Properties and hydration behavior of Ti-extracted residues-red gypsum based cementitious materials

The huge accumulations of Ti-extracted residues (TRs) and red gypsum (RG), which are the solid by-products from TiO2 pigment industry, have caused a series of on-site and off-site environmental problems. This work is concerned with the feasible recycling of TRs, RG, calcined RG (CRG) as main components in the production of a novel cementing material. The resulting mechanical strength, softening coefficient, hydration behavior (electrical resistivity) and microstructure (porosity and hydration products) of this binder were investigated. It was found that the flexural strength, compressive strength and softening coefficient of mortar samples at 28 days can reach up to 6.3 MPa, 47.9 MPa and 0.87, respectively. An electrodeless resistivity test, over a period of 72 h was also conducted: an increasing content of RG from 0 to 40% retarded the hydration within the first 24 h, whereas both the binders incorporating 20% CRG plus 20% RG and 40% RG were responsible for the later accelerating hydration. Moreover, an increase of RG's content (from 10% to 40%) and the substitution of CRG for RG decreased total porosity and narrowed pore size distribution of mortar samples. Results from X-ray diffraction and morphology analysis revealed that the intertwined hydrated products in the hardened binders were needle-like ettringite, unreacted bar-like gypsum and filamentous C-(A)-S-H, ensuring the blends with a good water resistance. In summary, this paper provides a cost-effective approach for utilizing TRs and RG to produce a high-strength cementing material as an alternative to ordinary Portland cement. (C) 2019 Elsevier Ltd. All rights reserved.