A Review on Fly Ash-Based Geopolymer Concrete

Ordinary Portland cement (OPC) is conventionally used as the primary binder to produce concrete. The environmental issues associated with the production of OPC are well known. Binders could be produced by a polymeric reaction of alkaline liquids with the silicon and the aluminium in source materials of geological origin or by-product materials such as fly ash. Low-calcium fly ash-based geopolymer is used as the binder, instead of Portland or other hydraulic cement paste, to produce concrete. The manufacture of geopolymer concrete is carried out using the usual concrete technology methods. As in the case of OPC concrete, the aggregates occupy about 75-80 % by mass, in geopolymer concrete. The silicon and the aluminium in the fly ash react with an alkaline liquid that is a combination of sodium silicate and sodium hydroxide solutions to form the geopolymer paste. Geopolymerisation in fly ash based concrete could be occur by using different solutions with the solution-to-fly ash ratio by mass of 0.25 to 0.30. The best compressive strength will be more than 60 MPa for mixtures that used a combination of sodium hydroxide and sodium silicate solution, after curing the specimens for 24 hours at 65oC. The proportion of alkaline solution to alumino-silicate powder by mass should be approximately 0.33 to allow the geopolymeric reactions to occur. Alkaline solutions forms a thick gel instantaneously upon mixing with the alumino-silicate powder. By the use of the mass ratio of the solution to the powder of about 0.39 and mixing 57% fly ash with 15% kaolin or calcined kaolin and the alkaline liquid comprise 3.5% sodium silicate, 20% water and 4% sodium or potassium hydroxide the maximum compressive strength will be 75 MPa when fly ash and builders’ waste are used as the source material.