Structural characterization of alkali-silica reaction gel: An x-ray absorption fine structure study
Introduction
An alkali-silica reaction (ASR) is the chemical reaction between aggregates and alkali hydroxides of concrete structures. The product of an ASR is a hygroscopic alkali-silica gel that, in the presence of water, increases its volume and cracks the concrete. The ways to prevent an ASR occurring in concrete are well established in several studies [[1], [2], [3], [4], [5], [6]], but new nanoscale studies are necessary to better understand the gel structure of the ASR, the mechanism of expansion and, also, to generate information that could be used to develop healing methods, which can be applied to previously ASR-affected structures [[7], [8], [9], [10], [11], [12], [13]]. Based on this context, several studies have been carried out.
Khouchaf et al. [14] proposed that an ASR occurs due to the rupture of Si-O-Si among tetrahedral SiO2 (Q4), thereby producing Q3 sites due to the attack of hydroxyl ions. Thus, amorphous phases co-exist in the aggregate during the alkaline attack. Over time, the number of Q3 (amorphous) and Q4 (crystalline) sites increases; Q3 sites lead to the formation of amorphous products within the aggregate particles, which could explain the expansion of the aggregate in concrete [15]. Further analysis of the ASR gel by Nuclear Magnetic Resonance (NMR) supports this proposition [16].
Kirkpatrick et al. [17] and Hou et al. [18] also investigated the structure of ASR gel by means of NMR analysis of the 29Si, which showed predominant Q3 sites but also significant concentrations of polymerized Si. The authors concluded that polymerization of the silicates from the gel is correlated with the alkali/silica ratio and found that 50% of the gel is associated with the H (Si-OH groups) and 50% is associated with alkaline cations. Moreover, they concluded that polymerized gel from an ASR is compatible with a CSH with low Ca content and that K and Na have similar behavior in the ASR.
Florindo [19] showed that the structure of the ASR gel is a lamellar disordered network, formed predominantly by Q3 sites with OH bonds and connected with Q1, Q2 and Q4 sites, and that K+ ions are uniformly distributed over the different Qn species. According to this author, there is an abundance of water in the structure of ASR gel, which gives it high mobility.
Ichikawa and Miura [20] developed a model of ASR reaction, in which they observed that OH−, Na+ and K+ ions, present in concrete solution, depolymerize silica from the aggregate and convert it into a hydrated silico-alkaline gel. Thus, the aggregate surface is converted into alkaline silicate; OH– consumption leads to dissolving the Ca2+ of the solution, which penetrates the structure of the alkaline silicate and promotes repolymerization. The aggregate is surrounded by ASR gel, which arises from the penetration of the Na+, K+, Ca2+ and OH−. The pressure is concentrated in the aggregate which then cracks, as does the cement grout around it.
Ling et al. [21] also concluded that ASR products have low calcium content. The morphology of the structure formed at early ages is more ordered in terms of polymerization and dominated by Q3 and Q2 sites. Probably the calcium ions in the solution enter the structure of the particle, depolymerize and develop a CSH phase, this being indicated by the dominant presence of Q1 sites [21]. In synthetic NaCa gels, a CSH phase and a gel phase co-exist [22].
Benmore and Monteiro [9] found intact silicates in the ASR gel with SiSi bonds, and that intact fragments are arranged in layers within the ASR gel, with a periodicity of 9.7 Å. They also found that a local K-kanemite-like structure persists in the gel and that water probably most likely resides in pores around these layered regions, but some water may also penetrate within and between the layers themselves.
Despite all the information about gel produced by an ASR, there are many doubts or assumptions about its atomic structure and, even with pieces of evidence indicating the existence of a structure similar to that of (Na,K)-kanemite in the ASR gel, there are also pieces of evidence suggesting that this is not the exact model. For example, experimental and theoretical studies developed by Kirkpatrick et al. [17] have shown that the interaction between water and gel may involve a chemical reaction, such as dissolution of the gel, in addition to sorption. Also, TGA results have shown that ASR gel contains a significant amount of water with structural environments that are not present in crystalline K-kanemite, and the form of the water sorption curve described in their study suggests that there is an important accumulation of water in pores, located between silicate nanoparticles. In addition, Molecular Dynamics modeling shows that the incorporation of large amounts of water into a kanemite-like structure is energetically unfavorable and, shockingly, K-kanemite does not accommodate any interlayer water. All this information indicates that a kanemite-like model is not enough to explain ASR in concrete, and that nanoscale studies to indicate possibilities and/or to better understand the atomic structure of the ASR gel are necessary.
Therefore, the purpose of this paper is to take advantage of the x-ray absorption fine structure (XAFS) technique to analyze ASR gel and obtain more information about its short-range order structure around silicon and potassium atoms, in order to validate or invalidate models, to better understand the ASR reaction and, if possible, to underpin the development of remedial actions for the damaged concrete structures.
Section snippets
Experiment
The gel sample studied in this paper was collected from the drain gallery walls of the Rio Grande Dam (Property of Furnas Centrais Eletricas S.A. and located in the state of Minas Gerais, Brazil), which has many cracks caused by ASR. Typical aggregates used for concrete were also chemically attacked. Therefore, using a solution rich in KOH, an attempt was made to reproduce the effects of ASR in laboratory conditions. Particularly, the effects of the chemical attack on quartz aggregates were
Results and discussion
Previous studies [9] suggest that the ASR gel structure has similarities with the (Na,K)-kanemite structure. On the other hand, there are also strong pieces of evidence suggesting this is not the exact model [17]. To enable the reader to better understand the results obtained, and how they validate or invalidate the suggested structural models for the ASR gel, the structure of (Na,K)-kanemite is briefly described below.
The Na-kanemite structure [28], shown in Fig. 2, is formed by (SiO4)
Conclusions
XANES simulation strongly indicates that the atomic structure around potassium atoms in the KOH attacked quartz is, in several aspects, similar to the atomic arrangement around the potassium atom #4 of the K2Si2O5 [30] but, more than that, it also insinuates that both structures present a similar oxidation state and coordination number. If the atomic structure around potassium atom #4 is the exact model for the average structure of the ASR gel at the potassium K-edge, XANES simulations suggest
Acknowledgements
The authors of this study wish to thank IMED, FURNAS, ANEEL Program of Research and Development (R&D), Electron Microscopy Center of Federal University of Rio Grande do Sul (CME/UFRGS) for making materials, equipment and laboratories available and the National Council for Research (CNPQ) for financial support. The authors would like to thank Dr. Paulo J.M. Monteiro for his collaboration for the study. This work has also been partially supported by the Brazilian Synchrotron Light Laboratory
References (37)
- et al.
Effects of lithium hydroxide on alkali silica reaction gels created with opal
Constr. Build. Mat.
(2007) - et al.
Influence of lithium hydroxide on alkali–silica reaction
Cem. Concr. Res.
(2010) - et al.
New observations on the mechanism of lithium nitrate against alkali silica reaction (ASR)
Cem. Concr. Res.
(2010) - et al.
A thermodynamic and kinetic model for paste–aggregate interactions and the alkali–silica reaction
Cem. Concr. Res.
(2015) - et al.
Alkali–silica reaction: current understanding of the reaction mechanisms and the knowledge gaps
Cem. Concr. Res.
(2015) - et al.
Utilization of quarry by-products for reduction of expansion due to alkali-aggregate reaction
Cem. Concr. Comp.
(2016) - et al.
Examination of the concrete from an old Portuguese dam: texture and composition of alkali–silica gel
Mater. Charact.
(2007) - et al.
Effects of aggregate size on alkali–silica-reaction induced expansion
Cem. Concr. Res.
(2012) - et al.
The structure of alkali silicate gel by total scattering methods
Cem. Concr. Res.
(2010) - et al.
Multi-scale analysis of alkali–silica reaction (ASR): impact of alkali leaching on scale effects affecting expansion tests
Cem. Concr. Res.
(2016)
Detecting alkali-silica reaction: a multi-physics approach
Cem. Concr. Comp.
Amorphisation mechanism of a flint aggregate during the alkali–silica reaction: X-ray diffraction and X-ray absorption XANES contributions
Cem. Concr. Res.
Modified model of alkali-silica reaction
Cem. Concr. Res.
Characterization of alkali silica reaction gels using Raman spectroscopy
Cem. Concr. Res.
Composition–rheology relationships in alkali–silica reaction gels and the impact on the gel's deleterious behavior
Cem. Concr. Res.
Alkali-silica reaction - a problem of the insufficient fundamental knowledge of its chemical base
Mater. Sci. Concr.
XANES, EXAFS and RMN contributions to follow the structural evolution induced by alkali-silica reaction in SiO2 aggregate
Phys. Scr.
Investigation of Concretes Affected by alkali-Aggregate Reaction and Advanced Characterization of Exuded Gel
Cited by (9)
Effect of type and quantity of inherent alkali cations on alkali-silica reaction
2023, Cement and Concrete ResearchEffect of improved autogenous mortar self-healing in the alkali-aggregate reaction
2021, Cement and Concrete CompositesCitation Excerpt :The tabular gel was also identified (Fig. 13b). According to Prado et al. [98], this formation reflects a high sodium content, as confirmed by this study through an EDS analysis. Regarding length change, another result from Fig. 8 is that combined mixtures (R1F1A and R1F3A) performed worse than the ones with non-combined materials (R0F1A, R0F3A, and R1F0A).
Intermolecular interactions of nanocrystalline alkali-silica reaction products under sorption
2020, Cement and Concrete ResearchCitation Excerpt :Their results suggest that water ingress in ASR products is limited at the molecular scale, which corroborates the experimental evidence [15,16]. However, recent studies show that kanemite molecular structure is not equivalent to the structure of ASR products found in concrete [15,17,18]. A recent experimental study [15] shows that crystalline ASR products are similar to the naturally occurring mineral shlykovite (KCa[Si4O9(OH)].3H2O]) [19], with isomorphic substitutions of potassium by sodium taking place according to the composition of the pore solution.
Atomistic structure of alkali-silica reaction products refined from X-ray diffraction and micro X-ray absorption data
2020, Cement and Concrete ResearchCitation Excerpt :It has been proposed recently [45], that K K-edge XANES spectrum of ASR product are similar to that of K2Si2O5 where K is coordinated by 7 O atoms [46]. It is likely that the measured bulk sample in the published work [45] contains dominantly amorphous product. As shown in Fig. 5a, the Na K-edge spectra resemble the features of K K-edge, in terms of the number and position of peaks.