Chloride transport and the resulting corrosion of steel bars in alkali activated slag concretes

As the relative performance of alkali activated slag (AAS) concretes in comparison to portland cement (PC) counterparts for chloride transport and resulting corrosion of steel bars is not clear, an investigation was carried out and the results are reported in this paper. The effect of alkali concentration and modulus of sodium silicate solution used in AAS was studied. Chloride transport and corrosion properties were assessed with the help of electrical resistivity, non-steady state chloride diffusivity, onset of corrosion, rate of corrosion and pore solution chemistry. It was found that: (i) although chloride content at surface was higher for the AAS concretes, they had lower chloride diffusivity than PC concrete; (ii) pore structure, ionic exchange and interaction effect of hydrates strongly influenced the chloride transport in the AAS concretes; (iii) steel corrosion resistance of the AAS concretes was comparable to that of PC concrete under intermittent chloride ponding regime, with the exception of 6 % Na2O and Ms of 1.5; (iv) the corrosion behaviour of the AAS concretes was significantly influenced by ionic exchange, carbonation and sulphide concentration; (v) the increase of alkali concentration of the activator generally increased the resistance of AAS concretes to chloride transport and reduced its resulting corrosion, and a value of 1.5 was found to be an optimum modulus for the activator for improving the chloride transport and the corrosion resistance

Performance of plain and slag-blended cements and mortars exposed to combined chloride-sulphate solution

The durability of reinforced concrete structures exposed to aggressive environments remains a challenge to both researchers and the construction industry. This study investigates the hydration, mechanical properties and durability characteristics of ground granulated blast-furnace slag (GGBS) - blended cements and mortars exposed to a combined sodium chloride - sulphate environment, at temperatures of 20°C and 38°C. The conditions were chosen so as to assess the performance of slag blends under typical temperate and warm tropical marine climatic conditions. Slags, having CaO/SiO2 ratios of 1.05 and 0.94, were blended with CEM I 52.5R at 30% replacement level to study the influence of slag composition and temperature. Parallel control tests were carried out with CEM I 42.5R. Pastes and mortar samples were cast using 0.5 water to binder ratio, pre-cured for 7 days in water before exposure. Flexural strengths were determined once the samples were 7, 28 or 90 days old. Hydration was followed using x-ray diffraction (XRD), thermal analysis, and calorimetry. Also, sorptivity, gas permeability and chloride diffusion tests were carried out on mortar samples to measure transport and durability characteristics. The results show improved mechanical and transport properties for slag blended cements exposed to environments rich in sodium chloride and sulphate

Uptake of chloride and carbonate by Mg-Al and Ca-Al layered double hydroxides in simulated pore solutions of alkali-activated slag cement

Chloride ingress and carbonation are major causes of degradation of reinforced concrete. To enable prediction of chloride ingress, and thus to improve the durability of structural alkali-activated slag cement (AAS) based concretes, it is necessary to understand the ionic interactions taking place between chlorides, carbonates, and the individual solid phases which comprise AAS. This study focused on two layered double hydroxides (LDH) representing those typically identified as reaction products in AAS: an Mg-Al hydrotalcite-like phase, and an AFm structure (strätlingite), in simulated AAS pore solutions. Surface adsorption and interlayer ion-exchange of chlorides occurred in both LDH phases; however, chloride uptake in hydrotalcite-group structures is governed by surface adsorption, while strätlingite shows the formation of a hydrocalumite-like phase and ion exchange. For both Ca-Al and Mg-Al LDHs, decreased chloride uptakes were observed from solutions with increased [CO₃²⁻]/[OH⁻] ratios, due to the formation of carbonate-containing hydrotalcite and decomposition of AFm phases, respectively