Coal fly ash alkalis content characterization by means of a full factorial design

Concrete structures durability is affected by the alkali ions amount in the pore solution when potentially reactive aggregates are employed. Then, a long-term durability indicator of concrete could be the coal fly ash alkali content. Such alkali ions are provided by the Portland cement, but also by the coal fly ash when is used as a main cement constituent. In this work, fly ashes from the combustion of a South African coal were investigated. The scope of this paper is to deep in the knowledge of coal fly ash alkalis content and, in particular, how it is affected by free CaO and reactive silica content and fineness. Evaluation of variables was done by using a 23 full factorial design. The maximum Na2Oequ of 1.68% was obtained at 1% free CaO, 40% reactive silica and 30% residue on 45µm (fineness). Reactive silicon is the main parameter influencing Na2Oequivalent amount in coal fly ash. •Alkalis in fly-ash acts as a concrete durability key parameter.•Influence of free CaO, reactive silica and residue fineness on alkalis content was assessed.•Alkalis content in fly-ash mainly depends on reactive silica amount.•Increasing reactive silica will increase alkalis content.•Alkalis changes in function of free CaO, reactive silica and fineness was examined.