The Microstructure Formation And Evolution Of C-S-H Gel In Cement Paste Under The Attack Of Sulfate In Weak Alkaline

Sulfate corrosion is one of the important environmental factors leading to degradation and service life shorten of cement-based materials.The diffusion and reaction of SO₄^2-in cement-based materials causes expansion,cracking,strength loss and microstructure damage and so on.Calcium silicate hydrate?C-S-H?gel is the main body of hydration products of cement-based materials also the basic units of the concrete.Its composition and structural have an important influence on the mechanical properties and durability of concrete in actual engineering.However,the C-S-H gel structure is complicated and changeable,and is susceptible to corrosion ions,temperature,mineral admixture and other factors.Therefore,in-depth analysis of the microstructure evolution of C-S-H gel owe to sulfate attack is great significance for the durability design and service life of concrete structures.Based on the National Natural Science Fund of China,“Concrete gypsum-type sulfate attack mechanism in weak alkaline”?No:51558004?.The effects of pH value of Na₂SO₄solution and concentration of Na₂SO₄ solution in weak alkaline environment on the structure of C-S-H gel in Portland cement paste under weak alkaline environment were studied by XRD,SEM-EDS,²⁷Al NMR,²⁹Si NMR and ¹H NMR,the composition and morphology of the products and the change in pore structure were analyzed.In addition,the C-S-H gels and C-A-S-H gels with different Ca/Si and Al/Si were synthesized by chemical synthesis.The mechanism of structure change of the pure C-S-H gel and C-A-S-H gel during sulfate attack was studied.Through the above research,the mechanism of sulfate attack on the microstructure of C-S-H gel in cement paste is revealed.The following conclusions are drawn:?1?In the weak acid and weak alkaline Na₂SO₄ solution,corrosion products of the paste are mainly gypsum phase.Compared with the strongly alkaline Na₂SO₄ solution,the weak acid and weakly alkaline environments inhibit the formation of ettringite and crystallize it into a larger"columnar"structure.At the same time,weak acid and weak alkali corrosion conditions effectively inhibited carbonization of the paste.In the process of corrosion in Na₂SO₄ solution with weak acid and weak alkaline,the Al[4]of C-S-H gel structure transforms into calcium aluminate phase Al[6]and then re-enters C-A-S-H gel,so that the Al[4]/Si in the gel structure decreases first and increases then.In addition,with the decrease of the pH value of the solution,the hydration of the paste is accelerated,the average molecular chain length?MCL?of the gel increases,the porosity of the paste increases,and the proportion of harmful pores increases.?2?In Na₂SO₄ solution with different concentration which constant the pH value is 8,the corrosion products of the paste are mainly gypsum.Increases the concentration of Na₂SO₄ solution,promotes the hydration of paste,promotes the growth of gypsum crystal,and transforms its crystal morphology from"vein"and"block"to"short sticks"gradually.After corrosion by Na₂SO₄ solution with different concentrations,the pore size of hardened cement paste is mainly between 0.01?m and 0.1?m,and a small part is larger than 0.1?m.In the process of sulfate attack,the sulfate excitation effect is obvious in the early stage of attack,and the sulfate attack gradually gradually occupied dominant position with the increase of age.In the early stage of corrosion,the porosity of hardened cement paste decreases with the increase of solution concentration,and lower than the hardened cement paste which without corrosion.When the corrosion reaches 180d,the porosity of hardened cement paste increases with the increase of concentration of solution.In addition,with the increase of Na₂SO₄ solution concentration,the proportion of harmful pores in cement paste increases.?3?During the corrosion process of different concentrations of Na₂SO₄ solution,the Al[4]/Si of C-S-H gel decreased first and increased then.In the early stage,the higher concentration of Na₂SO₄ solution promoted the release of Al[4]from the gel structure.With increase of corrosion age,Al[4]re-enters into the gel structure again,and the higher the concentration of Na₂SO₄ solution,the more Al[4]enters the gel structure.When the age of corrosion increased from 28d to 180d,With the increase of Na₂SO₄ concentration?0,3.5%,10%?,the relative intensity of of C-S-H gel increased 34.04%,38.95%,148.16%respectively.At the same time,the increase of Na₂SO₄ solution promoted the increase of MCLof C-S-H gel in the paste.?4?For the C-S-H gel and C-A-S-H gel by chemical synthesis,the structure,morphology,polymerization degree and MCL were changed under the attack of Na₂SO₄solution.Without corrosion,as increas Ca/Si of the C-S-H gel,the gel layer spacing increased,the structure became loose,and its morphology changed from"layered"to "granular"agglomeration.The degree of gel polymerization decreased and the MCL shortened.With the increase of C-A-S-H gel Al/Si,the degree of gel polymerization first increased and then decreased,and MCL increased.When Al/Si is 0.1,the degree of gel polymerization is the highest.?5?Sulfate attack made C-A-S-H gel and C-S-H gel dealuminate and decalcified,and corrosion products gypsum and Ettringite are formed.The spacing of the gel decreases,the degree of polymerization increases,and the MCL increase after corrosion in Na₂SO₄ solution.The capability of resistance to sulfate attack decreased with increase the Ca/Si of C-S-H gel.Compared C-A-S-H gel with C-S-H gel,the C-A-S-H gel had stronger capability of resistance to sulfate attack than the C-S-H gel of the same Ca/Si.The capability of resistance to sulfate attack increases first and decreases then with increase of Al/Si of the C-A-S-H gel.The sulfate resistance of decreased with the increase of Ca/Si.had stronger sulfate attack resistance than the C-S-H gel of the same Ca/Si,and its sulfate attack resistance increased with the increase of Al/Si.In this experiment,the capability of resistance to sulfate attack is the strongest when the Al/Si of C-A-S-H gel was 0.1.