Study On Manufacture And Properties Of Geopolymer-based High Performance Coral Aggregate Concrete

Marine concrete is an indispensable foundation for the development of Marine economy in China.In the environment of ocean reefs,the synthesis of coral aggregate concrete with local resources from coral reefs,which will effectively solve the problem of aggregate shortage in practical engineering applications.Internal curing of coral aggregate is similar to that of lightweight aggregate.However,the feature of light and porous showed a kind of rough surface and inclusion of impurities such as sediment,which increase the possibility of ions attack in the pores that not only will have a negative impact on the workability of the fresh coral concrete slurry,but also the mechanical properties and durability and volume stability of hardened paste.The weak long-term durability of silicate hydration products in harsh marine environments,the porous coral aggregates with a large amount of connected porosity,and the weak binding of the matrix-aggregate in the interface transition zone,which resulted in low strength and weak corrosion resistance of the traditional cement-based coral concrete.In this study,geopolymers are used as cementing materials,corals are used as coarse and fine aggregates.Geopolymer-based coral concrete with better strength and durability are designed by chemical micro-corrosion of coral coarse aggregates（9.5～4.75 mm）and synthesis of coral fine aggregates in directional gradation.The main research works and the conclusions are as follows:（1）With the increase of phosphoric acid concentration（0.44～0.84 mol/L）,the mass loss rate of coral coarse aggregate shows a trend from decline to rise.With the increase of phosphoric acid soaking time（30～90 min）,the mass loss rate of coral coarse aggregate is generally decreased.The flexural strength and compressive strength of coral concrete reached the peak value under the condition of 0.68 mol/L phosphoric acid soaking for 60minutes after 3 days.While the initial and the final setting time of coral concrete were extended by 34 minutes and 54 minutes in above conditions because geopolymers were retarded by PO₄^3-.The increase of phosphoric acid concentration and soaking time can effectively improve the acid corrosion.The decrease in strength using higher phosphoric acid concentration and prolonged soaking time clearly indicates that it is susceptible to the adverse impact of acid corrosion to the skeleton structure of coral coarse aggregates.The optimal acid corrosion parameter of coral coarse aggregate is 0.68 mol/L phosphoric acid soaking for 60 minutes.（2）Smaller diameter aggregate can form similarly shapes of sphere and ellipsoid with better mechanical reshaping and lower the degree of angularity,which was produced in large-diameter coral fine aggregates by ball mill.Coral sand with the lowest void fraction of 41.2%can be prepared by directional gradation.The geopolymer coral mortars prepared by repeatable mixing sand has good fluidity.Response surface methodology（RSM）was used to optimize the mix ratio parameters of geopolymer-based ultra high strength coral concrete.Fitting the second-order response surface model of coral sand fineness module（M_X）,coral sand-binder ratio(R_SB)and steel fiber volume content（D_S）,the mix of excellent performance geopolymer-based ultra-high-strength coral concrete was determined:M_Xis coral sand with a fineness module of 2.0,R_SBis a coral sand-binder ratio of 3.0 and D_Sis a steel fiber with a volume content of 2.0%.The concrete strength reaches its peak with 28days flexural strength of 31.9 MPa and the compressive strength of 146.7 MPa.（3）The surface of coral concrete deterioration can be detected in half immersion condition of sulfate.Ca²⁺in the C-A-S-H gel precipitates and reacts with SO₄^2-to form swelling gypsum,which damage the structure of the C-A-S-H gel and leads to deterioration of the properties of the geopolymer.Coral concrete did not show obvious surface deterioration during the chloride salt erosion process,the chloride diffusion coefficient of the coating group is only 2.88×10^-12m²/s.Geopolymer has relatively excellent pore characteristics,and the C-（A）-S-H gel and hydrotalcite-like phase in its products can also adsorb and fix part of Cl^-.So there is no obvious erosion and damage phenomenon.The combined attack is mainly caused by Mg SO₄.Na Cl has little effect on the deterioration of the microstructure of coral concrete,and more often leads to the corrosion of steel fibers.After coated the modified coarse aggregate with motars,the internal interface integration of concrete can be effectively improved,so that the development of compressive strength of coral concrete and resistance to the combined attack salt erosion can be enhanced.