Study On Durability Of Concrete Prepared With Coral Reef Sand And Sea-water

Concrete is an important engineering material for the construction of far-reaching island in China.Due to the island that is far from the mainland,however,raw material such as fresh water,sand and stone for concrete production are lackage.And construction progress can not be guaranteed because of high delivery fee that carry raw material from the inland to the island.Coral reef sand can be got from coral reef through the artificial processing like crushing and sieving.Use coral reef sand to produce concrete is not only one of the main technical approaches for the construction of marine engineering in China,but also an important innovation in the construction of distant island.In this paper,we use coral reef sand as the fine aggregate for the preparation of sea-water mixed coral reef sand concrete through sea-water mixing.And the differences in mechanical properties and microstructure of sea-water mixed coral reef sand concrete and ordinary concrete were studied.Based on this,the influence of water-cement ratio and mineral admixture about the volume stability of sea-water mixed coral reef sand concrete was researchered.Besides,the erosion damage rule in sea-water mixed coral reef sand concrete under the environment of sodium sulfate drying-wetting cycle and magnesium sulfate drying-wetting cycle were investigated emphatically.The change of macroscopic properties of concrete was based on the change of mass,compressive strength and dynamic modulus.In order to reveal the damage mechanism of concrete,XRD,TG-DSC and SEM were used to explore the species and morphology of the corrosive coral reef sand concrete in the coupling environment of sulfate and drying-wetting cycle.The main conclusions of the experimental study are as follows:1)Compared with the sea-water mixing river sand concrete,compressive strength of sea-water coral reef sand concrete increased rapidly at the early age,and slowly in the latter.Two types of concrete hydrated products were the same,mainly Friedel's salt,Ca?OH?₂ and ettringite.However,more Friedel's salt and less Ca?OH?₂ in river sand concrete compared with seawater coral reef sand concrete.2)The test results show that the degree of shrinkage of concrete increased with age. The greater water-cement ratio,the greater degree of drying shrinkage.Mineral admixtures were added to reduce the degree of dry shrinkage of sea-water mixed coral reef sand concrete.3)In different concentrations of sodium sulfate solution for drying-wetting cycle, when freshwater coral reef sand concrete is damaged,the mass loss and the degree of elastic modulus loss are both lower than ordinary concrete,but the compressive strength and the degree of loss of bending strength is higher4)From the analysis of XRD and TG-DSC,it can be seen that SO₄^2-eroded product is mainly ettringite.The damage degree of fresh water mixed coral reef sand concrete and ordinary concrete increases with the SO₄^2-concentration.The SEM morphology of the corrosion products was observed.It found that a large amount of ettringite formed in the pores of the concrete,the anhydrous Na₂SO₄ crystallized and expansion pressure,which cause the damage of the concrete structure.5)In the environment of sodium sulfate drying-wetting cycle,magnesium sulfate drying-wetting cycle,the lower the water-cement ratio is,the better the resistance to sulfate attack is.While mineral admixtures added,the concrete sulphate erosion process can be divided into two stages,the erosion performance optimization period and the deterioration period.It is found that complex fly ash and slag can improve the resistance of sulfate to concrete,and the effect is better than that of single fly ash and slag6)Microstructural analysis of the erosion products was carried out by XRD, TG-DSC and SEM.The main erosion products were gypsum and ettringite. Under the condition of sodium sulfate drying-wetting cycle,the corrosion products of concrete are dominated by ettringite.In the magnesium sulfate drying-wetting cycle,the concrete erosion products are mainly gypsum.When fly ash and slag mixed,the quantity of gypsum and ettringite is less.