Use of fly ash in the production of geopolymers: a literature review

Currently, the high pollution derived from the production of Portland cement is a major problem, therefore the development of alternative cements obtained by processes with lower gas emissions and low energy consumption is a new line of research of worldwide interest. It is proved that geopolymers can replace Portland cement in some application in the construction industry; however there are lack of critical reviews about finding of it during the last years. The main objective is to perform a critical analysis of the existing literature on material sources, physical and chemical characteristics of fly ash, combination, geopolymer manufacture and its properties; to complete that 91 indexed manuscripts from different data sources were analyzed. This review reveals that the optimal burning of the materials improves their characteristics and generates a higher compressive strength of the geopolymers, and it is also concluded that the most practical way to mix the inputs is by combining at the same time the alkaline solutions and the fly ashes. In the case of mortars and concretes, similar and higher values in compressive strength have been reported, but lower values in flexural strength with respect to conventional Portland cement. While alternative activators such as CCA, it is proven that their inclusion does not affect the strength of the mixture, finally it is found that fly ashes can contain heavy metals, and these are encapsulated and immobilized in the geopolymerization process, complying with environmental standards.