An overview of factors influencing the properties of alkali-activated binders

Abstract Alkali activated binders (AABs) are evolving as alternative to Portland cement. They are predominantly synthesized using aluminosilicate materials, such as fly ash, silica fume, metakaolin, etc. However, the synthesis of these binders is very sensitive to several factors. These include the source, constituents and fineness of the precursor materials, type and combination of alkaline activators, quantity of free-water, and type and duration of curing. Though the synthesis and properties of AABs were studied in the past, there is an apprehension towards using this technology by the construction industry due to practical difficulties. These include insufficient information and understanding of the mix design, lack of uniformity in the composition of precursor materials and its possible impact on the properties of these binders. In addition, there is lack of data on the long-term performance of AABs under aggressive exposure conditions, particularly chloride-bearing environment. The findings of research studies conducted so far indicate that the mechanical characteristics of AABs are either equivalent to or better than those of Portland cement concrete. However, some aspects related to the long-term durability of these binders are yet to be fully understood, including fire-resistance. Accordingly, this paper is aimed to present an overview of parameters that affect the properties of AABs and to elaborate the polymerization mechanisms of commonly used precursor materials. Moreover, the performance of AABs exposed to aggressive environmental conditions are presented. In the light of available literature, the review indicates that future research in this area has to be focused on the development of standard specifications and evaluation procedures for the precursor materials and combinations of alkaline activators for a particular source material. In addition, it is realized that the long-term study on the durability aspects of these binders, particularly exposed to chloride-bearing environment requires considerable attention.