minerals Article Effect of Gypsum Addition on the Mechanical and Microstructural Performance of Sulphide-Rich Cemented Paste Backfill Yu Tang 1 , Juanrong Zheng 1 , Lijie Guo 2,3, * and Yue Zhao 2,3   Citation: Tang, Y.; Zheng, J.; Guo, L.; Zhao, Y. Effect of Gypsum Addition on the Mechanical and Microstructural Performance of Sulphide-Rich Cemented Paste Backfill. Minerals 2021, 11, 283. https://doi.org/10.3390/min11030283 Academic Editor: Konstantinos Komnitsas Received: 3 February 2021 Accepted: 7 March 2021 Published: 9 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 School of Civil Engineering, Zhengzhou University, Zhengzhou 450002, China; tangyu@gs.zzu.edu.cn (Y.T.); zhengjr@zzu.edu.cn (J.Z.) 2 Beijing General Research Institute of Mining and Metallurgy, Beijing 100160, China; zhaoyue@bgrimm.com 3 National Centre for International Research on Green Metal Mining, Beijing 102628, China * Correspondence: guolijie@bgrimm.com Abstract: The present study investigates the effect of β-hemihydrate gypsum (HG) dosages on the mechanical and microstructural performance of cemented paste backfill (CPB) produced from sulphide-rich mine tailings using NaOH-activated slag (NAS) as the major binder. X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) analyses were carried out to elucidate the mineralogical composition and microstructure of NAS-HG-CPB samples. The results illustrate that the main hydration products of NAS-HG-CPB from sulphide- rich tailings are crystalline (CaSO 4 •2H 2 O and ettringite (AFt), 3CaO•Al 2 O 3 •3CaSO 4 •32H 2 O) and amorphous. The results also show that the 28 d unconfined compressive strength (UCS) of CPB with 30 wt. % HG replacing NAS increased by 52% compared to the UCS of CPB containing no HG, and both have stable long-stage (180 d) UCS (i.e., no strength loss). Excess HG addition (≧50 wt. %) reduced the early-stage (≦28 d) UCS of NAS-HG-CPB and led to unstable long-stage (180 d) UCS by the formation of secondary gypsum. The use of 30 wt. % HG replacing NAS in NAS-HG-CPB accelerates the hydration process of ground granulated blast furnace slag (GGBS) in the alkaline solution by forming ettringite (AFt), leading to the denser microstructure and improved mechanical performance in comparison with CPB containing no HG. The NAS-HG binder with low dosages of HG (≦30 wt. %) will be a promising binder for stabilising sulphide and non-sulphide tailings and CPB production. Keywords: cemented paste backfill; sulphide-rich tailings; NaOH-activated slag; β-hemihydrate gypsum; strength development 1. Introduction Sulphide-rich tailings are produced during the beneficiation or the treatment of sul- phide ores and concentrates. Conventionally, tailings have been deposited into tailings storage facilities, leading to severe environmental, geotechnical and economic problems. Acid mine drainage (AMD) [1,2] is a major environmental and economic hazard influ- encing the world’s mining industry. The sulphide minerals (e.g., pyrite FeS 2 ) present in tailings may be oxidised to form an acidic solution when in contact with air (or oxygen) and water. The acidic waters generated can significantly impact the local ecosystem and dissolve the heavy metals present in tailings [3]. The traditional means of mitigating AMD in tailings storage facilities involves the use of water or an impermeable cover to isolate them from the environment. Over the past few decades, cemented paste backfill (CPB) has become a promising alternative method for safe and environmentally responsible tailings disposal [4–6]. CPB is a mixture of dewatered tailings with a solid content between 70% and 85%, a hydraulic binder (often ordinary Portland cement, OPC) and water (fresh and/or mine processed). The mixed CPB slurry may then be pumped into mined cavities to form support systems and working platforms [7,8]. Minerals 2021, 11, 283. https://doi.org/10.3390/min11030283 https://www.mdpi.com/journal/minerals