Effect of Particle Gradation on Properties of Phosphogypsum based Cement Paste Backfill Suya He, Ying Shi and Qiyue Li

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1 nd International Conference on Sustainable Energy and Environment Protection (ICSEEP 2017) ISBN: Effect of Particle Gradation on Properties of Phosphogypsum based Cement Paste Backfill Suya He, Ying Shi and Qiyue Li Central South University, Changsha @qq.com, shiyingfriend@csu.edu.cn, @qq.com Key words:phosphogypsum;cement paste backfill;particle gradation;maximum particle fineness Abstract:Particle size of phosphogypsum (PG) is very small and uniform, which is poor in gradation. This feature can affect the quality of cement paste backfill (CPB) and may even lead to incomplete condensation. Given the problems mentioned, the main objective of this paper is to optimize the PG gradation. By mixing PG and quartz sand with different ratios, backfill samples were prepared based on the ideal gradation theory (Fuller grading curve). The results show that the optimization of PG gradation and the increase in the maximum particle size was beneficial to improve the quality of PG-based CPB. 1. Introduction PG is the major by-product in the process of phosphate fertilizer production via a wet-process [1]. At present, most of PG is deposited on the land, resulting in the occupy of a large land area and posing a great negative impact on the environment [2]. In order to solve the problem of PG accumulation, studies tried to apply PG in construction, agriculture and other industries [3-6]. PG-based cement paste backfill (PG-based CPB) mixing PG and binder for the underground is an efficient method, because of its advantages on great consumption of waste PG and the reduction of environment pollution. Gradation has an important effect on the quality of the backfill. Xing Ke et al [7]. studied two tailings with different sizes, and found gradation affected the quality of the backfill, such as fluidity, water content, porosity and compressive strength development. M Fall et al [8]. found that gradation also affect the moisture content and microstructure of CPB. The effect of particle size and gradation on PG-based CPB process has not been studied yet. The main purpose of this study is to investigate the effect of the maximum particle size and gradation on the quality of PG-based CPB. The properties of CPB in this study include initial and final setting time, water bleeding rate of fresh paste slurry, and unconfined compressive strength (UCS) of hardened paste backfill. 2. Experiment design The particle size distribution was tested by laser particle size analyzer (Ms3000). Particle size of PG was mainly distributed in the range of 5 μm-250μm, and its maximum particle size was no more than 0.5mm. It is clear that PG particles were very fine and its distribution was very concentrated. Coefficient uniformity Cu and coefficient curvature Cc are the common parameters for evaluating the gradation. Cu and Cc of PG in this study were about 3.4 and 1. Generally, the larger of Cu, smoother and wider are the particle size accumulation curve and the particle size distribution, and therefore it is better for consolidation. Cc represents continuity of particles, and poor particle continuity easily leads to segregation [7]. According to the mechanics theory, good gradation should satisfy 5 Cu 10 and 1<Cc<3. It is uniform particle when Cu<5 which is not good for slurry bleeding and backfill consolidation. Quartz sands were added in some certain ratios to improve the Cu of PG and to increase the maximum particle size, which could optimize PG gradation. Quartz sand was first sieved into X1 (<0.1mm), X2 ( mm), X3 ( mm), X4 (0.5-1mm), X5 (1-2mm). In this study, according to the ideal gradation theory (Fuller grading curve) (1), Cu of PG was adjusted to 5, 7 and 10, respectively, and its maximum particle size was increased to 1mm or 199

2 2mm. The percentages of quartz sand and PG are shown in table 1. =100( ) (1) In which, : is the passage percentage of aggregate particle size (%); : is the aggregate particle size; D: is the maximum particle size; n: is the gradation decreasing factor(0.3<n<0.5). No. Cu Table 1. Batches with different Cu and the percentages of quartz sands and PG in each batch. Maximu m size PG Conten t (%) X1 ( < 0.1) X2 ( ) X3 ( ) X4 (0.5-1) X5 (1-2) Materials and methods 3.1 PG, quartz sand and binder PG and binder used in this study were both from Guizhou province. Particle size distribution of PG is shown in Fig.1. For example, after adding quartz sand, the maximum particle size was controlled at 0.5while the particle size distribution was adjusted to 5, 7 and 10 of Cu,as shown in Fig.1. Binder used in this study consisted of cement clinker, yellow phosphorus slag, fly ash, calcium oxide. Figure 1. The size distribution before and after adding quartz sands into PG. 3.2 Paste backfill mix proportions According to Table 1, the PG, binder and quartz sands were mixed for 30min by JJ-5 type cement mixer. The ratio of aggregate and binder is 4.5:1, and concentration is 65%. The fresh paste slurry was immediately poured into 40mm 40mm 40mm molds. After 2 d the backfill samples were taken out of the molds and placed in chamber with constant humidity and temperature (90%, 25±2 ). 200

3 3.3 Water bleeding rate When preparing slurry, empty cylinder (100 ml) was first weighted. Then 100 ml slurry was poured into the cylinder. Every 30 min, bleeding water was sucked out by the syringe. After 24h, the rest in the cylinder was weighted. This method met the requirement of Chinese Standard DL/T Setting time Setting time was tested by cement consistency and coagulation time analyzer which meet the requirement of Chinese Standard JGJ/ UCS tests According to Chinese Standard JGJ/ , after curing 3 d, 7 d, 28 d, 60 d, the specimens were subjected to UCS tests using a machine with 200KN loading capability at a displacement rate of 3mm/min. Each data is obtained with three replications. 4. Test results and analysis 4.1 Effect of Cu on water bleeding rate Water bleeding rate is used to evaluate water demand of CPB and water retention capability. The relation between water bleeding rate and gradation is presented in Fig.2. Figure 2. The bleeding rates with different maximum particle size and Cu. According to Fig.2, the type of aggregate (PG and/or quartz sand) has a significant effect on water bleeding rate of the CPB. With the same size distribution, the water bleeding rate of PG and quartz sand were 5.2% and 12%, respectively. This might be attributed to the different chemical components between PG and quartz sand. The main component of PG is calcium sulfate dehydrate (CaSO 4. 2H 2 O) with microsolubility. calcium sulfate dehydrate could detained some water in PG-based CPB slurry. As a result, water bleeding rate of PG-based CPB was more than twice that of quartz sands based CPB. The Cu and the maximum particle size might have great effect on water bleeding rate. As shown in Fig.2, with the increase in Cu, the water bleeding rate also increased. Additionally, the water bleeding rate was in direct proportion to maximum particle fineness with the same Cu. This might be due to that the growth of pore size internal backfill could increase the outlet channels of excess water. 4.2 Effect of setting time The setting time is a necessary parameter to describe the quality of CPB, and it has important influence on early strength of CPB. The effect of gradation on initial setting time (IST) and final setting time (FST) is presented in Fig

4 Figure 3. The effect of particle gradation on IST and FST. Fig.3 shows that the IST and FST have been affected by the particle gradation. First, it is clear that the chemical components of aggregate had an important effect on IST and FST. Under the same gradation condition, IST and FST of PG-based CPB was much longer than that of quartz sand based CPB. The reason could be that calcium sulfate dehydrate formed a physical barrier in the process of hydration which was attached on the hydration products, and hence slowing down the hydration process. Calcium sulfate dehydrate could increase IST and FST and is not good for the development of early strength of CPB. In comparison, more quartz sand added, shorter was the IST and FST. The maximum particle size also had a certain influence on setting time. The increase in the maximum particle size could slightly reduce setting time. this could be explained by that the increase in particle size could relieve hydration barrier effect, and therefore shortening the setting time. 4.3 Development of unconfined compressive strength Compressive strength is one of the most important factors to measure the quality of mine backing. Some studies have shown that the required strength of CPB is about 1MPa at 28 days curing time [9]. The development of unconfined compressive strength is present in Fig.4. Figure 4. UCS development of CPB with different particle degradation. Fig.4 shows that UCS of CPB was closely related to aggregate composition. UCS of PG-only CPB was greater than 1MPa with 28 d curing, indicating that PG can meet the backfill needs. The addition of quartz sand could effectively improve the UCS of backfill. With 3 d curing time, the strength of quartz sand based CPB was about 1.7 times more than that of PG only CPB, which might be due to high water retention capability of PG. The maximum particle size could also affect the UCS. Mehta et al. [10] found that pores less than 100 nm have little effect on strength, while macropores are not good for strength development. As a result, increasing maximum particle size could, to some extent, help make the pore size distribution more uniform, and it is good for strength development. 5. Conclusion (1) PG-based CPB could provide sufficient support for underground mining activity, but the gradation of aggregate (PG) could be improved for better performance. 202

5 (2) The improvement of Cu and the maximum size distribution of PG-based CPB was good for shortening the condensation time, reducing water retention capability, and hence improving the quality of backfill. Acknowledgements This work was sponsored by Open-End Fund for the Valuable and Precision Instruments of Central South University (CSUZC201702) and Project of Innovation Driven Plan of Central South University (2015CXS016). We thank Ms Mei Wang of Kailin Mining company for assistance of field sample collection. References [1] Papageorgiou. F.: submitted to Environmental Monitoring and Assessment (2016). [2] Tayibi.: submitted to Journal of Envirionmental Management (2009). [3] Abril. J.M.: submitted to Journal of Environmental Radioactivity (2009). [4] Ali. M.A.: submitted to Waste Management (2009). [5] Singh. M.: submitted to Cement and Concrete Research (2002). [6] Xibing Li. J.D.: submitted to Journal of Cleaner Production (2017). [7] Ke. X.: submitted to Construction and Building Materials (2015). [8] M. Fall. M.B.S.O.: submitted to Minerals Engineering (2005). [9] Li. W. and M. Fall.: submitted to Construction and Building Materials (2016). [10] Jinxi Zhang and Sansan Jin, in: Micro-pore structure of cement concrete and its function, Science Publishing, Beijing, (2014), in press. 203

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