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good way to reduce fiber amounts at a given thickness. The addition of high-yield pulp (HYP) is one of the most frequently used methods to produce high-bulk paper (Resalati 2007; Xu and Zhou 2007; Zhang et al. 2011). Although the bulk of paper can be improved, the thick-walled HYP fibers may affect paper surface smoothness and can cause the surface to roughen upon rewetting when printing (Danby 2002; Nesbakk and Helle 2002). It is typically believed that paper bulk and strength are contradictory parameters. The inter-fiber hydrogen bonds, which are related to the number of free hydroxyl groups and the total area in molecular contact, provide the mechanical strength of paper (Retulainen et al. 1998; Mark 2002; Dongbo 2013). When the bulk of paper increases, the distance between fibers increases, which decreases paper strength. The aforementioned methods can enhance paper strength, but compromise paper bulk, or vice versa . Hence, it is necessary to investigate a method that improves both paper bulk and strength. Previous research has found that fly ash-based calcium silicate (FACS), an environmentally friendly by-product prepared from the silicate-rich fly ash of coal-fired power plants, has the potential to produce high-bulk paper (Song et al. 2012; Zhang et al. 2013). However, the strength of FACS-filled paper was sacrificed with improvements to the bulk. In this study, the co-flocculation of FACS particles and cellulosic fines with a high-molecular weight cationic polyacrylamide (CPAM) was employed to explore the possibility of improvement to both the strength and the bulk of FACS-filled paper. Bleached softwood kraft pulp was supplied by a pulp mill in Fujian province, China. The pulp was refined to a freeness of 425 mL (Canadian Standard Freeness) with a PFI refiner following TAPPI T248 sp-00 (2000). The pulp was diluted to a consistency of 0.3% before use. FACS was obtained from a coal-fired power plant in China. CPAM with a molecular weight of approximately (6.5±0.5)×10 6 g/mol was supplied by Nalco Chemical Company, Nanjing, China. CPAM solution (0.01% (w/v)) was prepared daily with deionized water and stirred by a magnetic stirrer at room temperature for 30 min. Fines were produced by extensively refining the bleached softwood kraft pulp to a freeness of 50 mL and separating the fraction that passed through the 200-mesh screen of a SWECO fiber classifier (Sweco division of M-l L.L.C.). Methods Filler and fines characterization The morphology and particle size of the FACS were tested with a scanning electron microscope (S-4800, Hitachi Ltd., Japan) and a BT-9300H particle size analyzer (Bettersize Instruments Ltd., China). The surface area of the FACS was measured by the multi-point Brunauer Emmett Teller (BET) nitrogen adsorption method (Gemini VII2390, Micromeritics Instrument Corporation, USA). The morphology of the fines was observed by a light microscope (DMB5-223IPL-5, Motic Electric Group Co., Ltd., China). Preparation and observation of composites Five grams of FACS (oven-dried weight) was diluted with deionized water to 5 wt%, followed by stirring at 300 rpm for dispersion. Then, various amounts of fines (Table 1) were added to the FACS slurry, and the solution was mixed for 1 min. After that, 0.05 EXPERIMENTAL Materials
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Zhang et al . (2016 ). “ Coflocculated fines & CaSiO 4 ,” B io R esources 11(3), 7406-7415.
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