Study On Preparation And Properties Of Basic Magnesium Sulfate Cement Based Coral Concrete

Marine concrete is an important foundation support for the development of marine economy.The use of dead coral waste to prepare concrete in the South China Sea ocean environment will effectively solve the problem of shortage of aggregates in engineering construction.Although coral aggregates have alleviated the huge demand for aggregates in marine engineering,their rough and porous surfaces,complex shapes and characteristics,and low cylinder compression strength severely limit the performance of coral concrete.Although the use of mineral admixtures,fiber toughening and admixture technology can greatly increase the strength and toughness of coral concrete,it does not completely solve the problem of restricting the strength of coral concrete.Treating the weak surface layer of coral aggregate（generating a shallow surface layer with a shorter age）,attachments（such as seaweed）and improving its porous properties are also effective ways to achieve high performance coral concrete.On the other hand,the existing cement-based coral concrete has poor stability in the marine environment due to cement hydration products,and the characteristics of coral aggregates also lead to weak concrete interface transition areas,resulting in low strength and poor corrosion resistance of cement-based coral concrete.In this paper,a new type of cementitious material,basic magnesium sulfate cement,is compounded with coral aggregate.By optimizing the coral sand gradation,adjusting the ratio of basic magnesium sulfate cement-based coral mortar,and exploring the efficient modification treatment of coral aggregate,the preparation basic magnesium sulfate cement based coral concrete with good mechanical properties and corrosion resistance.The main conclusions of this article are as follows.（1）Cleaning coral aggregate with citric acid can effectively remove the weak layer on the surface of the coral aggregate and the attachments in the pores,and improve the interfacial adhesion between the aggregate and the cementitious material.The coating of coral aggregate can significantly reduce the porosity of coral aggregate and improve its strength.The optimum technological parameters for citric acid treatment of coral aggregate are concentration 1%and soaking time 40 min.The best composition of the basic magnesium sulfate cement coating material isα-Mg O:Mg SO₄·7H₂O:H₂O=1.5:1:1.The modified coral aggregate can be used to prepare basic magnesium sulfate cement based coral concrete with a high compression ratio.The flexural strength and compressive strength at 28 d reach 15.2 MPa and 64.8 MPa,respectively.Compared with the control group,the flexural strength and compressive strength at 28 days are increased by 20.5%and 31.8%respectively,and the workability of concrete is increased by 35.3%.（2）Reasonable particle size distribution makes it easier for coral sand to reach the densest filling state,enhances the working and mechanical properties of basic magnesium sulfate cement-based coral mortar,optimizes its pore structure,and makes coral mortar has better comprehensive properties.The basic magnesium sulfate cement-based coral mortar with compressive strength greater than 70 MPa and a flexural strength greater than 12 MPa in 28 days can be made by using the particle size distribution of II_1/2coral sand.The second-order response surface model of the flexural strength and compressive strength of coral mortar based on basic magnesium sulfate cement is established under the interaction of molar ratio,mortar ratio and additive content.Proper molar ratio（8.5-9.5）and sand-cement ratio（such as 1.1-1.3）can make coral mortar have better mechanical properties.The flexural strength and compressive strength of basic magnesium sulfate cement based coral mortar for 28 days can reach 13 MPa and 79 MPa,respectively,by adopting the optimized ratio（molar ratio 8.5,sand-cement ratio 1.2,and additive content 1.8%）.（3）After being fully immersed in four corrosive solutions of 3.2%Na Cl,5%Na₂SO₄,5%Mg SO₄and 3.2%Na Cl+5%Mg SO₄composite solution for 120 days,the compressive and corrosion resistance coefficients of all samples except the Na₂SO₄group are all higher than 0.8.In the semi-immersion environment,there is no new phase formation in the immersion area,crystallization area and drying area of each group of samples.When Na⁺and SO₄^2-are contained in corrosive solution,it can cause corrosion damage similar to Na₂SO₄.The migration of Na⁺in the sample may be the main reason for the surface deterioration of the sample,and Mg²⁺may promote the hydration of basic magnesium sulfate cement.After using modified coral aggregate,the corrosion products above the liquid level of each group of samples in a semi-immersion environment are obviously reduced,and the compression and corrosion resistance coefficients of each group of samples in a full-immersion environment are higher than 0.85,with uniform compressive strength.The durability of coral concrete based on basic magnesium sulfate cement in corrosive environment can be significantly improved by increasing it by more than 20%.