Study Of The Influence Of Ions On The Structure And Performance Of Calcium Sulphate Dihydrate

Industrial by-product gypsum is an actual industria waste, which the main component is calcium sulfate. In recent years, with the promotion of green environmental development, the emission of the waste is growing quickly. Compared with natural gypsum, industrial by-product gypsum contains large amounts of impurity ions(Na⁺、K⁺、Mg²⁺、Al³⁺、F^-、Cland PO₄^3-etc), which had great hinder to its resource-oriented utilization.Calcium sulphate dihydrate is the main hydration products of gypsum building material,which determined the strength of gypsum based material.Generally, acicular like gypsum crystals can create effective cross-lap and enhance the flexural strength of products; while short columnar crystal is better for close packing and compressive strength of products.especially high bending strength of gypsum is very important. In the production process of gypsum-based construction materials, though introduction of retarders can improve its operability,but change of gypsum crystal morphology（from needle like into plate, sheet,irregular shape）,which would magnify the pore volume of hardened specimen and decrease the strength of products.Therefore, in this paper systematic studied the effects of types and dosage of different kinds of impurity ions(Na⁺、K⁺、Mg²⁺、Al³⁺、F^-、Cl- and PO₄^3-etc) on the structure and morphology of gypsum by the modern techniques of X-ray diffraction（XRD） and scanning electron microscope（SEM）; Investigated the retarding mechanism of citric acid and sodium tripolyphosphate and prepared a kind of high performance composite retarder. The main results were as follows:（1）Na⁺、K⁺、Mg²⁺、Al³⁺ etc cation can selectively adsorpt to gypsum（020） crystal plane,which had a certain impact on the intensity and position of（020） diffraction peak which changed its spacing and crystallinity degree. Due to the cloest of Mg²⁺ and Ca²⁺ and same valence state, the effect of K⁺、Mg²⁺、Al³⁺ on the intensity and position of（020） diffractionpeak was K⁺>Al³⁺>Mg²⁺;（2）When the content of Na⁺、K⁺was more than 1.0%, Na⁺、K⁺could combination with Ca²⁺ and SO₄^2- on the crystal surface to generate unstable complex salt particles, which could induce growth of gypsum crystal in this direction and appear small sheet of flake gypsum crystal; And Mg²⁺、Al³⁺ had bit effect on the needle like gypsum crystals;（3） F^-、Cl-、PO₄^3- etc can combination with Ca²⁺ on the crystal surface to generate CaF2、Ca3（PO4）2 precipitation, which covered on the（111） crystal plane and inhibited growth of c axis of gypsum crystal. And changed the needle like crystal into irregular shape（plate, sheet or block）. Under the same dosage, the effect order of phosphate on the gypsum crystal structure was PO₄^3->HPO₄^2->H₂PO₄^-;（4）With the adding of citric acid and sodium tripolyphosphate, the needle like gypsum crystal changed into low length to diameter ratio crystal, such as thick plate, block and other loose crystal shape; XRD showed that citric acid could react with Ca²⁺ to generate stable complex compound, which covered on the surface of gypsum crystals and inhibited the grow process of crystals; while sodium triphosphate reacted with Ca²⁺to generate calcium precipitation, which both covered on the hemihydrates and dihydrate crystals, suppressed dissolution of hemihydrates and formation process of dihydrate critical nucleus;（5） The order of retarding effect of three kinds of retarders was: citric acid>sodium tripolyphosphate>bone glue, bone glue had minimum influence on the strength of products.The orthogonal experiment concluded that the ratio of high performance retarder was: 0.06%citric, 0 sodium tripolyphosphate and 0.2% bone glue.The composite retarder not only had extension on the setting time of building gypsum, but also did not reduce the compressive strength of products.