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Study on the Use of Desulfurized Gypsum as a Cement Retarder Abstract: The cement properties when desulfurized gypsum is used as a cement retarder, as well as the mechanism of action of this gypsum, were investigated. Research shows that desulfurization gypsum contains a certain amount of calcium carbonate. When incorporated into cement, it does not affect the setting time of the cement; rather, it has a positive effect on the mechanical properties and stability of the cement. Therefore, it can be used as a substitute for natural gypsum in cement production. In addition, the granulation of desulfurized gypsum and the significant economic benefits it brings to manufacturing enterprises were also studied. Desulfurization gypsum is an industrial by-product gypsum produced during flue gas desulfurization in thermal power plants through the reaction of SO2 and CaCO3; its main component is CaSO4•H2O, with some impurities such as incompletely reacted calcium carbonate, other impurities present in limestone, and small amounts of potassium and sodium salts, whose concentration is generally no more than 0.5%. The large output of desulfurized gypsum, and the fact that it is not restricted by natural gypsum sources, have drawn widespread attention to its use in cement production. Abroad, there is successful experience in using desulfurization gypsum as a cement retarder; in China, desulfurization gypsum has only begun to be produced in recent years, and no systematic studies have yet been conducted on its use as a cement retarder. This study conducts in-depth research on the comprehensive utilization of 300,000 tons per year of desulfurization gypsum generated by power plants. Industrial by-product gypsum, as waste, can pollute the environment. Recycling waste, and using desulfurized gypsum as a cement retarder, is a very effective approach. This will bring good economic, social, and environmental benefits to waste-discharging entities and cement plants. Materials and experimental methods: The main raw materials include desulfurized gypsum, natural gypsum, cement clinker, slag, and fly ash; their chemical compositions are shown in Table 1. Desulfurization gypsum is a grayish-white powder with a residue of 1.0% on a 0.045 mm square-hole sieve. The main impurities are incompletely reacted CaCO3 and partially soluble salts. Chemical analysis shows that desulfurized gypsum contains no impurities that have a negative effect on its hydration properties, making it suitable as a cement retarder. Natural gypsum appears as grayish-white lumps. Fly ash is the dry ash discharged from power plants; its physical properties are shown in Table 2. The slag is water-quenched blast furnace slag. Cement clinker, gypsum, and various admixtures are measured according to the specified proportions and then mixed in a ball mill for 30 minutes, so that the fineness of the cement meets the **standard requirements. According to the actual measurements, the residue on a 0.08 mm square-hole sieve for cement is 7.0%–8.2%. The fineness of the composite cement is 2.8%–4.7%. In accordance with **standards**, comprehensive tests were conducted on the properties of Portland cement, ordinary cement, slag Portland cement, and composite cement. Results and Discussion: Effect of desulfurization gypsum on the properties of Portland cement and ordinary cement. The effects of desulfurization gypsum on the properties of Portland cement and ordinary cement are shown in Tables 3 and 4. According to the test results, when the amount of desulfurization gypsum used is greater than 2.0%, the setting time of the cement meets the standard requirements and its soundness is satisfactory. As the amount of gypsum increases, the setting time prolongs, but there is no significant change in strength. Compared with cement using natural gypsum at the same dosage, cement using desulfurized gypsum as a retarding agent has an earlier initial setting time; the final setting time remains similar, while its strength is about 5% higher than that of the latter. Cement performs well when the content of desulfurized gypsum ranges from 2.5% to 4%. When the addition level is below 2%, the retarding effect of the cement does not meet the requirements. It can be seen from the above results that desulfurized gypsum can be used in the production of silicate cement and ordinary cement just like natural gypsum. Effect of desulfurized gypsum on the properties of slag cement and fly ash cement: The effect of desulfurized gypsum on the properties of slag cement and fly ash cement is shown in Table 5. Based on the results of the above experiments, it can be seen that desulfurized gypsum is capable of properly regulating the setting time of cement, allowing the properties of the cement to develop normally. In particular, the strength parameters remain at the same level as those of cement using natural gypsum as a retarding agent; in some cases, they are even 5% to 7% higher. For low-grade cement, the increase is greater, around 10% to 20%. In this mixture, desulfurization gypsum serves not only as a retarder but also as a sulfate activator, thereby promoting the development of cement strength. The effect of desulfurization gypsum on the properties of composite cement is shown in Table 6, which presents the properties of composite cement containing slag and limestone, or a mixture of slag and fly ash, with desulfurization gypsum used as a retarder. The test results indicate that desulfurization gypsum can effectively regulate the setting time of composite cement, resulting in excellent cement properties. The properties of the composite cement with limestone incorporated are significantly better than those of the cement with fly ash incorporated. Note: The fineness of cement should be controlled between 2.8% and 4.7%. Based on the test results of the various cement types mentioned above, it can be seen that desulfurization gypsum contains some unreacted CaCO3 as well as certain soluble salts such as K+ and Na+ salts. The presence of these impurities helps to accelerate the hydration of cement and enables the full activation of the supplementary materials. Additionally, due to its fine particle size, desulfurization gypsum can come into full contact with both cement particles and supplementary material particles within the cement, leading to rapid reactions that can effectively regulate the setting time of the cement. For slag cement, fly ash cement, and composite cement, desulfurized gypsum acts as both a retarder and a sulfate activator during the hydration of the admixtures; moreover, the CaCO3 contained in it contributes to improving the structural properties of the cement. Granulation of desulfurization gypsum: Powdered desulfurization gypsum presents many disadvantages in terms of transportation and use in production; therefore, when it is used as a cement retarder abroad, the powdered gypsum is first granulated into solid forms. The imported granulation equipment for powdered gypsum is expensive, making it difficult to promote its use. In China, it has been proposed to use a mixture of half desulfurized building gypsum and half desulfurized dihydrate gypsum, and to form balls using a cement balling machine; this is technically feasible, but it is not economically viable for companies, as the cost of 50% of the desulfurized building gypsum used as a retarding agent is already comparable to the price of natural gypsum. The granulation process of gypsum is one in which the particle size of the powder is increased. During this process, the powder particles combine to form larger secondary particles through mechanisms such as interparticle bridging, capillary suction forces, adhesion, electrostatic attraction, and mechanical interlocking. The strength of these secondary particles is the key parameter used to evaluate the quality of granulation. It is advisable that the failure load requirement for gypsum particles be no less than 8 kg. Granulation methods can be roughly divided into two categories: one involves granulation through the cohesion between particles, such as the disk granulation method; the other involves using external forces to apply pressure to the powder in order to form particles, such as compression granulation. The first molding method requires the material to have good plasticity, whereas desulfurized gypsum has very poor plasticity, so this method is not suitable. Based on the properties of gypsum, repeated tests led to the selection of a technical approach that involves adding a small amount of binder to desulfurized gypsum and then compressing it into pellets. This binder has no impact on the properties of cement. The mixed material is compressed into shape using moderate pressures; the load required to break the pellets is at least 10 kg, which meets the required standards. Existing domestic equipment is capable of ensuring mass production. Calculations show that the cost of the cement retarder after granulation is 10–15 yuan per ton higher than that of desulfurized dihydrate gypsum, resulting in a price of 30–35 yuan per ton; this is significantly lower than the price of natural gypsum, which is 50–70 yuan per ton. Conclusion (1) Desulfurized gypsum is an industrial by-product gypsum produced by the reaction of limestone powder with sulfur dioxide; it contains no impurities that could negatively affect the properties of cement, making it suitable as a cement retarder. (2) Similar to natural gypsum, desulfurized gypsum can properly regulate the setting time of cement; when 3%–5% of desulfurized gypsum is added, the setting time of all types of cement meets the requirements of ** standards. (3) Desulfurization gypsum contains CaCO3 and a small amount of soluble salts, which is beneficial for promoting the development of cement strength and activating the activity of admixtures. The strength of cement using desulfurized gypsum as a retarding agent is comparable to that of cement using natural gypsum, with the strength of certain types of cement increasing to a certain extent. (4) By adding a small amount of binder to press the desulfurization gypsum into pellets, the requirements for transportation and use in production are met without significantly increasing its production costs. References: Liu Bingquan. The Current Situation and Development of Gypsum Used in Cement. Contemporary Cement, 1994, (1). Hu Shuguang. Development of High-Speed Railway Composite Silicate Cement. Contemporary Cement, 1994, (2). Lu Ping et al. The Influence of CaCO3 on the Hydration of C3S. Acta Ceramica Sinica, 1987, (4). Cong Gang et al. Research on the Utilization of Desulfurization Gypsum from Power Plants in Chongqing Area. Comprehensive Utilization of Fly Ash, 1996, (2). Lu Hougen. Introduction to Powder Engineering. Shanghai: Tongji University Press, 1993