summarizes various case studies of actual field applications of lithium compounds to control or mitigate ASR in new and existing concrete structures. Guidelines are provided later in this report on how to test, specify, and use lithium compounds efficiently.

3.2 THE BASICS OF LITHIUM

Lithium is an alkali metal found in Group IA of the periodic table and has an atomic number of 3. Lithium is a soft, silver-white metal and is the lightest dense metal, with a density about half that of water (0.53 g/cubic centimeter (cm³)). Lithium is a very reactive metal because of its tendency to expel its outer electron (it has a valence of +1). It does not occur freely in nature, but rather it is bound in stable salts or minerals.

The most common sources of lithium are pegmatite rocks-which are coarse-grained granites composed of quartz, alkali feldspar, and possibly mica-and salt brine lakes. The main lithium-bearing minerals include spodumene, petalite, amblygonite, lepidolite, and eucrypite; the chemical compositions of these minerals are shown in table 4 (Lumley, 1997). The table also includes the location of major mineral deposits, although other significant deposits also have been identified worldwide. Table 4. Principal Lithium Minerals and their Sources (after Lumley, 1997).

Of the minerals shown in table 4, spodumene is the most common lithium ore. However, because extracting lithium from spodumene is an energy-intensive process (requires heating to approximately 1100 ºC), most lithium production has shifted in recent years to the use of subsurface brine deposits (Ober, 2000). Lithium is extracted from brine sources by solar evaporation of concentrated brine, a process that is significantly less expensive that extracting lithium from ore deposits. Lithium is obtained from brine deposits found mainly in North and South America-Chile and Argentina are major sources (Ober, 2002).

Lithium carbonate (Li₂CO₃) is the most important lithium compound produced from brine and ore deposits, and in most cases, other lithium compounds require Li₂CO₃ as a feedstock for further processing (Ober, 2002). In addition to Li₂CO₃, other commonly produced lithium compounds include LiOH, lithium hydroxide (LiOH), lithium hydroxide monohydrate (LiOH•H₂O), lithium chloride (LiCl), lithium fluoride (LiF), and Li₂SO₄.

3.3 USING LITHIUM COMPOUNDS TO CONTROL ASR

3.3.1 History and Background

The use of lithium compounds to control expansion due to ASR was first reported by McCoy and Caldwell (1951). They conducted a comprehensive investigation on the potential use of chemical admixtures to prevent or minimize ASR-induced expansion and damage. Over 100 different compounds were included in this study, including metallic salts, acids, oils, organic chemicals, proteins, and proprietary admixtures. McCoy and Caldwell conducted a series of ASTM C 277 mortar bar tests (using Pyrex glass as the reactive aggregate) and reported that the most promising candidates in reducing ASR expansion were lithium compounds (LiCl, Li₂CO₃, LiF, lithium silicate (Li₂SiO₃), LiNO₃, and lithium sulfate (Li₂SO₄)), which essentially eliminated expansion after 8 weeks storage at 38 ºC, provided they were used in sufficient quantity.

For about 40 years after McCoy and Caldwell published the findings of their study, only a few studies were conducted on the effectiveness of lithium compounds to control ASR. In the past 10 years, however, there has been a resurgence in the interest in lithium-bearing compounds. This is reflected in an increase in scientific publications and field applications on the topic.

3.3.2 Mechanisms of ASR Suppression by Lithium Compounds

As previously mentioned, McCoy and Caldwell (1951) used mortar bars prepared with high-alkali cement and reactive glass aggregate to demonstrate the effectiveness of lithium additives in reducing expansion associated with ASR. These results and the results of more recent research (Chatterji, 1987; Sakaguchi et al., 1989; Stark, 1992; Lumley, 1997; Ramachandran, 1998; Diamond, 1999; Thomas et al., 2000), that show a reduction in expansion by ASR in the presence of lithium-containing additives, have generated much interest in using these compounds in concrete structures. While it is known that some lithium additives effectively reduce expansion in concrete affected by ASR, the mechanism or mechanisms by which these additives reduce expansion are not understood. Without understanding the control mechanism, it may be difficult to predict the effectiveness of a particular chemical additive or dosage, or to predict the duration of its control. However, several mechanisms have been proposed to describe the effect of lithium, including the following:

• Lithium may alter the ASR product composition, resulting in a less expansive product.
• Lithium may reduce silica dissolution.
• Lithium may decrease the repolymerization of silica and silicates.
• Lithium may reduce repulsive forces between colloidal ASR gel particles.

Proposed Mechanism: Formation of Less-Expansive Product

Several researchers have proposed that a less expansive or nonexpansive product may form during ASR in the presence of lithium. Stark (1992) proposed that during the ASR, in the presence of a sufficient concentration of lithium, a lithium-alkali (and possibly calcium) silicate forms that has little or no capacity for expansion. Because the research suggests that a minimum lithium threshold content is required to inhibit expansion, Stark reasoned that the product must contain a minimum proportion of lithium to be nonexpansive. Similarly, Diamond and Ong (1992) showed that, as the amount of lithium present in the gel product increased in proportion to the amount of sodium and potassium present, mortar bar expansion decreased, confirming the work of Stark (1992).

Lawrence and Vivian (1961) suggested that a lithium silicate forms by ASR in the presence of lithium, and that this product is less soluble and more stable than the ASR product in the absence of lithium. Due to its stability, it was proposed that the resulting LiOH silica complex may form an insoluble surface layer, protecting silica from attack by other alkalies (Lawrence and Vivian, 1961).

Lawrence and Vivian (1961) also showed that silica gel tended to be less reactive with NaOH with increasing concentrations of LiOH, to 2N LiOH equivalent in 2N NaOH equivalent solution. By measuring lithium and alkali concentrations in expressed pore solutions from mortar bars, Sakaguchi et al. (1989) found that the concentration of lithium decreased with time, while alkali concentrations remained nearly constant. In the absence of lithium, the alkali concentration of the expressed pore fluid decreased over time, suggesting that the lithium-silica reaction is more favorable than the sodium-silica or potassium-silica reaction in mortar bars. According to this theory, the formation of a nonexpansive lithium-containing product would be favored over the formation of a more expansive product containing relatively greater concentrations of the alkalies sodium and potassium.

Others (Chatterji, 1987) disagree, however, stating that the reaction of silica with sodium is favored over the reaction of silica with lithium. In a system containing Na⁺, K⁺, and Li⁺, the alkalies compete for adsorption at negatively charged sites on the silicate surface. Since adsorption affinity increases with bare cation radius, it is expected that sodium adsorption will be preferential to lithium adsorption, in disagreement with Sakaguchi (1989). Kurtis et al. (1998; 2000; in press), however, have proposed that strong field exchange behavior, where cations are in direct contact with a surface, may account for the preference of the ASR gel for Li⁺ as compared to Na⁺ and K⁺, as described by Sakaguchi (1989).

Proposed Mechanism: Suppression of Silica Dissolution

In examining the effect of various alkali-hydroxides on silica dissolution rate, Lawrence and Vivian (1961) found that the dissolution rate increased in this order:

Equation 6:

Wijnen et al. (1989) found that the rate of silica dissolution decreased in a similar order and proposed that this rate decreases with increasing hydrated ion radius of the alkali metal cations in solution surrounding a silicate surface. Considering lithium, sodium, and potassium, the rate of silica dissolution would, then, be slowest in the presence of lithium, which has a larger hydrated ion radius than sodium which is, in turn, larger than potassium. Chatterji et al. (1987) proposed that the size of the hydrated ion radius was important in determining the extent of chemical reaction during ASR, supporting research findings that degree of chemical reaction increased from lithium to sodium to potassium.

These results suggest that lithium may act to decrease the rate of silica dissolution, which would then limit the rate of product formation and potential for expansion. Over time, however, the concentration of dissolved silica for each alkali-hydroxide concentration examined by Lawrence and Vivian (1961) approached the same value, independent of alkali type, suggesting that lithium may reduce the rate of dissolution, not the solubility of silica. In examining silica gel in model pore solution, Collins (2002), however, found that silicon concentration in solution decreased with increasing lithium concentration over a period from 1 hour to 28 days. The results of Collins (2002) may demonstrate, again, the significance of dosage. If, as Sakaguchi's (1989) results suggest, the lithium reaction with the silica or ASR gel is preferential to the reaction with sodium and potassium cations, local concentrations of lithium near the silica may decrease the rate of dissolution, effectively decreasing the rate of formation of the expansive gel.

Proposed Mechanism: Decreased Repolymerization

Based upon microscopy, elemental analysis, and surface chemistry principles, Kurtis et al. (1998, 2000, in press) have suggested that, in addition to decreasing the rate of silica dissolution, lithium may limit repolymerization of dissolved silica species into a gel, effectively reducing the potential for expansion. Using x-ray microscopy to examine the reaction of silica in model pore solution in the presence and absence of lithium, Kurtis et al. (1998, 2000, in press) observed that significantly more gel product formed in samples containing no lithium than in those containing lithium. Companion results from elemental analysis showing a decrease in silicon concentration in solution indicated that the presence of lithium decreased silica dissolution to some extent, but the concentration of silicon in solution in the presence of lithium was on the order of the concentrations observed in the absence of lithium. These results suggested that the differences in behavior observed with lithium may be due more directly to changes in the amount of product formed rather than in the degree of reaction.

If, as the research of Sakaguchi (1982) suggests, adsorption of Li⁺ is favored over Na⁺ and K⁺ adsorption, a physical mechanism for preventing gel repolymerization may exist. Iler (1956) postulates that the highly hydrated lithium ions are not adsorbed as near to the silicate surface as a cation, which has a smaller hydrated radius, such as sodium or potassium. Thus, Kurtis et al. (1998, 2000, in press) proposed that the net repulsion between the silicate particles remains high in the presence of lithium. As a result, it is theorized that when lithium is present in sufficient concentrations, repolymerization into a potentially expansive gel does not occur. The effect of the lithium should, then, depend upon its relative concentration in the solution as well as the favorability of the silica-lithium reaction.

Proposed Mechanism: Reduction in Repulsive Forces within Product Prezzi et al. (1997) proposed the use of electrical double layer (EDL) theory to explain the expansion of the alkali-silica reaction gel, and the theory was extended to describe a proposed mechanism by which chemical additives, including lithium salts, may inhibit expansion (Prezzi, 1998). Applying these principles, the ASR gel is assumed to act as a colloid composed of negatively charged particles. According to the theory, swelling of the gel is attributed to double-layer repulsion effects between the colloidal particles. According to EDL theory, the valence and hydrated radius of cations in the colloid are important factors in determining gel expansion. The double-layer theory predicts that an ASR gel containing larger concentrations of cations with larger valences will exhibit less expansion. That is, higher proportions of trivalent (e.g., Al³⁺) and bivalent (e.g., Ca²⁺) cations relative to monovalent cations (e.g., Na⁺, K⁺, and Li⁺) should result in less expansive gels. Results from a series of mortar bar tests performed by Prezzi et al. (1998) agree with double-layer predictions according to cation valence. These tests showed expansion increasing with cation charge in the order:

Equation 7:

However, the size of the hydrated ion radius also will affect expansion as predicted by EDL theory. As stated previously, the radius of the hydrated lithium ion is larger than those for sodium or potassium. Therefore, EDL theory predicts that lithium will produce greater expansion compared to these other monovalent cations, not reduce expansion, as is generally evident. Therefore, additional mechanisms may need to be considered to explain the reduction in gel expansion associated with the use of lithium additives.

According to Prezzi (1997, 1998), a decrease in surface charge density (-s) effectively reduces the pressure (DP) generated by ASR gel expansion:

Equation 8:

Where: C_o is the bulk electrolyte concentration
   R is the molar gas constant
   T is the absolute temperature and

Equation 9:

Where:F is the Faraday constant
   ß=1.084x1016m/mol at 25 °C
   s is expressed per unit area

The presence of bivalent and trivalent cations provided by chemical salts has been theorized to lower surface charge density of the ASR gel as compared to systems where more monovalent cations are present. Rodrigues et al. (2001; unpublished) have performed potentiometric titrations to examine how the surface charge density of silicates, including ASR gel, is affected by the presence of various chemical salts, including LiCl. Their work showed that in a system with both sodium and lithium ions present, the surface charge density of opal (2001) and alkali-silica gel (obtained from a ASR-affected dam) (unpublished data) were decreased in the presence of LiCl, as compared to sodium chloride (NaCl). A decrease in the repulsive forces between colloidal particles of ASR gel in the presence of lithium would reduce expansive pressure generated by swelling of the gel. However, Rodrigues (2001) also observed that potassium chloride (KCl) produced an even greater reduction in surface charge density than LiCl. These results do not coincide with the findings of Prezzi et al. (1998) described above and suggest that further research is necessary.

3.3.3 Laboratory Studies Using Lithium to Control ASR: A Critical Review

This section reviews research to date on using lithium to control ASR, beginning with the initial investigation by McCoy and Caldwell and proceeding to recent studies. Because most of the laboratory (and exposure site) studies have dealt with lithium compounds as an admixture, only limited information is available on posttreating hardened concrete with lithium to mitigate further expansion.

Laboratory Research on Using Lithium as an Admixture

Many laboratory studies have focused on the use of lithium compounds to control ASR, some of which are discussed in section 3.3.2, as related to proposed mechanisms. The studies span more than 50 years and have used different test methods, lithium compounds, cementitious materials, and aggregates, making it somewhat difficult to compare one study directly to another. However, general comparisons are possible and trends in test results can be identified. These then can be synthesized and incorporated into guidelines and recommendations for efficiently testing, specifying, and using lithium to control ASR. Of particular interest in the review that follows is the documented dosage of lithium required to control ASR for aggregates of different levels of reactivity and for mixtures containing SCMs. Information is also provided on recommended methods of assessing lithium compounds in mortar and concrete, expanding upon the information provided in chapter 2 on testing methodologies. At the end of this section, the main findings from the various studies will be summarized and incorporated into specific guidelines for using lithium in new concrete.

As previously discussed, McCoy and Caldwell (1951) were the first researchers to identify lithium compounds as effective admixtures in controlling ASR. Their study included the use of LiCl, Li₂CO₃, LiF, Li₂SiO₃, LiNO₃, and Li₂SO₄ at various dosages. Testing was performed according to ASTM C 227, with Pyrex glass as the reactive aggregate. Each of the lithium compounds was found to be effective in minimizing expansion, provided that a high enough dosage was used.

Table 5 contains the expansion data (at various ages) for mortar bars containing different lithium compounds, where the dosages listed are based on mass of cement. A more convenient and useful method of displaying this data is to express expansion as function of the lithium-alkali molar ratio. Figure 12 shows the relative expansion of mortar bars containing lithium to a control without lithium (where a value of 1.0 reflects no effect on expansion), plotted against the lithium-alkali ratio (which is equal to the moles of lithium divided by the moles of sodium plus potassium). As the amount of lithium in mortar increased, the relative amount of expansion decreased. The data indicated that a molar ratio of lithium to alkali of 0.74 or above was sufficient to suppress expansion efficiently.

.

Figure 12. Relative Expansion of Mortar Bars Containing Lithium Compounds (after McCoy and Caldwell, 1951).

After the early findings of McCoy and Caldwell, it was approximately 40 years before research on controlling ASR with lithium compounds continued in earnest. A series of studies were initiated in the late 1980s and early 1990s, and more recent studies have been conducted or are still in progress. Several of these studies are reviewed briefly below. Most of these studies have used ASTM C 227 as the method of testing lithium, but some of the more recent investigations have used the tests recommended for testing lithium compounds in chapter 2, specifically the concrete prism test (ASTM C 1293) and a modified version of the accelerated mortar bar test (ASTM C 1260).

As described in section 3.3.2, Sakaguchi et al. (1989) proposed that the formation of a nonexpansive lithium-containing product would be favored over the formation of a more expansive product containing relatively greater concentrations of the alkalies sodium and potassium. Their study included the use of LiOH•H₂O, lithium nitrite (LiNO₂), and Li₂CO₃ in mortar bars containing Pyrex glass, and LiOH•H₂O in mortar bars containing a reactive pyroxene andesite sand from Japan. All of the lithium compounds were effective in reducing expansion, with increasing lithium dosages resulting in decreased expansion. For the tests performed using LiOH•H₂O with the reactive aggregate, the reductions in expansion are shown in table 6. A molar ratio of 0.9 was sufficient to completely suppress expansion.

Ohama et al. (1989) investigated the use of LiOH•H₂O, LiF, and Li₂CO₃ in mortar bars containing opaline amorphous silica as the reactive aggregate. The bars were cured for 1 day at 20 °C and 100 percent relative humidity, then subjected to autoclaved curing at 128 °C under a pressure of 2.5 kilogram (force) (kgf)/cm₂ for 4 hours, after which the bars were cooled back to 20 cm² and measured for expansion. Li₂CO₃ only slightly reduced expansion, whereas LiF and LiOH•H₂O at 0.5 percent and 0.7 percent (based on mass of cement), respectively, reduced the expansion to about half of that of the control. Because of the extreme conditions typical of high-temperature autoclaving, it may be difficult to relate these results to other laboratory investigations.

Considerable research on using lithium compounds to control ASR was performed under the Strategic Highway Research Program (SHRP), with the relevant findings reported by Stark (1992) and Stark et al. (1993). Because of the large scope of the SHRP study, only selected aspects of the study are discussed here. Stark and his coworkers produced and tested mortar bars according to ASTM C 227, using LiF and Li₂CO₃, and confirmed the efficacy of lithium in suppressing expansion, provided that a high enough dosage was used. Table 7 summarizes the results of these tests, which used a highly reactive natural aggregate (rhyolite). Expansion essentially was controlled at molar ratios (lithium to sodium plus potassium) of 0.6 for LiF and 0.92 for Li₂CO₃.

When testing the same rhyolitic aggregate as above and a less reactive granite gneiss, Stark et al. (1993) reported that molar ratios in the range of 0.75 to 1 were needed to suppress expansion adequately when using ASTM C 1260 (and adding LiOH to the 1N NaOH soak solution). Stark also recognized a pessimum effect, in which an insufficient dosage of lithium actually increased expansion (compared to a control). This was attributed to an increase in the alkalinity (OH^- concentration) of the pore solution, which is triggered by the addition of lithium (especially LiOH). As discussed later in this section, other forms of lithium, such as LiNO₃, do not tend to raise the pH of the pore solution in concrete, thereby eliminating the pessimum effect.

Diamond and Ong (1992) reported several interesting findings when using LiOH in a series of mortar bar tests. They reported that a substantial portion of the lithium (> 40 percent) added during mixing was removed rapidly from solution, presumably absorbed by the hydrating cement, and that this absorption was greater for lithium than for sodium or potassium. When mortar bars were cast and tested according to ASTM C 227, a Li:(Na + K) molar ratio of 1.2 was required to suppress expansion for mortar containing cristobalite as the reactive aggregate, but this dosage was not quite sufficient to suppress the expansion of similar bars containing beltane opal as the reactive aggregate. The amount of lithium required to suppress ASR-induced expansion was higher for this study than for other published studies, although the reasons for this difference are not clear. Diamond and Ong also confirmed the pessimum effect that was observed by Stark (1992), in which low and moderate amounts of LiOH actually increased expansion, compared to a control mortar without lithium.

In one of the few studies using LiNO₂, Qinghan et al. (1995) tested mortar bars containing reactive andesite aggregate. The mortar bars were cured for 1 day at 20 ºC, then placed in an autoclave under a pressure of 0.28 Megapascals (MPa) for 4 hours. After they were removed from the autoclave, the bars were placed in a curing container at 20 ºC for 4 months, then at an elevated temperature (40 ºC) for long-term measurements. A Li:(Na + K) molar ratio of 0.8 reduced expansion significantly for mortars with very high alkali contents (2 percent by mass of cement), but more lithium (based on molar ratio) was needed for mortars with lower alkali contents.

Lumley (1997) used the concrete prism test (ASTM C 1293) to assess the efficiency of LiOH•H₂O, LiF, and Li₂CO₃ in reducing expansion due to ASR. A calcined flint cristobalite was used as the reactive aggregate, and researchers covered a range of lithium dosages. The findings agreed with most published literature, suggesting that a ratio of equivalent Li2O to equivalent Na₂O of 0.33 to 1 (by mass) or a Li:(Na + K) molar ratio of 0.62 was sufficient to inhibit expansion.

An important trend in recent years has been the emergence of LiNO₃ as the preferred lithium compound in controlling ASR. Stokes et al. (1997) reported that a major advantage of LiNO₃ over other lithium compounds is that LiNO₃ does not increase the pH of the pore solution, thereby eliminating the risk of the pessimum effect as described earlier. Using LiNO₃ avoids this effect because its addition to cement paste results in an increase in the lithium and nitrate ion concentrations of the pore solution with no significant increase in the OH-concentration (Stokes et al., 1997). The implication of this behavior was confirmed in this study, in that all mortar bars containing LiNO₃, regardless of dosage, expanded less than the control, which was not the case for previous studies using other lithium compounds (e.g., LiOH). Another important advantage of using LiNO₃ as an admixture is that it is closer to a neutral pH than other lithium compounds, making it safer to handle.

A comprehensive study was initiated at the Building Research Establishment (BRE) in 1994 on using lithium compounds (LiOH and LiNO₃) to control ASR. Blackwell et al. (1997) reported on the preliminary findings and Thomas et al. (2000) gave a more recent update on the status of the project, which includes over 150 concrete mixtures, laboratory testing (using ASTM C 1260 and ASTM C 1293), and exposure block testing at an outdoor site located at BRE in the United Kingdom. The program involved the use of several reactive United Kingdom aggregates and also included the use of fly ash and slag. Figure 13 summarizes the 3-year expansion data for concrete prisms for the most reactive aggregate, plotted in a similar format to allow comparison with McCoy and Caldwell's findings (figure 12). A lithium to alkali molar ratio of approximately 0.70 was sufficient to control expansion when using LiNO₃, and a higher dosage, around 0.85 (molar ratio) was required for LiOH, mainly due to the impact of LiOH on pore solution pH (as previously discussed). The study also illustrated that the efficacy of lithium in reducing expansion is a strong function of aggregate reactivity (i.e., more reactive aggregates require more lithium), and that using fly ash in conjunction with lithium yielded synergistic reductions in expansion.

Figure 13. Relative Expansion of Concrete Prisms Containing Lithium Compounds.

Diamond (1999) provided further discussion of the work previously described by Stokes et al. (1997), including additional insight into the role of LiNO₃ in suppressing ASR-induced expansion. He noted that LiNO₃ does not raise pore solution pH and demonstrated that the Li+ ions in the pore solution were balanced mainly by NO₃- ions and, to a smaller extent, by SO4-2 ions. Diamond also illustrated that LiNO₃ tends to be removed from solution by hydration products, as do other lithium compounds, thus reducing the lithium available to suppress ASR expansion.

Durand (2000) presented the results of extensive testing using LiOH•H₂O, LiF, Li₂CO₃, and LiNO₃ in concrete prisms following ASTM C 1293. Three different reactive aggregates from Canada (from Sudbury, Potsdam, and Sherbrooke) were used in conjunction with different dosages of the four lithium compounds. The results indicate that a molar ratio Li:(Na + K) of 0.83 was required to suppress expansion (below 0.04 percent at 2 years) for LiOH•H₂O, LiF, and Li₂CO₃ when using the Sudbury aggregate. A molar ratio of 0.72 was found to be sufficient when using LiLiOH•H₂O with the Sudbury aggregate. None of the lithium compounds, even when used at their highest dosages (1.66 molar ratio for LiOH•H₂O, LiF, Li₂CO₃ and 0.72 for LiNO₃), were able to reduce sufficiently the expansion of concrete containing the Sherbrooke aggregate (a metamorphic schist). The only lithium compounds (and dosages) that adequately controlled expansion of concrete containing Potsdam sandstone were LiOH•H₂O and LiF, at molar ratios of 1.66. These findings confirm that the amount of lithium required to suppress expansion depends strongly on aggregate reactivity and also on the specific lithium compound used.

Thomas et al. (2001) provided preliminary findings from a comprehensive study focusing on the combined use of lithium and fly ash to control ASR. Testing was performed using ASTM C 227 and ASTM C 441, with a highly reactive aggregate from New Mexico and Pyrex glass. In addition, a modified version of ASTM C 1260 was used to investigate various combinations of lithium nitrate (30 percent solution), 6 fly ashes, and 3 cements. ASTM C 1260 was modified by adding LiNO₃ to the 1N NaOH soak solution to achieve the same lithium-alkali ratio as that used in the mortar mixture. This is an important modification and is recommended whenever testing lithium compounds with ASTM C 1260 to prevent excessive leaching of lithium from the bars into the surrounding solution.

The findings presented by Thomas et al. (2001) suggest that the beneficial effects of using low-calcium fly ash and lithium together to control ASR are cumulative. Specifically, when either the LiNO₃ or low-lime ash were used individually and were unable to suppress expansion completely (e.g., less than 0.10 percent at 14 days in ASTM C 1260), the combined action often was found to be sufficient. This approach requires less lithium, thereby reducing cost, and less fly ash, thereby increasing the early strength development. Although the combination of lithium and high-lime ashes (i.e., > 25 percent CaO) was not found to be synergistic, the combined use still sufficiently reduced expansion. Higher-lime fly ashes intrinsically are not as efficient as lower-lime ashes in controlling ASR expansion, and they require higher relative amounts to control expansion (Shehata et al., 1999). Using lithium in combination with such high-lime ashes helps control expansion, and also would provide other benefits in the field, including higher early strength (compared to low-lime ash), reduced permeability, and cost savings. It should also be noted that the lithium dose used by Thomas et al. (2001) was calculated on the basis of the portland cement alkalies alone, so even though the use of high-calcium fly ashes generally did not lead to a reduction in the dose rate (i.e. 75 percent to 100 percent of the recommended dose was needed), there is still less lithium added to these mixtures as they contain lower amounts of portland cement (due to replacement with fly ash).

Table 8 summarizes some of the information provided in this section on various laboratory studies using lithium compounds to suppress ASR expansion. The table particularly focuses on comparisons between the dosage of lithium needed to suppress expansion and does not include the combined use of SCMs with lithium. The findings, as summarized in table 8, will be used in chapter 5 to develop specific guidelines for using lithium in new concrete.

Other forms of lithium than those described in this section have also been investigated. For example, Thomas and Stokes (1998) investigated the potential use of decrepitated spodumene (a lithium-containing ore) as a preventive measure and reported that the material was effective when used in sufficient proportions. Cement kiln trials also have been conducted using spodumene as part of the raw feed, thus producing a clinker rich in lithium (Stokes et al., 2000a). A technology has been developed for producing a lithium-containing glass, which can be used as a concrete admixture or interground with portland cement, to suppress expansion due to ASR (Baxter, 2000). This approach of using lithium-bearing glass has been reported as a means of minimizing the uptake of lithium by hydration products, thereby increasing the efficiency of the active lithium compound in controlling ASR-induced expansion.

The findings summarized in this section have dealt exclusively with the effects of lithium compounds on expansion due to ASR. However, it is very important that there are no undesired side effects from any admixture added to concrete that impact other fresh or hardened concrete properties. Fortunately, it has been well-documented that lithium compounds, used in typical dosages to suppress ASR expansion, do not affect significantly other important concrete properties. Most of the more recent investigations have dealt specifically with LiNO₃, as it is the most common lithium compound being used today. Studies have shown that fresh concrete properties, such as air content, slump and setting time, and hardened concrete properties, such as strength and permeability, are not altered significantly by the use of LiNO₃, and that LiNO₃ is compatible with other chemical admixtures (Wang et al., 1994; Wang and Stokes, 1996; McKeen et al., 2000; Thomas et al., 2002).

* molar ratios are used, unless otherwise noted

Laboratory Research on Using Lithium to Suppress Expansion in ASR-Damaged Concrete

In comparison to the number of studies on using lithium as an admixture in new concrete, there have been very few laboratory-based studies on posttreating hardened concrete to arrest expansion due to ASR. There have been several field applications, as described in the next chapter, but the following briefly summarizes laboratory research on the topic.

Sakaguchi et al. (1989) allowed both high-alkali mortar bars (containing Pyrex glass) and concrete prisms (containing a reactive aggregate) to expand considerably (0.2 percent expansion for mortar, 0.1 percent for concrete), and then soaked the specimens in LiNO₂ or LiOH•H₂O solution. For both mortar and concrete, future expansion was essentially significantly retarded or, in some cases, prevented.

Stark et al. (1993) confirmed that treating hardened mortar that was previously subjected to ASR-induced expansion by soaking it in LiOH solution effectively suppressed further expansion. They noted that a key issue in field applications would be to ensure adequate LiOH penetration.

Recently, Stokes et al. (2000b) reported on the development of a novel material for controlling future expansion of hardened concrete. The material is a LiNO₃-based solution, but also contains a proprietary blend of surfactants to help penetrate hardened concrete. It was reported to be 50 percent more effective than LiNO₃, by itself, or 3 times more effective than LiOH, by itself. Stokes et al. (2000b) also noted that the specific time of treating hardened concrete plays a major role in the subsequent efficiency of lithium compounds in controlling expansion, presumably because of increased permeability and damage of ASR-affected concrete.

3.3.4 Specifications for Using Lithium to Control ASR in Concrete

There is relatively little guidance provided in specifications related to the use of lithium in concrete. Current specifications within ASTM and CSA do not include guidance on using lithium compounds, although the next version of CSA specifications is expected to provide information on using lithium in new concrete. AASHTO (2000) has provided guidance on using lithium compounds in new concrete as part of the guide specifications developed by the Lead States Program. Several State highway agencies in the United States allow for the use of lithium in their specifications, including Delaware, Texas, New Mexico, South Dakota, and Wyoming.

The BRE (2002) has published guidelines recently for using lithium admixtures in concrete, as shown in table 9. The BRE recommendations take into account aggregate reactivity, fly ash dosage (if used), type of lithium compound, and total alkali content of the concrete mixture. Table 9 is only applicable for concrete with total alkali content less than or equal to 5 kg/m³. The classification of aggregates (high reactivity or normal reactivity) generally is based on mineralogy, as opposed to performance tests, but a test similar to ASTM C 1293 may be used if aggregate reactivity is in question. BRE does not provide guidance on using slag in conjunction with lithium compounds. When fly ash is used, the alkali contribution of the fly ash in calculating the total mixture alkali is as follows:

• If fly ash content is greater than or equal to 25 percent (by mass of total cementitious materials)-alkalies in fly ash are not included in calculating total alkali content of concrete.
• If fly ash content is 20 to 24 percent-one-fifth of the total fly ash alkalies are included in calculating total alkali content of concrete.
• If fly ash content is less than 20 percent-all of the alkalies present in the fly ash are included in calculating total alkali content of concrete.

For convenient comparison to data presented earlier in this chapter, the last column of table 9 shows the molar ratio for each of the recommended lithium dosages. For the same aggregate type and fly ash content, higher dosages of lithium (based on molar ratio) are recommended for LiOH·H₂O than for LiNO₃, which recognizes the superior performance of LiNO₃ in controlling expansion due to ASR.

3.4 CONCLUSIONS

This chapter has reviewed a wide range of research performed using lithium to combat ASR-induced expansion. The following are some of the key findings:

• Lithium compounds have been used successfully in laboratory research to control expansion due to ASR for more than 50 years.
• Although all the mechanisms are not understood fully, it is evident that lithium affects the nature of the ASR gel, essentially rendering it nonexpansive. The reaction product (gel) still forms, but lithium greatly hinders its ability to absorb water and expand.
• Two test methods are recommended for assessing lithium compounds. ASTM C 1293 (2-year test) is the preferred test, as it better relates to field performance. ASTM C 1260 (modified by adding lithium to the soak solution) is recommended, but should be used with caution due to the limited research to date using this modified test. Specific guidelines for performing these tests to assess lithium are provided in chapter 5.
• In general, adding lithium at a molar ratio Li:(Na + K) of 0.74 is adequate to suppress expansion for most aggregates, although highly reactive aggregates may require more, and some less reactive aggregates may need less.
• LiNO₃ reduces expansion more efficiently than ot her lithium compounds, mainly because it does not raise pore solution pH.
• Lithium compounds have proven to be effective in posttreating hardened concrete that has already expanded from ASR, thereby reducing or eliminating future expansion.

There is a general lack of guidance and specifications for properly using lithium to control ASR-induced expansion. It is hoped that this document will be useful in providing technical guidance to practitioners interested in using lithium for new and existing concrete structures.