Applicability And Mechanism Of Steel Slag In Regulating The Setting Time Of β Hemihydrate Gypsum

There is a huge amount of industrial by-product gypsum heap in China,which not only occupies land,but also pollutes the environment.β hemihydrate gypsum can be prepared by calcining synthetic gypsum,which possesses the advantages of light weight,heat preservation,sound insulation and a certain degree of temperature and humidity regulation,and is recognized as green building materials.However,β hemihydrate gypsum hardens quickly when mixed with water,so appropriate retarding admixture is often added to adjust the performance in practical application.In practice,most retarders have some adaptability problems,and the cost is generally high.In this paper,β hemihydrate gypsum was modified by steel slag powder.It is found that steel slag can prolong the setting time of some β hemihydrate gypsum to a certain extent,and the cost is low.But it only works for some kinds of hemihydrate gypsum.Through the systematic study on the adaptability and influencing factors of steel slag regulating the setting time of β hemihydrate gypsum,it is concluded that the initial p H of gypsum slurry and the type of anions(mostly from acid)are the main factors.Thus,the method of controlling the hardening and setting of hemihydrate gypsum by using steel slag-phosphoric acid system is proposed.By studying the influence of steel slag-phosphoric acid system on the setting time and mechanical strength of β hemihydrate gypsum,the appropriate modification conditions are obtained: initial p H is 1.5,steel slag content is 3%.At this point,the initial setting time of β hemihydrate gypsum was increased from 9 min to 119 min,the loss of absolute dry flexural strength was 12.68%,and the loss of absolute dry compressive strength was 22.97%.Compared with the gypsum system with citric acid and sodium polyphosphate,the loss of mechanical properties is significantly reduced.The hydration rate,temperature rise,ion concentration and supersaturation of βhemihydrate gypsum were systematically studied.The results show that the hydration rate and hydration heat release rate of gypsum modified with steel slag-phosphoric acid system are significantly lower than that of blank,the dissolution exothermic period of hydration process is obviously prolonged,the accelerated exothermic period delayed and peak value decreases.The crystallization saturation of gypsum in dihydrate decreased first and then increased,then it continued to decrease slowly,and the decrease rate was lower than that of blank sample.The sulfate concentration in the early stage of hydration was significantly higher than that of blank sample,while the crystallization saturation of gypsum dihydrate was lower than that of blank.The hydration products of β hemihydrate gypsum mixed with steel slag-phosphoric acid were studied by XPS and XRD.It was concluded that the regulation mechanism was proposed.The calcium oxide in steel slag reacted with phosphoric acid to release dihydrogen phosphate and monohydrogen phosphate,which quickly combined with calcium ion to form calcium dihydrogen phosphate and calcium monohydrogen phosphate.In this way,the sulfate ion concentration in the early stage of hydration is increased and the crystallization saturation of dihydrate gypsum is decreased,which indicates that the higher concentration of sulfate ion in the early stage of hydration inhibits the dissolution of β hemihydrate gypsum.The insoluble calcium hydrogen phosphate is then covered on the crystal surface of dihydrate gypsum,which retards the formation and growth of dihydrate gypsum crystal nucleus,reduces the hydration rate of hemihydrate gypsum,and finally delays the setting and hardening time.SEM analysis shows that the pores in the microstructure of hardened body of steel slag-phosphoric acid modified gypsum increased compared with the blank,and the crystal morphology was still needle bar shape,with slightly smaller size,resulting in loss in mechanical strength of gypsum hardened body,but the loss is small.In order to investigate the adaptability of steel slag-phosphoric acid system,it was used in several kinds of β hemihydrate gypsum.The results show that steel slag-phosphoric acid can be used as gypsum retarder with better adaptability and less strength loss compared with the traditional retarders.The steel slag-phosphoric acid composite system proposed in this paper is used in the process technology of β hemihydrate gypsum to control the setting and hardening properties of gypsum products,which provides technical basis for low-cost control of the hardening properties of gypsum products,realizing the collaborative utilization of steel slag and industrial by-product gypsum,which conforms to the national industrial policy and sustainable development strategy,and has good social,economic and environmental benefits.