Study On Physical Salt Attack Of Concrete In Marine Environment

In the Marine environment,the most fundamental reason for accelerating the deterioration of building structures is the existence of seawater.This paper mainly studies the mechanism of erosion and degradation of concrete materials by physical crystallization of seawater,so as to predict the causes of erosion and degradation of concrete in the Marine environment.Firstly,the main ions in seawater were determined,and then the seawater was divided into single-salt solution and compared with seawater of the same concentration,so as to study the influence of each component in seawater on the physical crystallization erosion of concrete.Evaporation crystallization of single salt solution and seawater in air was carried out.The crystallization speed,macroscopic and microscopic morphology and composition of each solution were analyzed by Electron Back-Scatter?BSE-EDS?and X-Ray Diffraction?XRD?,and the crystallization situation in concrete was speculated.Then test mortar under the condition of partially-immersion and drying-wetting cycle,the macroscopical erosion caused by the physical crystallization and the degradation,combined with the Scanning Electron Microscope?SEM?,BSE-EDS,XRD,X-Ray Fluorescence?XRF?and capillary test.The crystallization distribution of each solution in mortar was tested,and the physical crystallization erosion mechanism of each single salt solution and seawater on concrete was speculated by combining the degradation situation and dynamic elastic modulus test.To focus on the effects of physical crystallization,low aluminate cement?Super sulfate resistant cement?is used to remove as much of the chemical action of the concrete in the Marine environment as possible.The composition of seawater was divided into sodium chloride?NaCl?,magnesium chloride?MgCl₂?,calcium chloride?CaCl₂?and sodium sulfate?Na₂SO₄?and magnesium sulfate?MgSO₄?solutions based on the principle of the same total number of ions.All solutions except CaCl₂ and MgCl₂ can be crystallized into solids by evaporation in the air.Although seawater can form solid crystals,the crystal is wetted due to the influence of CaCl₂ and MgCl₂.The order of crystallization speed was:Na₂SO₄>MgSO₄>NaCl>seawater,and the order of crystal climbing height was:NaCl>Na₂SO₄>MgSO₄>seawater.Under the condition of partially-immersion,the surface crystallization amount and the crystallization time sequence of the samples in each solution are the same as those in the air for the mortar samples.The grain size of internal crystallization is smaller than that of surface crystallization.Only the specimens in MgCl₂ and CaCl₂ solutions ruptured,and a large number of expansible crystal products were observed internally,while no cracking was observed in the specimens in other solutions.Thus,it can be seen that the formation of expansive crystal products is a necessary condition for cracking,while the damage degree of physical crystallization itself to the specimen is not obvious.The position and degree of physical crystallization are closely related to capillarity under the condition of half-immersion.The order was deionized water>NaCl>Na₂SO₄>CaCl₂>MgCl₂>MgSO₄>seawater.This indicated that the capillarity was inhibited as a whole under the coexistence of multiple ions in seawater.With the increase of capillary pore diameter,the difference of capillarity of each solution becomes smaller and the rising height becomes less obvious.Under the condition of drying-wetting cycle,each solution has little effect on the properties of mortar.In the two magnesium salt solutions,the surface of the mortar was obviously denuded,which was caused by the dissolution of Mg?OH?₂ generated by the chemical reaction of magnesium salt in the mortar.NaCl,Na₂SO₄,MgSO₄ and seawater solutions will generate a small amount of crystallization on the surface of the specimens,which is related to the concentration distribution of ions in the mortar.In general,the deterioration of the specimens under the condition of drying-wetting cycle was slightly lower than that under the condition of half-immersion.The physical crystallization itself doesn't lead to the cracking of concrete.The observed cracking in the partially-immersion environment is actually caused by the carbonation expansion of some salts in seawater?CaCl₂ and MgCl₂?,while the accelerated deterioration in the drying-wetting cycle is caused by the dissolution of magnesium salts in seawater.Therefore,in order to avoid cracking caused by physical crystallization,it is important to strengthen the concrete surface so as to weaken carbonization and flow erosion.