Calcium hydroxide in geopolymers – a critical overview

As known, calcium hydroxide Ca(OH)2 is a basic material, that can be used in numerous applications. Recently, many trials have been performed to modify some properties of alkali-activated materials (AAMs) by introducing Ca(OH)2. In general, Ca(OH)2 can be introduced into AAMs in three forms: as a part of a precursor or as an additive, as a sole activator or combined with an auxiliary activator. The incorporation of Ca(OH)2 into different matrices may modify some properties or worsen others. This essentially depends on curing conditions, the ratio of Ca(OH)2, precursor type and fineness, activator type, activator concentration, and testing age. This review aimed to collect, summarize, and analyze the available studies regarding the effect of Ca(OH)2 on the properties of AAMs in its three forms. The review showed that most studies preferred using Ca(OH)2 as a part of precursor followed by using it as an activator combined with an auxiliary activator, with its use as the sole activator being least preferred. The review found that Ca(OH)2 incorporation in precursors reduced workability and setting time but increased carbonation resistance, with 5-30% Ca(OH)2 being the most effective activator. Auxiliary activator with Ca(OH)2 enhanced the strength. Additionally, some recommendations for the future were mentioned.