Fly ash often is used as a filler in concrete. When used at excessive levels, fly ash and concrete has been cited as one of the reasons for excessive moisture in concrete slabs.

In Durock™ EcoCap™ Self-Leveling Underlayment, the fly ash is not used as filler but rather as a source of aluminum and silicon in the geopolymer reaction that takes place. In this case, the fly ash contributes significantly to the strength, self drying and setting characteristics of the product due to the geopolymer reaction.