The Effects of Combined Retarders on the Properties of Desulfurized Gypsum

Abstract

As a relatively fast-setting building material, desulfurized gypsum is difficult to use. Citric acid, sodium gluconate, and sodium tripolyphosphate were used to design and prepare a desulfurized gypsum paste containing single or combined retarders. The effects of the retarder type and content on the setting inhibition and mechanical properties of desulfurized gypsum were evaluated. The experimental results showed that all three retarders had different degrees of retardation on desulfurized gypsum; among them, citric acid had the best retarding effect. When the citric acid content was 0.10 %, the initial and final setting times were 25 and 31 min, respectively. However, the mechanical properties of desulfurized gypsum could be decreased by more than 60 %. Compared with single-doped retarders, combined retarders had obvious advantages. They not only significantly lengthened the setting time and hardening time two-fold or even more but also the compressive strength of desulfurized gypsum with combined retarder was 50 % higher than that with single retarder under the same retarding effect as a single retarder. The hydration heat curve provided insight into the setting inhibition of desulfurized gypsum by retarders. Scanning electron microscopy images showed the change in the crystal morphology of desulfurized gypsum-hydrated crystals with the addition of retarders. The addition of a retarder lengthened the induction period of desulfurized gypsum and changed the morphology of desulfurized gypsum-hydrated crystal, thus extending the setting time and reducing the strength.

Effect of borax on the hydration and hardening of β-hemihydrate gypsum at high water–plaster ratio

“…However, there are obvious defects in β-hemihydrate gypsum blocks with large water-plaster ratio as building materials. For example, it coagulates easily after mixing with water, the early strength changes greatly [21][22][23], and it cannot meet the needs of normal construction operations [24]. Therefore, before its use in construction, the method of adding gypsum retarder is adopted to prolong the setting time of the gypsum slurry [25].…”