Coal fly ash geopolymer: study of weight variation and shrinkage as a function of relative humidity change

In the last twenty years a new class of materials known as geopolymers has rapidly grown in interest in order to reduce the CO2 emissions for cement and ceramic materials productions. Geopolymers are based on alkali activation of precursors able to consolidate at room or slightly higher temperatures. One of the main advantages of geopolymers is the possibility to use waste-based powders, such as coal fly ashes, thus promoting a circular economy approach. Nevertheless, the scope to classify geopolymers as a new binder for the construction sector can be obtained only by a good knowledge of both differences and similarities between this new class of materials and ordinary portland cement. As known, shrinkage is a critical aspect to take into account for a proper characterization of a new material in the construction industry. Indeed, shrinkage has a crucial importance for both durability aspects and structural long-term maintenance, due to possible cracks formation. At the present state, only a few researches on engineering properties and shrinkage of geopolymers have been made. In this work, three different coal fly ash-based geopolymer mortars were studied with an attempt to investigate their drying shrinkage behavior (as opposed to chemical or autogenous). For comparison sake, an ordinary portland cement mortar was used as reference. Consistency at the fresh state and physical properties (i.e., bulk density, water absorption and total open porosity) of the mortars were determined. Then, two different behaviors were studied on specimens: 1) the first shrinkage during the first months of curing; 2) the dimensional variations related to humidity change after a long period of time. All tests have been made at a constant temperature and varying the relative humidity: weight change and shrinkage of all specimens were measured regularly. Preliminary obtained results could be useful for the future set up of a predictive model for the shrinkage of geopolymer that at the present state does not exist.