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This research investigates unit weight and compressive strength of water treatment sludge improved with sand and fly ash lightweight cellular geopolymer. The water treatment sludge (SL) was obtained from the Metropolitan Waterworks Authority of Thailand (MWA). Fly ash (FA) was collected from Mae Moh power plant of the Electricity Generating Authority of Thailand (EGAT) at Lampang Province. The fly ash : fine aggregate (SL and sand (SA)) ratio was fixed at 30:70, while the SL:SA ratios of 70:0, 60:10, 50:20, 40:30, 30:40, 20:50, 10:60 and 0:70 were investigated. Liquid alkaline activator (L) is a mixture of sodium hydroxide solution (NaOH) and sodium silicate solution (Na2SiO3) which is 30:70, 50:50 and 70:30, respectively. The NaOH concentration was 5 Molars. Foam content was 0, 1, 3 and 5 % by weight of FA. Test results show that the unit weight of SL improved with sand and fly ash lightweight cellular geopolymer varied with SL and SA ratios, L content and foam content. The unit weight of SL improved with sand and fly ash lightweight cellular geopolymer decreases with increasing in the SL content, L content and foam content. The compressive strength of sample increases with the increase of SA and L content. The maximum compressive strength of sample was found at SL:SA:FA of 0:70:30, NaOH:Na2SIO3 ratio of 30:70, L/B ratio of 0.95 and foam content of 1%, which gave the compressive strength of 12.8 MPa.
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