Experimental Study On The Effect Of Freshwater Seawater Mixing On The Dissolution And Permeability Of Coral Sand

Coral sand is a special rock medium in the tropical Marine environment.The main chemical composition is calcium carbonate,also known as calcareous sand.Coral sand is usually formed by the long geological processes of corals,algae,shellfish and other Marine biological remains.Due to the special physical,chemical and biological effects,coral sand has the characteristics of multi-pore development,high compressibility,fragile particle fragility,easy cementability and structural strength,which makes the physical and chemical properties of coral sand,especially the infiltration property,different from the ordinary continental sand.Coral sand is the main component of reefs and reef freshwater resources lack,enrichment in coral sand layer is the only fresh water resources,fresh water body in the infiltration characteristics of coral sand is the important factors influencing the formation and change of water body,and coral sand in the long-term of different salinity water rock under the action of seepage characteristics of a certain change,for the reef engineering construction and the formation of the fresh water body has a very important influence.In this paper,a preliminary experimental study on coral sand samples taken from a certain reef is carried out.Firstly,the particle size and particle composition of coral sand samples were analyzed,and three different salinity solutions were used as solution filtrates.In each cycle,a 500g coral sand column was dissolved,and the water samples were used for the anionic test.The dissolution test process was simulated by PHREEQC and compared with the XRD test results after the test,,the main geochemical and solution composition changes in the process of qualitative and quantitative analysis.In order to determine the maximum salinity range of dissolution,we set up a batch shock test to further clarify the effect of different salinity on the dissolution.The permeation characteristics of coral sand were tested by using the constant head osmosis device,and the effect of dissolution on the permeability of coral sand was clarified.Through the dissolution test,the concentration of ions in the solution increased,and in the test condition of water quality ratio of 2:1,the Ca²⁺and Mg²⁺reached 40mg/L in the fresh water body,and the SO₄^2-reached 100mg/L.By PHREEQC simulations under different salinity conditions of water rock interaction including dissolution,precipitation,cation exchange,increased with salt concentration solution is more and more intense,calcite dissolution,and dissolved quantity increased with the increase of salt concentration,ion exchange effect is becoming more and more intense;It was found that the salinity was the strongest between 0.2-0.25mol/L and the water quality was 2:1 by the indoor concussion test.Through the constant head penetration test,the water rock effect of different salinity conditions changed the permeability of sand and increased the permeability of coral sand.The above studies provide a reference for the establishment of the osmotic parameter variation and the water chemical reaction of the simulated freshwater body of the islands,providing important reference for the stability of the foundation of the reef works.