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

Geopolymer represents a new class of materials with low environmental impact and physical- mechanical properties similar to the cement ones. The objective to classify geopolymer as new binder for the construction sector can be obtained only by means of a good knowledge of differences/similarities between this new class of materials and ordinary portland cement. The scope of the present work is to investigate the shrinkage of geopolymers. Shrinkage represents a critical parameter in the characterization of a material for the construction industry because of its crucial importance in durability aspects and structural long-term maintenance, due to cracks formation. Studies on engineering properties and shrinkage of geopolymer are still at a starting phase. In this work the behavior of three coal fly ash based geopolymer mixes is studied and compared with a cement mortar as reference. Consistency at the fresh state of the mixes and physical properties of the obtained mortars are detected. Concerning the study of weight change and drying shrinkage as a function of relative humidity change, it is studied both the behavior of the first shrinkage on specimens during the first months of curing and the dimensional variations on specimens after a long period of time. Tests are made at a constant temperature, varying the relative humidity, and regularly measuring weight change and drying shrinkage of specimens. 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.