Recovery of thermally treated fluorine-containing sludge as the substitutions of Portland cement

Abstract

New solid waste with increasing output has provoked the exploration and development of new alternative SCMs. The enormous production of wet-base fluorine-containing sludge from electronic device manufacturing plants triggered consideration of land occupation, environmental pollution and resource utilization, the reasonable treatment came to draw research interests. In this study, the effect of thermal treatment at different temperature (600 °C, 700 °C and 800 °C) on the wasted fluorine-containing sludge was investigated. The recovery of thermally treated fluorine-containing sludge (TTFS) as the substitutions of Portland cement was evaluated systematically by measurement of the setting time, compressive strength, hydration products and microstructure in blended system. The results indicated that the thermal treatment would result in the new formation of fluorapatite and re-flocculation of crystalline phases, particle size distribution of TTFS was changed. It was noticed that the dried fluorine-containing sludge under 105 °C could seriously increase water requirement, shorten setting time and weaken compressive strength of blended system. Thermal treatment from 600 °C to 800 °C could inhibit the negative effects, the 2.5 wt % incorporation of TTFS thermally treated by 800 °C for 2 h would actualize the suitable water requirement 27.0%, the appropriate setting time (initial setting time: 80 min; final setting time: 250 min) and the optimum compressive strength (3 d: 33.63 MPa; 7 d: 38.28 MPa; 28 d: 49.56 MPa). The analysis of XRD, TG as well as SEM-EDS expounded the weakened effect of organic matter and the formation of petal flake-shaped brushite and rod-like hydroxylapatite. The interaction of ettringite and hydroxylapatite was observed to strengthen the hydrated matrix. The most probable aperture and porosity were reduced, the pore distribution of blended pastes was also improved the 2.5 wt % TTFS treated by 800 °C was added.