Hydration of sustainable ternary cements containing phosphogypsum

Abstract

Phosphogypsum (PG) is a large environmental issue generated by phosphate fertilizer industries. Its annual disposal worldwide is about 160 million tons, representing a high risk of contamination of the environment. The content of dihydrate (CaSO4.2H2O) in PG is generally above 90%, presenting recycling potential in cement production. The objective of this research is to investigate the applicability of unprocessed PG in the composition of sustainable ternary cement (TC) containing Portland cement (PC) and wastes of marble and clay brick. The effects on hydration properties and phase contents of PG added in ratios 1, 3, 5, and 10 wt% to cement were studied. The technical feasibility of applying PG in TC with different WCB/WM ratios (2.0 and 3.6) was also evaluated. Isothermal calorimetry was applied to study the hydration kinetics of the TC pastes. The hydration products were evaluated using Fourier transform infrared spectroscopy (FTIR), thermal analysis (TG/DTG/DTA), and X-ray diffractometry using the Rietveld method. The evolution of axial compressive strength was assessed at ages 1, 3, 7, 28, and 91 days. The increase in PG content satisfactorily prolonged the cement setting time and the induction period. Phosphorus impurities in the form of CaHPO4.2H2O did not cause any deleterious effects in the first hours of hydration. Although the presence of PG affects the hydration reactions by leading to higher levels of hydrated calcium silicate (C-S-H) and monocarboaluminate (Mca). TC pastes are mainly composed of C-S-H gel, portlandite, ettringite, Mca, calcite, quartz, dolomite, β-belite, and periclase. TC pastes after 28 days reached at least 70% of the PC compressive strength. The WCB/WM ratio affected the compressive strength, although it did not significantly influence the formation of phases. By evaluating the properties of TC paste it was concluded that the unprocessed PG can sustainably be used as an alternative to a natural source of calcium sulfate in the manufacture of the ternary cement.