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It was conjectured that the presence of Al in the SCMs possibly enhance the removal of alkalis from the concrete pore solution resulting a reduction of its pH, which would be beneficial for ASR mitigation. Hong and Glasser (Hong and Glasser, 2002) reported that incorporation of Al into C-S-H leads to formation of C-A-S-H phase, which markedly increases its alkali binding capacity. They suggested that this partially contributes to the potential of Al-rich SCMs in reducing pore solution alkalinity and subsequently ASR expansion. Later, Sun et al. (2006) showed significant increases in basal spacings when Al was incorporated into C-S-H phase. They explained that alumina was incorporated into C-S-H and present in the interlayer and that the substitution of Si by Al provides a negative site that must be charge balanced by a net positive charge (e.g., Na⁺, K⁺) leading to alkali binding. Thomas (Thomas, 2011) also stated that there is evidence that the alumina content of the SCM also affects its alkali-binding capacity as its silica content.

On contrary, an earlier study from Diamond (Diamond, 1981) showed that class F fly ash reduced hydroxyl and alkali ion concentration of the pore solution by a factor equivalent to its OPC replacement level of 30%, indicating a simple dilution effect. This observation is supported by the studies of Chappex and Scrivener (Chappex and Scrivener, 2012a). In their study, they compared an OPC-metakaolin paste with a paste containing silica fume and an inert filler to maintain an equivalent silica fraction and could thus isolate the effect of alumina. They found that the pore solution alkalinity was reduced with increasing the replacement level of OPC. However, such reduction of pore solution alkalinity was a mere dilution effect, and the alumina did not actively consume hydroxyl ions or alkalis.

Most of the aforementioned studies focused on cement mixtures containing Al-rich SCMs, where the joint effects of alumina and silica in SCMs could not be directly separated. To isolate the role of Al in mitigating ASR, Szeles et al. (2017) replaced OPC with 20% Al(OH)₃ and measured the pore solution pH of the cement paste mixtures. They observed that the pH of the Al(OH)₃ mixture was initially similar to the 80% of the control mixture, indicating a pure dilution effect. At later ages up to 1 year, slight reduction of pH (0.13 pH unit) was observed beyond the dilution effect, the pH reduction was not significant enough to be the sole mechanism for Al to mitigate ASR. A drop of pH from 12.4 to 11.9 was also recently captured by Shi et al. (2021) during the synthesis of ASR products at 40°C containing SiO₂, CaO, and KOH, where Al(OH)₃ precipitated as a separate phase. However, the pH increased at 80°C from 12.1 to 12.8, where the formation of an alumino-silicate phase prevented the formation of ASR products. The influence of Al on formation and structure of ASR products will be discussed in detail in the next section.

The above seeming contradictions has been clarified based on synthetic C-S-H (L’Hôpital et al., 2016), where the uptake of K in C-S-H and C-A-S-H at different Ca/Si ratios were compared as shown in Figure 2. The results show that the K/Si ratio in C-S-H is not significantly increased in the presence of aluminum. The scattered results reported in the aforementioned studies was attributed to the experimental error associated with the measurement of alkali concentrations, the synthesis protocol, and equilibration time (L’Hôpital et al., 2016). Therefore, the mechanism of significantly increased alkali fixation with subsequent pH reduction can be excluded.

For a long period of time, it was debated whether dissolved aluminum ions affect the formation and structure of ASR products or not. Many studies reported the incorporation of Al in ASR products based on scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM/EDS) analysis (Fernandes et al., 2007; Fernandes, 2009; Šachlová et al., 2010; Hagelia and Fernandes, 2012; Leemann and Lura, 2013; Shafaatian et al., 2013; Guo et al., 2018), while many others reported no Al or hardly detectable amounts in ASR products (Aquino et al., 2001; Leemann and Lothenbach, 2008; Katayama, 2012a; Katayama, 2012b; Leemann, 2017). It should be noted that the observation of Al in ASR products could be caused either by the close intermixing with Al-containing phases (hydrated cements or Al-containing minerals within the aggregates) or by cross-contaminations during sample preparation for SEM/EDS analysis. The absence of a significant amount of Al in ASR products was recently confirmed (Shi et al., 2021) based on laboratory synthetized ASR samples, where Al precipitated as a separate phase [gibbsite, Al(OH)₃] at 40°C, i.e., under temperatures relevant for field conditions, while the structure and composition of ASR products was not affected by aluminum.

At high temperature (80°C) and high alkali concentration (0.5–1 mol/L NaOH or KOH), however, the formation of zeolites or zeolitic precursors (alkali alumino-silicate phase) was observed in concrete (Hünger, 2007; Shi et al., 2018) as well as in synthetic samples (Shi et al., 2021). Hünger (Hünger, 2007) found an inverse relation between the silica releasing rate of aggregate and the amount of zeolite formed at the same temperature and suggested that the formation of alkali alumino-silicate and zeolite would reduce the concentration of so-called “free” silica available for ASR and thus inhibit ASR. Also laboratory experiments showed the suppression of the formation of ASR products in the presence of sufficient Al. While the formation of alkali alumino-silicate or zeolite could in fact lower the potential to form ASR products at high temperatures (80°C or above), at ambient temperatures the formation of alkali alumino-silicate and zeolite is usually slow (decades) (Sand et al., 1987; Lothenbach et al., 2017). Thus, the formation of alkali alumino-silicate and zeolite is under most conditions too slow to be relevant in suppressing ASR formation in field concretes.

A possibility to prevent ASR is to suppress or strongly slow down the dissolution of SiO₂ within the aggregate. Aluminum can drastically reduce silica dissolution rates as aluminum sorbs on the silica surface, which passivates the active silica sites and slows down dissolution (Iler, 1973; Bickmore et al., 2006; Nicoleau et al., 2014). Relatively low concentrations of Al (1–5 mM) can reduce the dissolution rate of silica by as much as 90%. However, the sorption of Al(OH)₄ ^- on the surface of silica and the slowing down of silica dissolution silica is more distinct at intermediate pH values (<12), while at pH 13 and above the sorption of Al(OH)₄ ^- on silica becomes weak resulting in only feeble suppression of the dissolution rate (Yokoyama et al., 1988; Chappex and Scrivener, 2012b; Nicoleau et al., 2014) as illustrated in Figure 3 (Bagheri et al., 2022). A comparison with the Al concentration in the pore solution of Portland cement and blended cements, show that blending of Portland cement with fly ash or metakaolin could in fact increase the Al concentration to 1 mM and above, i.e., to Al concentrations efficient in slowing down silica dissolution (see Figure 3). Recent study from Zhou et al. (2022) showed that the Al concentration could potentially reach about 80 mM after dissolution of 2 g metakaolin (ca. 1.0 g Al₂O₃) for 100 days in 75 ml simulated pore solution [0.6M NaOH, Ca(OH)₂ saturated], although in a concrete pore solution the concentrations might be significantly lower due to the formation of AFt and AFm phases. In fact, drastic reduction in damage to reactive siliceous aggregates is observed in metakaolin, slag or fly ash blended cements (Chappex and Scrivener, 2012a; Tapas et al., 2021), although this reduction might also relate to the decrease of pH in the pore solution due to the reaction of metakaolin, slag or fly ash.

As Al slows down, but does not prevent silica dissolution, one could expect a slower buildup of ASR with time. However, as demonstrated in Figure 1, the presence of Al(OH)₃ or Al-rich SCMs seems to prevent ASR completely up to 700 days (Ramlochan et al., 2004; Szeles et al., 2017), indicating that other effects in addition to a slow down of dissolution could play an important role.

Lithium salts to inhibit ASR were first used 70 years ago by McCoy and Caldwell (McCoy and Caldwell, 1951). McCoy and Caldwell investigated the potential of over 100 different compounds to prevent ASR of mortars containing Pyrex glass as reactive aggregate. All the studied lithium salts containing LiCl, Li₂CO₃, LiF, Li₂SiO₃, LiNO₃, and Li₂SO₄ were found to be more effective than the other inorganic salts and the other compounds such as acids, oils, organic chemicals, proteins, and proprietary admixtures. Since even the almost insoluble LiF seemed to be beneficial, they also suggested that the nature of the accompanying anion of the lithium salts was not particularly important. Later, Stark (Stark, 1992) confirmed the effectiveness of LiF and Li₂CO₃ in inhibiting ASR expansion, and reported that also LiOH seemed to be effective. Several other studies further reported that also LiBr (Qinghan et al., 1995; Bian et al., 1996; Demir and Arslan, 2013; Demir et al., 2018) and LiH₂PO₄ (Bian et al., 1996) are effective in reducing ASR expansion. Bian et al. (Bian et al., 1996) reported LiCl, LiBr, LiNO₂, LiNO₃, Li₂SO₄, and LiH₂PO₄ actually produced similar suppressive effects on ASR expansion, whereas Li₂CO₃, LiOH, and LiF only exhibited certain effects at a [Li]/[Na + K] molar ratio of 0.8.

Among all types of lithium salts, LiNO₃ emerged as the preferred lithium compound for inhibiting ASR (Diamond, 1999), due to its neutrality, high solubility and good compatibility with other admixtures (Wang et al., 1996). It was also reported that LiNO₃ has a benign effect on the concrete properties of strength, electrical resistance, drying shrinkage, and resistance to freezing and thawing, whereas LiOH retard the strength development (Lane and Board, 2002). In Japan the use of lithium nitrite (LiNO₂) as an ASR inhibitor has been widely studied (Sakaguchi et al., 1989; Saito et al., 1992; Kobayashi and Takagi, 2020) due to its positive effect on preventing rebar corrosion as well as due to its high solubility.

Although lithium salts are used to inhibit ASR expansion, Stark et al. (Stark et al., 1993) also reported that insufficient dosages of lithium compounds may even increase the ASR expansion in some aggregates, known as “pessimum effect”. The dosage of lithium salts to be added for inhibiting ASR is commonly expressed as the molar ratio Li/(Na + K). Only at relative high Li/(Na + K) ratio can completely suppress the ASR expansion. A molar ratio of Li/(Na + K) of 0.74 seems to be required to suppress ASR expansion as found in many studies (McCoy and Caldwell, 1951; Bérubé et al., 2004; Leemann et al., 2014), while others also reported that the optimum Li/Na molar ratio is in the range of 0.9–1.2 if LiOH is added (Sakaguchi et al., 1989). In fact, the exact value of the Li/(K + Na) ratio is not fixed but varies depending on the following factors:

a) Type of lithium salts: Thomas et al. (2000) observed that in the case of for LiNO₃ a [Li]/[Na + K] molar ratio of 0.74 sufficiently suppressed ASR-induced expansion with most aggregates, whereas higher ratio of [Li]/[Na + K] of 0.85 was required in the case of LiOH·H₂O in concrete prisms containing a crushed siltstone aggregate. Collins et al. (2004b) reported that the required minimum threshold [Li₂O]/[Na₂O_e] ratio varies from 0.5 to 1.0 for each additive examined (LiOH, LiCl, or LiNO₃), with respect to the particular reactive aggregate used in their work.

Regardless of the above-mentioned factors, it is generally accepted that a dosage of Li/(Na + K) of >0.6 is required to inhibit ASR (Collins et al., 2004b; Leemann et al., 2014; Islam and Ghafoori, 2016) as illustrated in Figure 5 reproduced based on the data from literature (Qinghan et al., 1995; Thomas et al., 2000; Collins et al., 2004b; Mo et al., 2005; Folliard et al., 2006; Kobayashi and Takagi, 2020) and in Table 1.

The proposed Li/(K + Na) molar ratios from most of the studies discussed above are based on freshly prepared mortars and concretes where lithium salts have been used as admixtures. In practice, lithium salts are often used afterwards to slow down expansion in ASR affected concrete. The determination of the optimum amount of lithium for concrete is thus not straightforward. The even distribution of lithium ions within the entire concrete plays a key role in effectively suppressing ASR expansion. Several methods have been suggested to apply lithium salts to ASR-affected concrete, e.g., sprinkling lithium solution on concrete surface (Zapała-Sławeta and Owsiak, 2018), using electrochemical method by applying voltage at 40 V (Souza et al., 2017), injecting lithium solution into concrete under vacuum (Thomas et al., 2007) or by pressure through drilled holes of a 10–30 mm diameter using a compressor (Kobayashi and Takagi, 2020), or by soaking ASR affected concrete elements in lithium solutions. Most of these methods do not achieve sufficient ingress of the lithium salts into concrete (Thomas et al., 2007). The electrochemical process showed higher Li penetration, but reduced the pH of the pore solution near rebar (cathode) due to electrochemical reduction of water. Surface overlay is commonly used on road and airfield pavements and on highway dividers. In this case, better penetration of Li can be achieved by several applications of smaller amounts, e.g., 0.06 L/m² to 0.40 L/m² of 30% LiNO₃ solution with multiple applications (Folliard et al., 2003; Thomas et al., 2007). Injection of lithium salts has been applied to almost 100 ASR-affected concrete structures in Japan (Kobayashi and Takagi, 2020). Since it can only prevent further deterioration and not recover the lost performance, Kobayashi and Takagi suggested that it is desirable to perform lithium injection in an as early as possible stage of deterioration (Kobayashi and Takagi, 2020). However, they found that lithium injection into concrete at early deterioration stage, which indeed suppressed the ASR expansion to some extent, resulted in a larger final expansion than that of the concrete treated with lithium after developing ASR cracks. Moreover, injection of lithium also took longer time for concrete at early stage of deterioration, thus the authors suggested that it would be reasonable and economical to apply this treatment only to concrete already suffering ASR with expected further deterioration. Of course, one should avoid over development of ASR cracks, as it will affect the serviceability of concrete structure. An optimization of injection time is therefore very important. For this purpose, precise evaluation of the stage of ASR is critical for lithium treatment.

ASR is a very slow process at ambient temperature, thus many studies applying lithium salts to mitigating ASR expansion are based on accelerated or ultra accelerated (autoclave) testing methods (Ohama et al., 1989; Qinghan et al., 1995; Bian et al., 1996; Mo et al., 2003). Feng et al. (2005) concluded that the results from autoclave methods involving lithium are not directly comparable to those from studies at lower temperatures and pressures. In contrast, Berra et al. (2003) found that LiNO₃ was effective at both low (38°C) and high (150°C) temperatures and reported a linear relationship of the effective LiNO₃ dosage (Li/[K + Na] molar ratio) between the two temperatures as shown in Figure 6. However, they also found that Li₂CO₃ was only effective at 38°C.

Most published studies on ASR with respect to the effect of lithium on silica dissolution is based on the findings of Lawrence and Vivian (Lawrence and Vivian, 1961), who found that the dissolution of reactive silica strongly depended on the type of alkali hydroxides (i.e., NaOH, KOH, or LiOH) following the order LiOH < NaOH < KOH. Twenty years later, Wijnen et al. (1989) reported similar results and suggested that the dissolution rate of reactive silica decreased with an increase in the effective cation radius of the alkaline species following the order K⁺ < Na⁺ < Li⁺. While many later studies supported the observation of reduced silica dissolution in the presence of various lithium compounds (Plettinck et al., 1994; Kurtis and Monteiro, 2003; Collins et al., 2004b; Feng et al., 2005; Tremblay et al., 2010; Rajabipour et al., 2015), others also observed no significant influence of Li on silica dissolution (Dove and Nix, 1997; Dove, 1999; Leemann et al., 2014; Oey et al., 2020; Leemann, 2021; Bagheri et al., 2021). In some cases, even an increase in silica dissolution in presence of lithium was observed (Kurtis and Monteiro, 2003; Collins et al., 2004b; Bagheri et al., 2021). Kurtis and Monteiro (Kurtis and Monteiro, 2003) and Collins et al. (2004b) studied the dissolution of silica gel in simulated pore solutions with and without lithium salts and observed that in the slurries prepared with LiCl and LiNO₃, the dissolved Si concentration decreased with increasing lithium dosages, while for LiOH they observed an increase in silica dissolution with increasing lithium dosage. A recent study (Bagheri et al., 2021) indicated that lithium in the absence of calcium could in fact accelerate SiO₂ dissolution rate at high pH values by 20–50%.

The above contradictory observations reported in literature are found to be related to the influence of pH and Ca²⁺ on the effect of Li⁺ the dissolution rates (Bagheri et al., 2021). Many studies, which investigated the effect of Li on the dissolution of SiO₂, were carried out under acidic to neutral conditions (Plettinck et al., 1994; Dove and Nix, 1997; Dove, 1999), where no or only a slightly retarding effect of Li⁺ in comparison to Na⁺ or K⁺ was observed and related to the lower tendency of Li⁺ to sorb on the SiO₂ surface and faster ligand exchange rate (Dove, 1999). Bagheri et al. (2021) investigated the effect of Li⁺ under high pH conditions (in 400 mM KOH) and found a clear acceleration of SiO₂ dissolution in the presence of Li⁺, which might be related to the ability of Li⁺ to form surface complexes on silica. It should be noted that lithium can precipitate in the presence of silica as Li₂SiO₃ (Zhou et al., 2018; Bagheri et al., 2021), which can lower the measured silicon concentration or mass loss in dissolution experiments (Kurtis and Monteiro, 2003; Tremblay et al., 2010; Leemann et al., 2014; Zhou et al., 2018; Oey et al., 2020; Leemann, 2021), leading to an apparent decrease in the observed dissolution rate. In the presence of calcium, the effect of Li on the dissolutions changes drastically. A significant decrease in SiO₂ dissolution rate in 100 and 1,000 mM NaOH in the presence of both Li and Ca and the formation of a dense C-S-H containing Li were observed (Leemann et al., 2014; Zhou et al., 2018), indicating a destabilization of Li₂SiO₃ in the presence of calcium. It has been speculated that this Li containing C-S-H layer is responsible for the slowdown of the silica dissolution in the presence of both Li and Ca. More detailed discussions on influence of the lithium on the change of ASR products and formation of Li-Si complex are presented in the next sections.

The detailed study of the literature has indicated that Li might somewhat accelerate SiO₂ dissolution, but only in the absence of calcium, while in the presence of Ca and Li a clear decrease of the silica dissolution has been observed (Leemann et al., 2014; Zhou et al., 2018), which might contribute together with other factors to the lower the expansion observed for Li containing concretes.

Along with the studies on dissolution of silica in presence of lithium, also changes in the ASR product has been reported in many of the studies mentioned above. Several studies reported that presence of lithium lowers the CaO/SiO₂ as well as the (Na + K)/Si ratio in the ASR products (Kawamura and Fuwa, 2003; Feng et al., 2010; Leemann et al., 2014). Feng and co-workers (Feng et al., 2010) observed a dense rigid alkali–silica gel composed of Li with low-Ca contents. The low content of Ca in reaction products was also confirmed in the extensive study by Leemann et al. (2014). They suggested that Ca can be replaced by lithium due to comparable radius of the hydrated cation. It also has been suggested that Li⁺, due to its smaller ionic radius and higher charge density, is more readily incorporated in ASR products than K⁺ and Na⁺ (Kawamura and Fuwa, 2003; Mo, 2005).

In addition to lower the Ca content of ASR products, the precipitation of amorphous Li-Si products has been observed (Sakaguchi et al., 1989; Schneider et al., 2008; Guo et al., 2019). Due to the lack of characterizations of the chemical composition and molecular structure, the amorphous Li-Si products are often described as “Li-Si complex” following the work of Lawrence and Vivian (1961). In some cases, also the precipitation of crystalline Li-containing products has been observed (Mo et al., 2003; Collins et al., 2004a; Feng et al., 2010), generally Li₂SiO₃, which seems to form in the absence of Ca and at low Ca but high Li contents (Zhou et al., 2018; Bagheri et al., 2021). However, so far no systematic investigations about the formation conditions of the amorphous and crystalline Li-containing products have been conducted. Leemann et al. (2014) found no evidence of crystalline Li₂SiO₃ formation and argued that crystalline lithium silicate is unlikely to form under dosages of Li/Na + K < 1.0.

Most studies reported that those Li-containing products are not expansive (Sakaguchi et al., 1989; Diamond, 1992; Kawamura et al., 1994; Leemann et al., 2014), although a minimum proportion of lithium must be reached to be non-expansive (Stark, 1992), which could explain the “pessimum” effect of lithium dosages on controlling ASR expansion. However, a recent study showed that formation of crystalline Li₂SiO₃ could actually cause expansion and cracks in concrete after long periods of exposure (Liu et al., 2019).

Several researchers (Mitchell et al., 2004; Schneider et al., 2008; Leemann et al., 2014) have used nuclear magnetic resonance (NMR) spectroscopy to study the effects of lithium on the chemical structure of ASR gels. The incorporation of Li⁺ ions into the ASR gel changes its structure from a product with a layered structure (containing mainly Q³ sites) to a product with more disordered networks containing mainly Q¹ and Q² sites. This was put forward as a possible reason, why Li based ASR products show little expansion. It was claimed that expansive gels are typically characterized by large presence of Q³ sites as layered silicates, while the depolymerized products containing mainly Q¹ and Q² sites are not considered to swell upon contact with water (Kirkpatrick et al., 2005; Tambelli et al., 2006). Also Kurtis and coworkers (Kurtis et al., 2000; Kurtis and Monteiro, 2003) suggested that the suppressive effect of lithium on ASR expansion should be attributed to the limitation of ASR gel re-polymerization, rather than reduced dissolution of silica. By dispersing the ASR gel extracted from ASR-affected structure to NaOH solution alone, they observed that ASR is partially dissolved and re-polymerized as a potentially expansive gel. However, when the ASR gel was exposed to the mixture NaOH and LiCl solution, re-polymerization into an expansive gel was limited.

The understanding of ASR mitigation mechanisms by Li is simply based on the assumption that conventional ASR products, which have a layered silicate structure, will swell upon uptake of water while Li-containing ASR products with mainly Q¹ and Q² sites will not swell. However, as recent work based on synthetic ASR products (Shi et al., 2019) and field ASR products (Leemann et al., 2020) demonstrated that both amorphous and crystalline ASR products do not swell upon uptake of water, rather alternative mechanisms seem responsible for ASR expansion as well as their suppression by Li, indicating an urgent need for clarifying the real cause of ASR expansion.

Another mechanism of ASR mitigation suggested for lithium is that these products serve as physical barrier preventing the further dissolution and reaction of reactive silica. This mechanism was first suggested by Lawrence and Vivian (1961), who reported that the lithium silicates formed had low solubility producing a coating effectively protecting the reactive aggregates from further participation in ASR. This observation has been supported by many later studies (Kawamura and Fuwa, 2003; Mitchell et al., 2004; Feng et al., 2010; Leemann et al., 2014; Leemann et al., 2015; Kim and Olek, 2016; Guo et al., 2019). Leemann et al. (2015) observed that the mitigating efficiency of LiNO₃ lessens with increasing the specific surface area of the reactive aggregates, due to the increased area to be covered by the lithium products. Based on this finding, Kawamura (2017) assumed that the mitigating effects of lithium on ASR affected structures may be related to the amount of ASR products already formed. Few studies reported that only the Li products containing Ca could serve a physical barrier. Zhou et al. (2018) observed that quartz glass slices immersed in solution containing both LiNO₃ and Ca(OH)₂ were well protected by the precipitation of a dense layer, while samples were seriously damaged in the solution with only LiNO₃ or Ca(OH)₂. In contrast, Tremblay et al. (2010) considered the formation of physical barriers unlikely based on detail analysis of the surface and suggested an increased chemical stability of silica due to a presently unknown mechanism as the probable cause.

Several additional mechanisms for the ASR mitigation of lithium have been proposed. Prezzi et al. (1997) introduced an electrical double layer (EDL) theory to explain the suppressive effect of lithium on ASR expansion. ASR gels are negatively charged (Krattiger et al., 2021), and are thus surrounded by a positively charged electrical double layer where cations accumulate. Theoretically, cations with the same valence but smaller hydrated ionic radii will result in a thinner double layer, which would cause based on the swelling theory less gel expansion. As the hydrated radius increases in the order K⁺ < Na⁺ < Li+ (Conway, 1981), this would mean that Li would result in higher expansion, which is contradictory to the results generally observed in expansion testing, and thus also to the explanation suggested by (Prezzi et al., 1997). It was also proposed that presence of lithium may reduce the repulsive forces between colloidal ASR gel particles (Mohd et al., 2017). Bian et al. (1996) proposed that the suppressive effect of cations on ASR expansion depended on the ionic surface charge density. The higher value of surface charge density, the stronger the bonding between the cation and anions in the gels, the less tendency to expand. Others (McCormick et al., 1989; Gaboriaud et al., 1999) studied the mitigating mechanisms based on sol-gel principles, they observed that presence of lithium enhanced the formation of large silicate species in solution, which took longer time to destabilize (Gaboriaud et al., 1998).

Based on the literature reviewed above it becomes clear, that the presence of some Ca as well as of sufficient Li are prerequisites for an effective mitigation by Li. Different mechanism such as blocking of dissolution, formation of a non-expansive solid, prevention of swelling due to other reasons have been suggested, however the findings reported in literature are contradictory and the experimental evidence pointing in any direction is circumstantial and inconclusive. This may not be surprising as also the mechanism of ASR expansion itself is under debate since recent investigations have suggested that the swelling theory does not agree with the observed changes in the ASR product (Shi et al., 2019; Leemann et al., 2020; Shi et al., 2020a; Geng et al., 2021).