Publications

This study aims to predict the autogenous shrinkage of alkali-activated concrete (AAC) based on slag and fly ash. A variety of analytical and numerical models are available for the prediction of autogenous shrinkage of ordinary Portland cement (OPC) concrete, but these models are found to show dramatic discrepancies when applied for AAC due to the different behaviours of these two systems. In this study, a new numerical approach is developed to predict the autogenous shrinkage of alkali-activated slag (AAS) and alkali-activated slag-fly ash (AASF) concrete from the experimental results on corresponding paste. In this approach, the creep of AAS and AASF and the restraining effect of the aggregate are particularly considered. By this approach, a fairly good prediction is obtained. Moreover, the microcracking in paste caused by restraining aggregates is evaluated. The results indicate that AAC is subjected to high tendency of development of microcracking.

The influence of the Ground granulated blast-furnace slag (GGBS) on the microstructure of the interfacial transition zone (ITZ) and the bulk matrix was determined by using MIP measurements. The ITZ quantity was controlle by systematically varying the aggregate volume fraction. Correlating the chloride migration coefficients with the variations in the microstructure, the relative importance of the improved ITZ to the enhancement of the overall properties was discussed. Results indicated that the incorpooration of GGBS as a partial substitute of OPC tends to refine the pore structure, particularly in the range superior to 100 nm. The Dnssm values of the series containing GGBS decrease significantly when the replacement rate of GGBS increases regardless of aggregate volume fraction and size. From the results provided in the transport measurements, the differences in the transport coefficients between the reference series and the blended series were more apparent for the neat paste samples. Moreover, the variations of transport coefficient were highly dependent on the changes in the capillary porosity rather than on changes in the coarse pore size range. This is presumably because the large pores at the interfacial zones are not interconnected with each other but rather the relatively small pores of the matrix. The observed improvement of chloride resistance of the GGBS blended samples must be more importantly due to the marked densification of the bulk cement matrix.

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.

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.

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.

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.

Because of environmental and economic benefits, a fraction of cement is increasingly replaced by limestone fillers raising a question on to what extent limestone fillers affect the durability of cementitious materials. This work aims at understanding the effects of water/powder (w/p) ratio and limestone filler replacement on water permeability of cement pastes. A newly proposed technique using a controlled constant flow concept was applied to measure permeability of hardened cement paste samples following a factorial experimental design. It was observed that both limestone filler and w/p ratio significantly influence the water permeability. At a given w/p ratio, adding limestone filler made the microstructure coarser, especially for high w/p ratio. Nevertheless, if the comparison is based on a given water/cement (w/c) ratio instead of w/p ratio, the limestone filler replacement refined the microstructure in terms of capillary porosity and pore size distribution, resulting in permeability decreases of cement pastes. Furthermore, a modified Carmen-Kozeny relation was established which enables prediction of the permeability from capillary porosity and the critical pore diameter.