Unit weight, strength and microstructure of a water treatment sludge-fly ash lightweight cellular geopolymer

A water treatment sludge–fly ash lightweight cellular geopolymer (WTS–FA LCG) is investigated in this research with the intention to develop an alternative green construction and building material, without using Portland cement as a cementing agent. Two waste by-products: WTS from the Bang Khen water treatment plants of the Metropolitan Water Work Authority of Thailand (MWA) and FA from the Mae Moh power plants of the Electricity Generating Authority of Thailand (EGAT) were used as an aggregate and a precursor, respectively. The liquid alkaline activator (L) used was a mixture of sodium silicate solution (Na2SiO3) and sodium hydroxide solution (NaOH). The unit weight and strength of WTS–FA LCG heated at 65 °C for various influential factors are investigated and presented in this paper. The various influential factors studied include mixing ingredient (air content (Ac), liquid alkaline activator content (L) and Na2SiO3/NaOH), heat duration and curing time. Scanning electron microscopy (SEM) analysis was undertaken to investigate the role of influential factors on unit weight and strength. The test results indicate that the L content at liquid limit state (LL) is optimal for manufacturing WTS–FA LCG for all Na2SiO3/NaOH ratios, heat durations and air contents tested for which the highest strength is attained. Although the unit weight of WTS–FA LCG significantly reduces when L > LL, it is not economical to manufacture WTS–FA LCG at L > LL due to the drastic strength reduction. The addition of Ac at L = LL is found to be an appropriate means to reduce the unit weight and minimize the strength reduction. The maximum strengths at L = LL for various air contents are found at Na2SiO3/NaOH of 80:20 and heat duration of 72 h. The longer heat durations of 96 and 120 h cause the loss of moisture, thereby resulting in micro-cracks and strength reduction. The WTS was found to be viable alternative aggregate to develop WTS–FA LCG, thereby resulting in this waste material traditionally destined for landfills to be used sustainably as a valuable resource