Study of the NaOH concentration and aluminium compound on the ettringite formation in fluidized bed combustion (FBC) fly ash geopolymer

Abstract

Abstract

Fluidized bed combustion (FBC) fly ash contains high amount of calcium sulfate (CaSO4) due to feeding of lime into the combustion process to capture the SOx gases. The CaSO4 can be formed and react with alumina in the FBC fly ash under base condition resulted in calcium sulfoaluminate (ettringite). Therefore, effect of sodium hydroxide (NaOH) concentration on ettringite formation in FBC fly ash geopolymer was studied. In addition, the properties of geopolymer was improved by the addition of aluminium compound. The geopolymer was prepared from FBC fly ash mixed with aluminum hydroxide (Al(OH)3),
sodium silicate and sodium hydroxide solution which Al(OH)3 could convert the calcium compound in FBC fly ash to calcium aluminate hydrate and aluminosilicate. It exhibited the strength gain of geopolymer. It was found that there is no sign of ettringite formation when 15 M NaOH was used. Use of Al(OH)3 not exceed 2.5% by weight of FBC fly ash in geopolymer preparation gave the compressive strength of 35 MPa at 90 days, while that of Al(OH)3 at 5% by weight resulted in sodium aluminate
formation and strength reduction.