Effect of alkali-mixed content and thermally untreated phosphate sludge dosages on some properties of metakaolin based geopolymer material

In the present work we investigate the possibility of reusing phosphate sludge, which is considered as a mining by-product from beneficiation plants of the Moroccan phosphate rocks (OCP, Youssoufia), through the geopolymerization process. This process allows to evaluate the alkaline behaviour of thermally untreated phosphate sludge (UPS) as a partial replacement of metakaolin. The geopolymers were synthesized with various activator modulus (Ms = 1.40, 1.30, 1.24, 1.20, and 1.15) made from a mixture of different alkaline (NaOH and KOH) solutions and sodium silicate. The effect of 0-60% untreated sludge (UPS) powder in metakaolin based geopolymer materials cured at 60 degrees C was investigated. Compressive strength and apparent porosity were determined after 28 days of curing and the reaction products were tested by different techniques including: X-Ray Diffraction (XRD), Fourier Transformed Infrared spectroscopy (FTIR), thermal gravimetric analyses (TGA) and Electron microscope scanning coupled with energy dispersive spectroscopy (SEM-EDX). For up to 50% of UPS, physical properties of the hardened products are induced, especially when the activator modulus (Ms = 1.30) is used. In these blends, the presence of UPS is favorable to ensure the better connectivity between grains in the system. In fact, the sludge has been suggested to have a dual role, it has a filler effect and/or a small precipitation of Ca-bearing phases co-existing with (Na, K)-A-S-H type gel.