14 style and contributes to the resource conservation and environmental protection. In this paper, the 15 orthogonal test and two curing methods including 7-day non-soaking curing and 28-day periodical-16 soaking curing are used to investigate the effect of cement, activators and waste like blast furnace slag, 17 silica fume and fly ash on the properties of FGD gypsum-based composites. Three typical specimens 18 from the orthogonal test are chosen, a pure gypsum specimen, a specimen with

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... nt combination 19 properties and a specimen with a big fall in strength after the 28-day periodical-soaking curing, to 20 explore their composition and structure characterisation in two curing methods by XRD, SEM-EDS and 21 DTA-TG. It was found that the properties of the composites, especially the waterproof property, can be 22 improved by adding an appropriate amount of OPC and waste due to the synergistic effect between them. 23 The periodical-soaking curing accelerates the hydration of the active substances in the composites, which 24 promotes forming more C-S-H gel, AFt crystals or tacharanite. After the 28-day periodical-soaking, the 25 flexural and compressive softening coefficient are 79.8 % and 107.8 % higher than that of 28-day pure 26 FGD gypsum specimens, respectively. However, improper composition can produce excessive ettringite, 27 which can damage the mechanical properties and water resistance of the specimens due to the 28 expansibility of ettringite. 29 30 31 32 INTRODUCTION 33 The development of industrial activities such as steel and iron manufacturing, smelt and power 34 generation lead to the release of large quantities of flue gas desulfurisation (FGD) gypsum [1], blast 35 furnace slag (BFS) [2], silica fume (SF) [3], and fly ash (FA) [4], which occupies much land and poses a 36 threat to the environment if disposed in landfills. Therefore, it is a hot topic to use and recycle these 37 industrial wastes instead of landfilling [5-8]. 38 FGD gypsum is a by-product of power station where FGD equipment is installed to remove SO2 from 39 the flue gas by adding limestone/lime. The higher content of calcium sulphate dihydrate, fewer impurities, 40 smaller and more uniform particle size, wider range of sources, better fire resistance, lower thermal 41 expansion coefficient, lower cost and sounder insulation properties are the obvious advantages of FGD 42 gypsum [1,[9][10][11]. It is applied widely in construction and building engineering materials nowadays, for 43 example, as a cement-coagulation agent or raw material in the production of cement and concrete to control 44 the hydration rate of cement and improve the early strength, or as a body of cementitious materials after 45 calcined to prepare lightweight gypsum-based building elements [12] [13][14]. The latter, appearing in the form 46 of gypsum-based blocks, gypsum particleboards or gypsum plasterboards, is a better way to reuse or 47 recycle, as far as the consumption quantity of FGD gypsum waste [15][16][17]. However, the fact that is not 48 desirable in the FGD building gypsum is the kind of air-hardening of the binding materials which are 49 commonly used indoor because of its low mechanical strength, high solubility and poor resistance to water 50 [18, 19]. 51 FA, BFS and SF are by-products from coal burning thermal power stations, manufacturing of pig iron 52 and the electrometallurgy industry, respectively [2, 3, 4, 20, 21]. Due to their good pozzolan and water-53 hardening nature, they are usually utilised in the manufacturing of cement and concrete, which has a low 54 post-performance loss [22][23][24]. For example, Naishu et al. [25] investigated when the silica fume is 55 migrated from the cement matrix and diffused to the concrete interface to form amounts of hydration 56 products. So, some researchers attempted to add one or two of them into gypsum to improve the partial 57 properties of the gypsum and explored the hydrate mechanism of these gypsum-based mixtures by SEM, 58 XRD or DSC [13, 26, 27] in the meantime. Zhao et al. [19] utilised FGD gypsum, granulated blast-furnace 59 slag and high calcium fly ash to prepare water resistant blocks, finding that the softening coefficient of the 60 product prepared is over 0.80 after the curing temperature of 60 °C and curing time of 16 h due to the 61 formation of ettringite (AFt). Ettringite is a compound characterised by satisfactory mechanical strength, 62 water insolubility and fire resistance [28]. Antonio et al. [13] investigated the hydration ternary systems 63 consisting of 40 % FGD gypsum, 35 % calcium hydroxide and 25 % fly ash, presenting that the samples 64 have better mechanical properties and water resistance at curing temperatures up to 85 °C for 7 days 65 because of the combined action of AFt and C-S-H. But AFt is responsible for an expansive behaviour, 66 which will reduce the properties of the material if it contains too much. Generally speaking, the curing age 67 of the gypsum products is generally short (within several days) even if some inorganic minerals are added. 68 This is because gypsum can basically hydrate within hours. However, those water-hardening materials 69 have two characteristics of curing, one is that the curing period is longer; the other is that water or alkaline 70 environment will make the hardening get better. Hence, Zhao et al. [19] found that there are still some 71 obviously non-hydrated components, such as silicon dioxide, in the composite system. In addition, even 72 at an extended age (increased to 28 days), the active components of the inorganic minerals react slowly 73 due to the insufficient amount of water or activators [29]. Therefore, the selection of the composition and 74 curing method of the specimens are very important. It is perfect if a sufficient amount of water and a kind 75 of alkaline environment is given to the gypsum-based composites during the curing, then the water-76 hardening materials added in the system can be fully hydrated. 77 On the basis of the above-mentioned considerations, we are going to adopt a new curing method, 78 which is called the 28-day periodical-soaking curing. It is different from the normal curing method of 79 gypsum and includes 7 days of air curing and a subsequent 3 cycles, each cycle including 2 hours soaking 80 curing in an alkaline solution and 166 hours of air curing. The 2-hour soaking in an alkaline solution in 81 every cycle makes the blocks obtain the necessary water and prevents the loss of the internal activator and 82 accelerates the hydration of the non-hydrated active substances in the composites. Meanwhile, the 83 structure of gypsum does not subject a lot of damage over a short soaking time of only 2 hours. In this 84 paper, we carry out the experiment on comparing the FGD gypsum-based specimens that experienced the 85 28-day periodical-soaking curing with those that experienced the 7-day non-soaking air curing in their 86 micro-structure and the properties including compression strength, flexural strength and water-resistance. 87 Before this experiment, the orthogonal test with five variables of OPC, FA, BFS, SF and activators are 88 conducted to prepare our target specimens with the appropriate amounts of waste BFS, FA and SF. It is 89 assured that every sample consists of more than 90 % waste. The hydration mechanism, the synergistic 90 effect between the inorganic minerals and its performance characteristics after the 28-day periodical-91 soaking curing are investigated in detail. 92 93 MATERIALS AND METHODS 94 Raw materials 95 The FGD gypsum used in this work was from the power station in Zibo (China) and was calcined 96 into FGD building gypsum with a standard consistency of 57%, an initial setting time of 9.5 min, and a 97 final setting time of 13 min, by Zibo Lvneng Building Materials Corporation (China). The OPC (42.5 98 grade), fly ash (FA), blast furnace slag (BFS) and silica fume (SF) were also produced in China. The 99 detailed chemical constituents of these raw materials are shown in Table 1 . The Sodium citrate, calcium 100 chloride and calcium hydrate were all made at the Sinopharm Group (China) and the activators were 101 prepared by calcium chloride and calcium hydrate. Figure 1 shows the XRD analysis of the FGD building 102 gypsum from industrial calcining, which indicated that the building gypsum mainly consisted of β-103 hemihydrate (CaSO4•1/2H2O), located around 2θ of 14.7°, 30°, 31.8° and 32°。In addition, there was a 104 small amount of anhydrite (CaSO4Ⅲ) located around the 2θ of 14.6°, 29.6° and 32° based on the PDF 105 database. The particle diameter of the FGD building gypsum powders is given in Figure 2 . The scanning 106 electron microscopy (SEM) images of the various inorganic modifier materials are shown in Figure 3. 366 19. Zhao F.-q., Liu H.-j., Hao L.-x., Li Q. (2012): Water resistant block from desulfurization gypsum. Construction and