Publications

Micromechanics-guided development of a slag/fly ash-based strain-hardening geopolymer composite

Strain-hardening geopolymer composite (SHGC) lately emerged as a promising alternative to traditional strain-hardening cementitious composite with added advantages of industrial by-product utilization and enhanced sustainability. However, as the design of SHGC requires multi-factor optimization, the application of traditional trial-and-error method is inefficient and hinders the development of this material. This paper aims at the development of a slag/fly ash-based SHGC with low slag content using a micromechanical model to guide the composite mixture design. To this end, experimentally characterized physical properties of fiber, matrix and interface are used as input for the micromechanical model, which serves as a predictive tool for the tensile performance of SHGC. Following the guidance, a slag/fly ash-based SHGC with tensile strain capacity of 4.8% and ultimate tensile strength above 3.8 MPa was systematically developed. The feasibility and effectiveness of using micromechanics as the design basis of SHGC are demonstrated and experimentally verified.

Prediction of mechanical properties and durability of civil engineering structures

A coupled transport-reaction model for simulating autogenous self-healing in cementitious materials. Part I : Theory

In this study, a coupled transport-reaction model was specifically developed for simulating autogenous self-healing in cementitious materials. The diffusion of ions between the solution in the crack and in the bulk paste is simulated with a transport model. The concentration of ions in the crack can be calculated. As the kinetics of chemical reactions is faster than diffusion, the chemical reactions taking place in the crack can be simulated by thermodynamic calculation. With the thermodynamic calculation, the amount of reaction products formed in the crack and the remaining ion concentrations after the chemical reactions can be determined. In this way, autogenous self-healing in cementitious materials is simulated iteratively by means of diffusion calculation then thermodynamic calculation at each time step.

Numerical study on predicting the performance of repair materials under differential volume changes

Differential volume changes between repair material and substrate concrete can induce high stresses in repair systems and result in failure of concrete repairs. An analytical model has been developed to investigate the performance of repair materials. This model can be used to calculate the failure risk of concrete repairs. With this model, the performance of nine commercially available repair materials was investigated in the case of differential volume changes due to shrinkage of repair materials, ambient temperature change or sudden cooling.

A methodology to study carbonation of cement paste and its effect on permeability

Carbonation is mainly known as a deterioration phenomenon for reinforced concrete structures. However, it has also a beneficial effect when we consider transport of aggressive species. This study aims at developing an optimized technique, in terms of quantification of carbon dioxide uptake, to control the carbonation of cement-based materials and to examine the permeability of the carbonated specimens. Samples were embedded in a special cell and subjected to an elevated CO2 pressure at the upstream side. The CO2 uptake was measured in the up-and downstream part by precise mass flow controllers. The experiments were performed on hardened cement. Post-analysis techniques like TGA, XRD and pH measurents were used to determine the phase changes, hence the carbonation front, while MIP and nitrogen adsorption were used to characterize the microstructure changes The effect of carbonation on the general transport behaviour was studied by measuring changes in the water permeability. Results show that the carbonation of fully saturated sample was very slow. Even this limited carbonation leaded to a decrease in permeability by one order of magnitude.

Experimental and numerical evaluation of mechanical properties of interface between filler and hydration products

Mechanical properties prediction of blast furnace slag and fly ash-based alkali-activated concrete by machine learning methods

Effect of Limestone Fillers on Ca-Leaching and Carbonation of Cement Pastes

Because of its environmental and economic benefits, part of cement is replaced by limestone fillers (LS). However, the effect of LS on the chemical degradation of cement-based materials is still unclear. In this study, accelerated leaching and carbonation were applied on cement pastes to study the effects of LS replacement on the degradation rates and microstructural alterations of degraded materials. Ammonium nitrate solution was used to accelerate the leaching process, while carbonation was speeded up by applying an elevated pressure gradient of pure CO 2 on samples with 65% relative humidity. The carbonation rate was characterized by phenolphthalein carbonation depth and CO 2 uptake, while leaching rate was quantified by phenolphthalein leaching depth and Ca-leached amount. Leached/carbonated samples were analyzed by a series of post-analysis techniques to characterize the microstructural and mineralogical changes. Results showed that, for a similar w/c ratio, a higher LS replacement resulted in lower leaching rate. For carbonation, LS replacement promoted the CO 2 uptake despite similar carbonation depth. Furthermore, LS replacement led to less C-S-H carbonation compared to samples without LS.

Review on concrete under combined environmental actions and possibilities for application to alkali activated materials

3D Microstructure Simulation of Reactive Aggregate in Concrete from 2D Images as the Basis for ASR Simulation

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 40×40×40 mm3 concrete cube with irregular coarse aggregates was then simulated with the aggregate assembled by the five representative microstructures. The average pore (at microscale μm) 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.

Effect of curing condition on mechanical properties and durability of alkali-activated slag mortar

While alkali-activated slag (AAS) has emerged as a promising alternative binder in construction engineering, a consensus on the optimal curing condition for this material has not been reached yet. It is well known that AAS can harden at ambient temperatures, but the influence of humidity on its properties remains poorly understood. Herein, we considered five curing conditions with different relative humidities (RH), including ambient/dry condition (RH=55 %), sealed condition (RH=80–95 %), fog condition (RH>95 %), water immersion condition (RH=100 %), and saturated limewater immersion condition (RH=100 %). Various properties have been examined, including flexural and compressive strengths, elastic modulus, shrinkage, pore structure, carbonation resistance, and freeze-thaw resistance of AAS mortars (AASM). Two types of activators, sodium hydroxide and sodium silicate (modulus at 1) solutions were used. The experimental results indicate that drying at early ages is detrimental to almost all the properties investigated. Sealed curing can deliver desirable mechanical properties and durability, but considerable shrinkage. Fog and water curings are highly effective at mitigating early shrinkage in AASM, but the problem of leaching adversely affects its long-term properties. Generally, limewater curing offers limited benefits compared to other high-humidity curing methods.

Effect of superabsorbent polymers (SAP) on the freeze–thaw resistance of concrete: results of a RILEM interlaboratory study

Enhancing the reaction of municipal solid waste incineration (MSWI) bottom ash in blast furnace slag-based alkali-activated blends: A novel strategy and underlying mechanism

Durability and service life of concrete repairs in the presence of cracks

Engineered Cementitious Composite (ECC) has been proposed to be one of the most promising repair materials due to its unique high ductility and tight crack width control. In concrete repairs, the shrinkage of repair materials is restrained by concrete substrate, and the repair material therefore often cracks. When ECC is used as repair material, the crack width is much smaller compared to normal concrete. The tight crack width of ECC retards the penetration of water and harmful substances and thus enhances the durability of concrete repairs. This paper is aimed to explore the chloride penetration in cracked ECC repairs and to assess the service life of the repair systems. Rapid chloride migration tests was conducted to investigate the chloride penetration profile. Based on the experimental results, the service life of repair systems was evaluated.

Elastic Modulus of the Alkali-Silica Reaction Rim in a Simplified Calcium-Alkali-Silicate System Determined by Nano-Indentation

This work aims at providing a better understanding of the mechanical properties of the reaction rim in the alkali-silica reaction. The elastic modulus of the calcium alkali silicate constituting the reaction rim, which is formed at the interface between alkali silicate and Ca(OH)2 in a chemically-idealized system of the alkali-silica reaction, was studied using nano-indentation. In addition, the corresponding calcium to silica mole ratio of the calcium alkali silicate was investigated. The results show that the elastic modulus of the calcium alkali silicate formed at the interface increased with the increase of the calcium to silica mole ratio and vice versa. Furthermore, the more calcium that was available for interaction with alkali silicate to form calcium alkali silicate, the higher the calcium to silica mole ratio and, consequently, the higher the elastic modulus of the formed calcium alkali silicate. This work provides illustrative evidence from a mechanical point of view on how the occurrence of cracks due to the alkali-silica reaction (ASR) is linked to the formation of the reaction rim. It has to be highlighted, however, that the simplified calcium-alkali-silicate system in this study is far from the real condition in concrete.

Engineered cementitious composite with blast furnace slag and limestone powder

Evaluation of rheology and strength development of alkali-activated slag with different silicates sources

Element zonation in carbonated alkali-activated slag