Investigating carbonation and other forms of chemical attack on GGBS based alkali activated materials

The growth in global population leads to an increase in the need for construction materials. Portland cement (PC) based materials are the most widely used due to their efficiency, price and durability. However, PC production is responsible for 5 to 8% of global carbon dioxide emissions. There is an urgent need to find (and use) alternative materials that have similar properties while having lower carbon footprint. One of the most encouraging alternatives is alkali activated materials (AAM).
An AAM is made by the reaction between an alkaline activator (such as sodium silicate or sodium hydroxide) and an aluminosilicate precursor (such as slag or fly ash). Unfortunately, there remains a gap in our knowledge concerning the durability of these materials. The durability of cementitious materials can be challenged by chemical and mechanical attack, either separately or in combination. Even though AAM microstructure and chemistry are slightly different from PC based materials, the main mechanisms of attack are believed to remain similar. In reinforced concretes based on PC, one of the main service life-limiting factors is carbonation.
Carbonation is the reaction between atmospheric CO2 and cementitious materials. In PC based materials, carbonation causes a permeability change, by a redistribution of the pores structures, and a pH drop, potentially leading to the corrosion of the steel rebar in reinforced concrete. The corrosion products that form on steel rebar have a higher volume and can cause cracking of the concrete. Even if the carbonation process is slow, around a few millimetres per year, it is still one of the main durability challenges for concretes in service. Previous work has shown that the consequences of AAM carbonation are slightly different from those of PC carbonation.
Although the consequences of AAM carbonation depend on the mix design, past studies show that in these materials, it is possible that the pH drop is not low enough to lead to steel corrosion but the mechanical performance may decrease. To overcome the lack of service life data regarding AAM carbonation, there remains a need for an efficient and accurate standard test method to evaluate AAM carbonation, as the use of methods designed for measurement of PC carbonation may lead to inaccurate predictions.
The purpose of this PhD project is to understand the process of carbonation on GGBS based AAM, in order to advise on the design of structures containing these materials. To achieve this goal, it is necessary to understand the steps and outcomes of this reaction on AAMs, depending on the composition, to be able to predict service life under carbonation. In this work, the impact of carbonation alone, or coupled with efflorescence and/or soft water leaching on the GGBS based AAM microstructure will be observed with various test methods. This study presents the impact of the carbonation on both the reaction of the carbonatable species and the diffusion coefficient of AAM. This study also shows that when carbonation is coupled with efflorescence, the microstructure of both the non-carbonated and carbonated parts are affected, while it is not the case of when carbonation is alone or coupled with leaching. These findings should be considered while designing a structure containing GGBS based AAM. Finally, this study highlights that the change of colours in these materials is due to the oxidation of the sulphur species, and while these species are oxidised when the material is carbonated, it is not linked to the carbonation, and consequently the change of colours should not be associated to the carbonation front.