Utilisation of waste material in geopolymeric concrete

Fly ash, silica fume and rice husks are hazardous waste materials that have no use and in the past have been landfilled. However, landfilling is becoming expensive and causes contamination to soil and ground water. Utilisation of waste material in concrete is also very effective in overcoming the problems caused by the production of cement; namely the emission of carbon dioxide and degradation of the environment due to the quarrying of raw materials (limestone and clay) for the production of cement. This research study was based on the complete elimination of ordinary Portland cement from concrete that can achieve 28 days target cube strength in the range of 40–50 MPa with the emphasis on the curing techniques applicable for in situ construction; namely ambient and external exposure curing. Fly ash was utilised as a base source material and silica fume and microwave-incinerated rice husk ash were used as replacements for the fly ash by 3, 5 and 7%. Alkaline activators, namely sodium hydroxide and sodium silicate solution were used as activators of silica and aluminium in the source material and sugar was incorporated in the mix to increase the hardening time of the concrete. Compressive strength, flexural strength and scanning electron microscopy tests were conducted on the specimens and the results showed that at 3, 7, 28, 56 and 90 days the fly ash along with silica fume, microwave-incinerated rice husk ash and alkaline activators could be a good replacement of cement. The compressive strength of external exposure curing for the geopolymeric concrete reached up to 48·7 MPa at 28 days and this concrete had a well-developed microstructure shown by scanning electron microscopy analysis. The flexural strength showed values which were comparable with ordinary Portland cement concrete.