Effect Of Gypsum Formation On Performance Of Cement-based Materials During Sulfate Attack

Sulfate attack is one of the pivotal factors affecting the durability of concrete,which will not only shorten the service life of the materials,but may even threaten the safety of the structure.Due to the wide distribution of sulfate environment,many countries in the world are facing serious economic and property losses.So far,many scholars have done a lot of research on the project.The ettringite form of sulfate attack is well studied and established in the existing researches.Ettringite is the main corrosion product of concrete formed in the environment of high alkaline sulfate solution,and it will cause expansion and cracking of materials when it grows in narrow or closed pores.The corresponding expansion mechanism includes water swelling theory,crystallization theory and solid phase reaction theory.In addition,in the prevention of ettringite-type sulfate attack,it was found that by controlling the tricalcium aluminate content in the cement can reduce the expansion hazard caused by the formation of ettringite effectively.Thaumasite is another sulfate attack product similar to ettringite in crystal structure.The experiment shows that its formation will cause the decomposition of C-S-H gel in the cement paste,which will lead to surface muddy and strength loss.But its occurrence needs to meet the conditions simultaneously such as sulfate,carbonate,low-temperature and humidity,so the prevention measures are relatively simple.Gypsum is also a corrosion product of cement-based materials during sulfate attack,but gypsum is generally considered to be formed under high-concentration sulfate conditions,so the gypsum form of sulfate attack has acquired little attention.However,in recent years,more and more field research has found that even if the concentration of sulfate is very low in the concrete service environment,gypsum is still found on the structural surface,this indicates that the formation of gypsum is related to other factors,and the pH is the biggest difference between service environment and laboratory conditions.In addition,in real engineering environment,besides to SO₄^2-,it may also contain other erosive ions,such as Mg²⁺,Cl^-and NH₄⁺,which also affect corrosion products and deterioration mechanisms.The ammonium salts is considered to be the most threaten to cement-based materials.It is not only widely existed in the industrial and agricultural fields,but also found that NH₄⁺has an important influence on the formation of erosion products,especially gypsum phase.Finally,the exists research not only has different views on the damage types of material that caused by gypsum formation,but also fails to provide reasonable and effective recommendations for the prevention of gypsum-type sulfate attack.In this paper,the thermodynamic conditions of gypsum formation and stability during sulfate attack were studied.In addition,the relationship between leached OH^-or Ca²⁺and exposure time in cement mortar immersed in Na₂SO₄ solution with different pH value and concentration was investigated.The effect of calcium leaching on the pore structure of matrix was analyzed by low-field NMR technique.The composition and contents of erosion products were determined by XRD and DSC,and combined with the results of sample appearance,SEM and EDS patterns,we have revealed the internal relationship between corrosion products and degradation characteristic of cement mortar.Finally,the deterioration process and degradation mechanism of mortar immersed in?NH₄?₂SO₄solution were also studied,and the compressive strength,water absorption ratio and resistance to chloride ion penetration of cement mortar blended with silica fume were evaluated.The following conclusions can be deduced for the present paper:?1?The solution pH will significantly affect the formation and stability of gypsum.The low pH value attributes to the formation of gypsum,but NH₄⁺can raise the pH conditions of gypsum formation and stability.In addition,the lower the pH value,the lower the SO₄^2-concentration required for gypsum formation,which provides a new idea for the prevention of gypsum-type erosion,i.e.in the low pH field environment,the surface of the structure needs to be strictly protected so that ensure that the internal alkalinity of the material is maintained at a high level.?2?For Na₂SO₄ attack,the leached OH^-and Ca²⁺in cement mortar is linear with the square root of exposure time.Decreasing the pH or increasing the concentration of the solution can increase the amount and rate of leached OH^-and Ca²⁺.The continuously calcium leaching will not only increase the porosity and weaken the microstructure of cement mortar,but also accelerate the diffusion and invasion of SO₄^2-in external solution so that more and more harmful products such as gypsum are formed.?3?Compared with concentration,the solution pH has a more significant effect on the corrosion products of cement mortar under Na₂SO₄ attack.The corrosion products were mainly AFt When the pH value of the solution is high,the solid products are AFt under high pH conditions,there are also a large amount of Ca?OH?₂,and the specimens were slightly cracked by corners.Decrease the pH value of the solution,the amount of gypsum is significantly increased,and the formation of gypsum is accompanied by the consumption of Ca?OH?₂ and the decalcification of the C-S-H gel,which will reduces the gelation of the matrix,so that the appearance is characterized by surface paste spalling and structural collapse.?4?Under the attack of?NH₄?₂SO₄,the deterioration mechanism of cement mortar is the gypsum form of sulfate attack,and a large amount of gypsum crystals are formed in the corrosion products.At the same time,the sand become uncovered and the paste tend to decomposition.In addition,the deterioration rate and degree of cement mortar immersed in?NH₄?₂SO₄ is significantly higher than that in Na₂SO₄ solution.Finally,the incorporation of silica fume can effectively improve the performance of cement mortar resistance to?NH₄?₂SO₄ attack,and the compressive strength,water absorption and chloride ion permeability of 10%silica fume cement mortar are better than those contains 5%silica fume cement mortar.