Structural characterization of alkali-silica reaction gel: An x-ray absorption fine structure study

https://doi.org/10.1016/j.cemconres.2019.05.019Get rights and content

Abstract

This paper sets out to acquire information on the atomic structure of the alkali-silica reaction gel by using the x-ray absorption fine structure (XAFS) technique to improve the understanding of the mechanism of expansion. The gel generates mechanical stress in the concrete, which cracks it. XAFS enables the study of the local atomic structure around each of the atomic species in the material, even in disordered structures. Analyses were made at the potassium (3608 eV) and silicon (1839 eV) absorption K-edges. Information was obtained about the local order (first and second neighbors) around the absorbing atoms and the nanoscale arrangement in the gel. The results show that most accepted structural models that are used to describe the gel are inaccurate and that a chemically inhomogeneous nanostructured material can be the better choice to explain the ASR gel structure and its chemical behavior.

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 (Sisingle bond-OH groups) and 50% is associated with alkaline cations. Moreover, they concluded that polymerized gel from an ASR is compatible with a Csingle bondSsingle bondH 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 Osingle bondH 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 Csingle bondSsingle bondH phase, this being indicated by the dominant presence of Q1 sites [21]. In synthetic Nasingle bondCa gels, a Csingle bondSsingle bondH phase and a gel phase co-exist [22].

Benmore and Monteiro [9] found intact silicates in the ASR gel with Sisingle bondSi 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)

  • M. Rashidi et al.

    Detecting alkali-silica reaction: a multi-physics approach

    Cem. Concr. Comp.

    (2016)
  • J. Verstraete et al.

    Amorphisation mechanism of a flint aggregate during the alkali–silica reaction: X-ray diffraction and X-ray absorption XANES contributions

    Cem. Concr. Res.

    (2004)
  • T. Ichikawa et al.

    Modified model of alkali-silica reaction

    Cem. Concr. Res.

    (2007)
  • C. Balachandran et al.

    Characterization of alkali silica reaction gels using Raman spectroscopy

    Cem. Concr. Res.

    (2017)
  • A.G. Vayghan et al.

    Composition–rheology relationships in alkali–silica reaction gels and the impact on the gel's deleterious behavior

    Cem. Concr. Res.

    (2016)
  • W. Wieker et al.

    Alkali-silica reaction - a problem of the insufficient fundamental knowledge of its chemical base

    Mater. Sci. Concr.

    (1998)
  • L. Khouchaf et al.

    XANES, EXAFS and RMN contributions to follow the structural evolution induced by alkali-silica reaction in SiO2 aggregate

    Phys. Scr.

    (2005)
  • N.P. Hasparyk

    Investigation of Concretes Affected by alkali-Aggregate Reaction and Advanced Characterization of Exuded Gel

    (2005)
  • Cited by (9)

    • Effect of improved autogenous mortar self-healing in the alkali-aggregate reaction

      2021, Cement and Concrete Composites
      Citation 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 Research
      Citation 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 Research
      Citation 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.

    View all citing articles on Scopus
    View full text