Preparation of Geopolymer Bricks from Alum Waste

Geopolymers are conventionally prepared by the reaction between a pozzolanic binder and an alkaline activator. In the present work, the waste form alum industry, commonly known as "dealuminated kaolin" (DK) was used as binder and sodium hydroxide solutions (with molarities ranging from 2 to 14 mole.L-1) as activator. The waste powder was first dried, finely ground and characterized by X - Ray fluorescence (XRF) and X - Ray Diffraction (XRD) analyses as well as SEM micrography. This was then mixed with the alkaline activator in a mass ratio of DK to NaOH solution equals 0.7 and molded in 50 mm steel molds. The effect of curing time and molarity of the alkaline solution on the properties of the produced bricks were assessed. It was observed that the degree of polymerization after 28 days generally increases with the increase of NaOH solution up to 8.10 M to reach about 90% after which the effect of NaOH concentration is less pronounced. Water absorption was found to drop considerably following the increase in NaOH concentration from 4 M to 14 M. The results also showed that the 28 days compressive strength reaches the minimum allowable value of strength of building bricks (8.7 MPa according to ASTM C 62) starting from NaOH concentrations of about 12 M.