Study On Improvement Method Of Water-resistance Of Hemihydrate Phosphogypsum

Phosphogypsum is a solid waste produced in the production of phosphoric acid.Due to its large emissions,and low utilization rate,has formed a huge amount of storage and caused serious environmental pollution problems,and has become the focus of research at home and abroad.To improve the water-resistance of phosphorus building gypsum,based on the analysis of the root causes of its poor water resistance,the following methods were adopted to improve the water-resistance of phosphorus building gypsum:1)Adding mineral materials（lime,silica ash,kaolin,slag）to generate water-soluble materials or adding ultrafine materials（Ca SO₄whisker,nano Si O₂）to fill the gap and reduce the porosity of hardened body;2)Adding water reducing agent to improve the structural compactness;3)Mixed with mechanical waterproof agent（sodium methyl silicate,polymethyl triethoxysilane）to change the hydrophilicity of the hardening body.The influence of the above minerals or admixtures on the properties of phosphorus-building gypsum（flexural strength,compressive strength,water absorption,and softening coefficient,etc.）was studied.On this basis,the phosphorus-building gypsum composite with high water resistance was obtained from the above materials.XRD,SEM,EDS,and water contact Angle tests were used to analyze the action mechanism of the additives on the properties of phosphorus-building gypsum.The main conclusions are as follows:（1）When mineral materials are used to generate insoluble substances or ultrafine materials are added to fill the gap and reduce the porosity of the hardened body.1)Adding an appropriate amount of wollastonite,kaolin or slag can improve the water resistance and strength of phosphorus-building gypsum,and the effect of kaolin is the most significant.The best properties of phosphorus building gypsum composite can reach the flexural strength of3.14MPa,compressive strength of 12.59MPa,and softening coefficient of 0.48.They were increased by 56.2%,53.3%,and 60%compared with the group without addition,respectively.The water absorption rate was 28.79%,which decreased by 33.6%compared with the group without addition.2)Adding an appropriate amount of Ca SO₄ whisker or nano-sio₂ can improve the water resistance and strength of phosphorus-building gypsum,and the effect of nano-sio₂ is the most significant.The best properties of phosphorus building gypsum composite can reach the flexural strength of 4.55MPa,compressive strength of 17.1MPa,and softening coefficient of 0.5.They were 108%,38.7%and,22%higher than those in the no-addition group,respectively.（2）When water reducing agent is used to improve the structural compactness,the optimal dosage is 0.3%～1.0%.The optimal performance of phosphorus-building gypsum can reach the flexure strength of 4.75MPa,compressive strength of 22.94MPa,and softening coefficient of0.6,which are 84.4%,69.4%and 40%higher than that of the group without adding water reducing agent,respectively.The water absorption rate was 21.8%,which was 49.4%lower than that of the group without addition.（3）When using a mechanical waterproof agent to change the hydrophilicity of the hardened body,adding an appropriate amount of sodium methyl silicate or polymethyl triethoxysilane can improve the water-resistance of phosphorus-building gypsum,among which the effect of sodium methyl silicate is the most significant.The best performance of phosphorus-building gypsum can reach the flexural strength of 2.18MPa,compressive strength of 9.52MPa,and water absorption rate of 22.7%.It was 33.7%less than the group without addition.The softening coefficient was 0.46,which was 27.8%higher than that of the group without addition.（4）Given the above measures to improve the water resistance,the most effective method is adding mineral materials to generate insoluble hydration products,followed by adding water reducing agent to enhance the structure density,adding mechanical water repellent agent to change the structure hydrophilicity,and finally adding ultrafine materials to fill the gap and reduce the porosity of hardened body.（5）When various methods were used to improve the water-resistance of phosphorus-building gypsum,the results showed that the best performance of phosphorus building gypsum composite was when the mixture of 1%polycarboxylic acid water-reducing agent,2.5%nano-sio₂,25%kaolin,and 1.2%sodium methyl silicate,and the flexural strength was 4.61MPa and the absolute dry compressive strength was 19.54MPa.The softening coefficient was 0.64,which was increased by 100.4%,157.1%,and 113.3%,respectively.The water absorption rate was 19.85%,which decreased by 81.2%compared with the group without addition.Microscopically,it was confirmed that in addition to Ca SO₄·2H₂O,more insoluble calcium silicate hydrate gel and ettringite were also generated during mixing.Moreover,the water contact Angle on the surface of the hardened body is 109.01°,which effectively improves the hydrophobicity of the material surface.The change of these microscopic components and contact Angle is the internal reason for the obvious improvement of the water resistance of the phosphor-building gypsum composite.