Abstract
The preparation of building gypsum from phosphogypsum is one of the ways to realize its resource utilization. However, the precondition is first to reveal the influence mechanism of the impurities in phosphogypsum on the properties of plaster. Thus, the influence of the form of soluble P2O5 (H3PO4, H 2 PO 4 − , and HPO 4 2 − ) that is considered the most significant impurity on the hydration and hardening of hemihydrate phosphogypsum was studied in this paper, which included the setting time, mechanical properties, hydration rate, crystal structure, and micromorphology of hydration products. The results showed that both H3PO4 and H 2 PO 4 − can accelerate the early hydration of hemihydrate phosphogypsum, shorten the setting time of paste, and improve the mechanical properties of hardened paste. In addition, H3PO4 had a more significant effect than H 2 PO 4 − . Different from H3PO4 and H 2 PO 4 − , HPO 4 2 − significantly delayed the early hydration reaction of hemihydrate phosphogypsum, significantly prolonged the setting time of the paste, and significantly reduced the mechanical properties of the hardened body. The test data of XRD, IR, and SEM showed that two negative effects can be caused when soluble P2O5 existed in the form of HPO 4 2 − . On the one hand, HPO 4 2 − can lead to the formation of Ca3(PO4)2 insoluble substance in the hydration reaction of hemihydrate phosphogypsum, which significantly hindered the hydration reaction and prolonged the setting time of the paste. On the other hand, HPO 4 2 − can change the crystallization habit of the hydration product of CaSO4·2H2O and make it change from needle-columnar to semiflake and irregular shape, resulting in a significant reduction in the mechanical properties of hardened paste.
Highlights
Phosphogypsum is an industrial by-product produced in the process of wet production of phosphoric acid [1, 2]
Soluble P2O5 exists in four forms: H3PO4, HPO24−, HPO24−, and PO34−, which are produced by the ionization of phosphoric acid in an aqueous solution
Phosphoric acid was bought from Yantai Shuangshuang Chemical Co., Ltd, which had a H3PO4 concentration of 85 wt. %. e analytical pure reagents of NaH2PO4 and Na2HPO4 for the experiment were purchased from Tianjin Hengxing Chemical Reagent Co., Ltd. e polycarboxylic acid water reducer (PCE) was purchased from Shandong bock Chemical Co., Ltd. e deionized water was always used in the experiment
Summary
Phosphogypsum is an industrial by-product produced in the process of wet production of phosphoric acid [1, 2]. 4–6 tons of phosphogypsum can be produced for each ton of phosphoric acid [3]. E annual output of phosphogypsum in China’s phosphoric acid industry has exceeded 70 million tons, but the utilization ratio is only 40% according to China Phosphate and Compound Fertilizer Industry Association. Soluble P2O5 exists in four forms: H3PO4, HPO24−, HPO24−, and PO34−, which are produced by the ionization of phosphoric acid in an aqueous solution. Erefore, soluble P2O5 in phosphogypsum mainly exists in three forms: H3PO4, HPO24−, and HPO24−. Most studies on the influence of soluble P2O5 on building phosphogypsum mainly focus on macroscopic properties, while there are still few studies on the influence mechanism of soluble P2O5 on the hydration rate of hemihydrate phosphogypsum and the crystal structure of hydration products. Based on the above research deficiencies, this paper intended to study the influence of a soluble P2O5 form on the hydration reaction rate, crystal structure, and micromorphology characteristics of hemihydrate phosphogypsum by introducing different forms of soluble P2O5, establish the relationship between the hydration hardening characteristics of hemihydrate phosphogypsum and its physical and mechanical properties under the influence of soluble P2O5 form, and reveal the internal influence mechanism of soluble P2O5
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