Preparation Of Cementitious Material Based On Recycled Fine Powder From Construction Waste And Its Effect On Properties Of Phosphogypsum

The application of solid waste is an important way for the building materials industry to realize the national"double carbon"strategy.Based on the concept of"treating waste with waste,turning waste into treasure,"the activation process of recycled fine powder prepared from construction waste was explored for the four solid wastes of construction waste,phosphogypsum,carbide slag and sulfide alkali slag.The effects of carbide slag and sulfide alkali slag on the excitation efficiency of recycled fine powder and the effects of carbide slag/recycled fine powder based cementitious materials on the properties of phosphorus building gypsum were studied.The synergistic mechanism of multi-component solid waste was analyzed by modern testing methods such as laser particle size,XRD,SEM and MIP.（1）Recycled fine powder was prepared by two conventional activation methods:mechanical grinding and high temperature calcination.Compared with the traditional dry grinding,the advantage of wet grinding was that the micro-powder particles were in full contact with the grinding beads during ball milling.With the increase of ball milling time,the size of powder particles decreased significantly but the energy consumption increases.High temperature calcination of recycled powder could make the hydrated cement stone in the powder dehydrated and decomposed into a secondary hydration phase.The optimum activation temperature of recycled powder was 800℃.（2）The pretreated carbide slag and sulfide alkali slag were used as activators to chemically activate and regenerate recycled fine powder.The adaptability of brick powder and carbide slag was better than that of concrete powder.The strong alkalinity of carbide slag not only stimulated the pozzolanic property of brick powder,but also provided calcium source for the hydration reaction of brick powder.The compatibility of concrete powder and sulfide alkali slag was better than that of brick powder.Sodium salts such as Na₂SO₄ and Na₂CO₃ in sulfide alkali slag promoted the hydration of active substances in concrete powder.The purity of the sulfide alkali slag after flotation treatment was improved,and it could be ground into fine powder and used as a cement admixture,but the dosage should not exceed 20%.（3）Low carbon cementitious materials were prepared by using carbide slag as calcareous raw material,recycled brick powder as siliceous raw material and metakaolin as aluminum raw material.According to the orthogonal test results,the cementitious material with the ratio of 55%recycled brick powder,30%carbide slag and 15%metakaolin had the best performance.The initial setting time was 111 min,the final setting time was 218 min,and the28 d compressive strength was 16.57 MPa.MIP test results showed that with the increase of carbide slag content,the number of harmful pores in the test block decreased,the total porosity decreased,but the average pore size did not change much.XRD test results showed that the hydration products of brick powder based composite cementitious materials were mainly hydrated calcium silicate and aluminum calcium silicate.The compressive strength of the cementitious material matrix was further improved by reducing the water-cement ratio and pressure molding.When the water-cement ratio was 0.25,the compressive strength of the test block was up to 25.23 MPa.（4）Phosphorous building gypsum was modified with activated recycled powder as admixture.Single-doped recycled fine powder was not beneficial to the water resistance of phosphogypsum.Because the particle gradation of brick powder was better than that of concrete powder,the 28 d compressive strength of phosphogypsum after single-doped brick powder was higher than that of single-doped concrete powder group.With the increase of brick powder content,the compressive strength of phosphogypsum increased first and then decreased,and the optimum content of brick powder was 10%.The compressive strength and water resistance of brick powder/phosphogypsum test block with carbide slag as activator were lower than those of pure phosphogypsum.The high alkalinity of carbide slag destroyed the growth environment of gypsum crystal,resulting in poor crystallization of gypsum,porous inside gypsum and reduced performance.When cement was used as the activator,the compressive strength of the test block was reduced but the water resistance was improved.The hydraulic product after cement hydration was wrapped on the surface of gypsum crystal,and the contact surface between gypsum crystal and free water was reduced,so the water resistance was improved.The incorporation of polypropylene fiber could improve the compressive and flexural strength of brick powder/phosphogypsum-based cementitious materials,but the content should not exceed 1.6%.