Effect of the incorporation of spent diatomaceous earths on the properties of alkaline activation cements based on sewage sludge ash

Abstract

The aim of this research is to evaluate the synergistic effect of the incorporation of spent diatomaceous earth (SDE) from beer filtration on the technological properties of alkaline-activated cements (AACs) based on sewage sludge ash (SSA). Pastes have been manufactured by mixing different amounts of precursor, SSA (30–70% by weight) and SDE (30–70% by weight). The cements were activated using a solution of sodium silicate (50% by weight) and sodium hydroxide (50% by weight NaOH with 8 M) as activator. A liquid/binder (l/b) ratio of 1.2 were used for cements with 30–40% by weight of SSA and 1.0 for binders with 50–70% by weight of SSA. The specimens were cured at room temperature for 7, 28 and 56 days. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the microstructural characterization of the AACs. The mix proportion of the precursors or the CaO/SiO ratio exerts a substantial impact on the physical, mechanical, and thermal characteristics of the examined AACs. Binders containing 40% by weight SSA and 60% by weight SDE show optimal technological properties, reaching compressive strengths of 32.8 and 36.8 MPa at 28 and 56 days of curing, respectively. This may be due to the formation of different gel types according to SEM analysis: a compact gel phase characterized by a fibrous structure corresponding to the N-(A)-S-H gel surrounding flatter and more uniform zones corresponding to the calcium aluminosilicate gel, C-A-S-H, and a mixed amorphous N,C-A-S-H gel. The results indicated that is possible to produce more sustainable alternative binding materials from the recovery of two types of waste: one from the wastewater treatment, and the other originating from the filtration process in the brewing industry.