A 3D reactive transport model at microscale is proposed for simulating the chemical reaction process of alkali silica reaction (ASR) thermodynamically and kinetically including the dissolution of reactive silica, the nucleation and growth of ASR products and the dissolution of calcium hydroxide (CH) and calcium silicate hydrate (C-S-H) as a buffer of Ca²⁺ and OH⁻ to ASR. The implementation methodologies are firstly explained. Sensitivity analyses are done to calibrate some important parameters. The model is then applied to investigate the influence of the silica microstructural disorder degree on ASR. The simulation results show that the model is able to simulate successfully two typical patterns of the expansion sites location depending on the silica reactivity (inside the aggregate or in the aggregate-cement paste zone) found in field concrete and laboratory samples. A possible mechanism is provided based on the quantitative data captured by the model. The model can be extended to a multiscale ASR model for physic-chemical simulation to bridge the gap between the fundamental chemical mechanisms and the physical response of concrete.

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Prediction of alkali silica reaction is still difficult due to the lack of a comprehensive understanding of its chemical fundamentals. In-site experimentally revealing the fundamentals is not realistic as ASR shows over several years or even decades and is affected by many factors. In this paper, by utilizing a 3D reactive-transport simulation model at microscale, we have numerically explored the fundamentals of ASR in the early stage under the influence of reactive silica fraction, alkali concentration, silica disorder degree and aggregate porosity. Based on the simulation results, the chemical sequences of ASR, the initial location of ASR products, the mechanism behind and the role of calcium under the influence of the above factors are elaborated. Furthermore, a comprehensive mechanism to explain the pessimum reactive aggregate content is derived. The results of this paper give some insights about ASR in the early stage such as the initial expansion locations.

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The microstructure of alkali-reactive aggregates, especially the spatial distribution of the pore and reactive silica phase, plays a significant role in the process of the alkali silica reaction (ASR) in concrete, as it determines not only the reaction front of ASR but also the localization of the produced expansive product from where the cracking begins. However, the microstructure of the aggregate was either simplified or neglected in the current ASR simulation models. Due to the various particle sizes and heterogeneous distribution of the reactive silica in the aggregate, it is difficult to obtain a representative microstructure at a desired voxel size by using non-destructive computed tomography (CT) or focused ion beam milling combined with scanning electron microscopy (FIB-SEM). In order to fill this gap, this paper proposed a model that simulates the microstructures of the alkali-reactive aggregate based on 2D images. Five representative 3D microstructures with different pore and quartz fractions were simulated from SEM images. The simulated fraction, scattering density, as well as the autocorrelation function (ACF) of pore and quartz agreed well with the original ones. A mm concrete cube with irregular coarse aggregates was then simulated with the aggregate assembled by the five representative microstructures. The average pore (at microscale m) and quartz fractions of the cube matched well with the X-ray diffraction (XRD) and Mercury intrusion porosimetry (MIP) results. The simulated microstructures can be used as a basis for simulation of the chemical reaction of ASR at a microscale.

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ASR (alkali-silica reaction) is one of the toughest durability problems in engineering. However, the damage induced by ASR is still fairly unpredictable due to the lack of microstructural information of cement-based materials affected by ASR, while the microstructure determines the global performance. In order to fill this gap, a multiscale simulation model of ASR is under development. The basic theory and assumptions about this multiscale model can be found in [8]. This paper illustrates how the microstructure evolution of cement-based materials induced by ASR is achieved by this model. In the model, the entire chemical process including dissolution of reactive aggregate, nucleation and growth of ASR products (alkali silicate complex, calcium alkali silicate complex), is quantitatively simulated based on the kinetic and thermodynamic parameters. Furthermore, the 3D heterogeneous aggregate is numerically simulated using stereology based on the data from 2D thin-section. As a result, the dissolution degree of aggregate, the amount and location of ASR products and the porosity change can be traced. These micro parameters can be used for the simulation of crack formation in mesoscale. Similarly, the macroscale damage can be predicted based on the simulation results from mesoscale.@en