| Phosphogypsum is a solid waste produced in the wet-process phosphoric acid process and currently faces the problem of high emissions and low utilization.Adding phosphogypsum into concrete not only solves the problem of shrinkage cracks caused by cement hydration reaction,but also opens up a new way for the resource utilization of phosphogypsum and protects the ecological environment.In this study,the original phosphogypsum without pretreatment is used.Only the phosphogypsum is screened and refined.Compared with the pretreatment process of phosphogypsum,the treatment process of phosphogypsum is simplified,the cost is saved,and the environmental pollution caused by the secondary treatment is avoided.In this paper,the mechanical properties of cement mortar and pumice concrete specimens made with different amounts of undisturbed phosphogypsum are tested for compression and bending resistance,and the effect of the amount of undisturbed phosphogypsum on the mechanical properties of cement mortar and concrete is studied.Then,the carbonation and freeze-thaw resistance tests of concrete were carried out to study the effect of undisturbed phosphogypsum on the durability of concrete.After that,the SEM scanning test was carried out on the cement mortar and concrete specimens,and the modification mechanism of the original phosphogypsum on the cement mortar and pumice concrete was studied from the microscopic point of view.Finally,the test results are analyzed.Through the comprehensive analysis of the impact of phosphogypsum on the shrinkage compensation effect,durability and mechanical properties of pumice concrete,the optimal dosage of phosphogypsum is found,and the optimal mix proportion of pumice concrete under this dosage is obtained.First of all,the cement mortar test with single phosphogypsum,compound phosphogypsum and fly ash,compound phosphogypsum and mineral powder,and three phosphogypsum,fly ash and mineral powder was carried out.The curing period was 3d,7d,14 d and 28 d respectively,and the curing method was mold curing.The test results obtained through this test are that when the content of single phosphogypsum is 4%,the content of compound phosphogypsum and fly ash is2% and 15%,the content of compound phosphogypsum and mineral powder is 2% and 20%,and the content of three phosphogypsum,fly ash and mineral powder is 2%,15% and 20%,respectively,the mechanical properties of cement mortar are the best.Through various analysis methods of orthogonal test,it is found that compared with fly ash Mineral powder is a major factor affecting the mechanical properties of cement mortar.Secondly,five groups including the group with the best mechanical properties in the cement mortar test and the blank control group without adding phosphogypsum,fly ash and mineral powder were selected to make pumice concrete test blocks and carry out compressive strength tests to study the effect of the amount of phosphogypsum and other additives on the mechanical properties of pumice concrete.The pumice concrete studied in this paper is lighter than the traditional concrete,which makes up for the shortcomings of the traditional concrete with large dead weight,and is more in line with the requirements of the current urban construction and development for lightweight,high-strength,green and environmental protection of concrete.The design strength of the concrete in this study is LC30.It is found that the 28 d compressive strength of the five groups of mix proportions reaches the standard of LC30.Finally,the concrete freeze-thaw test,concrete carbonization resistance test and freeze-thaw test were carried out for the above five groups.The test results showed that the A1B1C1 group had the best freeze-thaw performance and carbonization resistance performance,that is,the content of phosphogypsum was 2%,the content of fly ash was 15%,and the content of mineral powder was20%,indicating that the addition of mineral admixtures such as phosphogypsum had a positive effect on the durability of concrete.Then,the specimen was tested by SEM,which is a conventional way to observe the micro-morphology,compactness,structural uniformity,and micro-structure defects of various hydration products in cement concrete.In this experiment,the blank group mixed with any admixture was compared with the best group A1B1C1 in the orthogonal test.The curing period was 28 days,and the SEM images were obtained at a magnification of 2000 times.It was found that prismatic and flaky ettringite crystals were formed on the surface of% of the test blocks in A1B1C1 group,while obvious cracks appeared on the surface of the blank control group without any admixture.Relatively speaking,the integrity and structural compactness of A1B1C1 group test block is slightly better than that of the test block without phosphogypsum,and there is no crack.Through the above tests,it is found that the addition of undisturbed phosphogypsum greatly improves the working performance of cement mortar and pumice concrete,and reduces the cracking problem caused by shrinkage of mortar and concrete... |
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