Municipal solid waste incineration (MSWI) bottom ash, due to its high mineral content, presents great potential as supplementary cementitious material (SCM). Weathering, also known as aging, is a treatment process commonly employed in waste management to minimize the risk of heavy metal leaching from MSWI bottom ash. Using weathered MSWI bottom ash to produce blended cement pastes is considered as a high-value-added and sustainable waste disposal solution. However, a critical challenge arises from the metallic aluminum (Al) in weathered MSWI bottom ash, which is known to induce detrimental effects such as volume expansion and strength loss of blended cement pastes. While most metallic Al in weathered MSWI bottom ash can be removed with eddy current separators in metal recovery plants, the residual metallic Al, owing to its small particle size, cannot be removed with the same technique. This study is dedicated to addressing this issue. An in-depth analysis was conducted on residual metallic Al embedded in weathered MSWI bottom ash particles, aiming to guide the removal of this metal. This analysis revealed that mechanical removal was the most suitable method for extracting metallic Al. The specific processes and mechanisms underlying this method were elucidated. After reducing metallic Al content in weathered MSWI bottom ash by 77 %, a significant improvement in the quality of blended cement pastes was observed. This work contributes to the broader adoption of mechanical treatments for removing residual metallic Al from weathered MSWI bottom ash and facilitates the application of treated ash as SCM.

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In our former paper, based on a published 3D reactive transport model at microscale with the capability of simulating the chemical reactions involved in ASR, the location of expansive ASR gel related to the reactivity of aggregate, temperature, aggregate porosity and silica content in aggregate, is clarified. Based on the simulation results, in this paper, the cracking process at mesoscale in concrete induced by ASR in the early stage is investigated. The results show that the cracking process can be divided into four stages and three cracking routes are generalized with the behind chemical exposed environments specified. The cracking routes are found to be comparable with the experimental observed routes. For the first time, the cracking patterns induced by ASR in concrete at mesoscale is linked with the chemical reactions at microscale, which is the first step towards building a complete computational tool to predict ASR as realistic as possible.

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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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Portland cement is the most produced material in the world. The hydration process of cement consists of a group of complex chemical reactions. In order to investigate the mechanism of cement hydration, it is vital to study the hydration of each phase separately. An integrated model is proposed in this paper to simulate the dissolution of alite under different hydrodynamic conditions at microscale, coupling Kinetic Monte Carlo model (KMC), Lattice Boltzmann method (LBM) and diffusion boundary layer (DBL). The dissolution of alite is initialised with KMC. Two Multiple-relaxation-time (MRT) LB models are used to simulate the fluid flow and transport of ions, respectively. For solid-liquid interface, DBL is adapted to calculate the concentration gradient and dissolution flux. The model is validated with experiment from literature. The simulation results show good agreements with the results published in the literature.

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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

Cracking is inevitable during the service period of concrete structures. They are preferential ingression channels for aggressive ions. It is difficult or even impossible to repair all the cracks due to the limitation of practical conditions. However, cracks have potentials to self-heal due to further hydration and carbonization. The effect of autogenous self-healing on the properties of cementitious materials has been studied experimentally by many researchers. However, researches on modelling of the autogenous self-healing processes are still limited. In this paper, a Lattice Boltzmann single component model is proposed to simulate the self-healing caused by further hydration in cement paste matrix at mesoscale. The model simulates not only the healing efficiency but also the geometry change. The simulation result shows that even when the filling efficiency is low, some locations in the crack could be completely blocked. This may lead to lower effective diffusion coefficient of ions via the cracked sample.@en

Microcracks play vital roles in the prediction of the service life of concrete structure. Because microcracks in concrete structure are the preferential ingression channels for aggressive ions, e.g., chloride, sulphate, etc. However, microcracks have potentials to self-heal autogenously due to the continuous hydration of unhydrated cement, especially when ultra-/ high strength concrete is used. To quantify the autogenous self-healing effects of microcracks in cement paste, our experiment is designed to monitor the self-healing process of microcracks in cement paste continuously by using optical microscope. The healing products are quantified by image analysis with newly implemented software in MATLAB. The results indicate that the microcracks are not filled evenly along the crack length and most healing products are Ca(OH)2, which dissolve partly from the paste matrix and re-nucleate in the microcrack, in addition to its counterpart from the continuous hydration of unhydrated cement. Furthermore, the sample cracked at earlier age shows higher potential to heal, while the sample with smaller crack width experiences greater filling efficiency. The obtained autogenous selfhealing mechanism will be used in the future simulation. @en