The Influence Of Magnesium Sulfate Attack On The Microstructure Of Cement-slag Paste Under Dry-wet Cycling

Concrete materials are widely used in marine infrastructure construction.Hydration product composition and microstructure of concrete determines its durability and thus the service life of the project.Among the seawater corrosive ions,Mg²⁺and SO₄^2-have the most destructive effects on the cement paste.Especially under the action of dry-wet cycle,the composition and microstructure of the hydrated products in the paste have evolved,leading to the destruction of the cement paste microstructure and the decrease of the binding force.Based on the"973"project,“Basic Research on Long Life of Concrete Materials and Structures in Harsh Environment-Formation and Regulation of the Microstructure of Long Life Concrete in Harsh Environment”?2015CB655101?,formation and evolution mechanism of C-S-H gel microstructure and composition of hydration products of cement-slag cement paste under the joint action of MgSO₄ erosion and MgSO₄ erosion in dry-wet cycle was researched.This paper found that MgSO₄ erosion and dry-wet cycle have a significant effects on the microstructure of C-S-H gel,slag can weaken the decalcification and dealumination effects of the cement paste caused by MgSO₄corrosion and dry-wet cycle,inhibits the migration of Al[4]on silicate chains of C-S-H gel and transition of AFm to AFt in the paste,and reduces the produce of AFt,which improve the stability of the C-S-H gel microstructure.The results of the innovations are as follows:The formation and evolution of the cement-slag paste microstructure under the action of water conservation and dry-wet circle were studied.With the curing age growing?28¹80d?,the MCL of C-?A?-S-H gel in Portland cement paste increases,Al[4]/Si decreases.At the same age,with the increase of slag content and water to cement ratio,the amount of Al³⁺produced by depolymerization entering the bridge Si-O tetrahedron in C-?A?-S-H gel as Al[4]increases,MCL and Al[4]/Si of C-?A?-S-H gel increase,and the increase amount is advanced with the increase of slag content.Wet-dry circle promotes the hydration of the cement paste,which results that MCL increase and Al[4]/Si decrease of C-?A?-S-H gel in Portland cement paste,and the increase of MCL and Al[4]/Si of C-?A?-S-H gel in cement paste mixed with slag are bigger compared with those cured in water in the same age.The formation and evolution of the cement-slag paste microstructure under the action of 5.0%MgSO₄ corrosion and dry-wet circle were studied.With the curing age growing?28¹80d?,the increase of Portland cement paste C-?A?-S-H gel MCL and the decrease of Ca/Si and Al[4]/Si are significant.Compared with water conservation,MgSO₄ corrosion promotes Al[4]in the bridge site of C-?A?-S-H gel silicate chains transiting to Al[6]in AFt or AFm.With the increase of slag content,the increase of MCL and the decrease of Al[4]/Si in C-?A?-S-H gel attacked by MgSO₄ are reduced.Slag can weaken the dealumination and decalcification effects of MgSO₄ on C-?A?-S-H gel,which improves the microstructure stability of C-?A?-S-H gel.At the same age,attacked by MgSO₄ with dry-wet circle,microcracks on the paste surface increases,Mg?OH?₂ layer is destroyed,the diffusion rate of Mg²⁺and SO₄^2-accelerates,which promotes the hydration of slag.Slag is depolymerized into a large number of Al³⁺.Part of it is combined with SO₄^2-and Ca²⁺in the pore solution and generates AFm and AFt,the other part enter the bridge site of Si-O tetrahedron in C-S-H gel as Al[4],of which the amount is more than attacked by MgSO₄.As a result,MCL and Al[4]/Si of C-?A?-S-H gel is higher than attacked by MgSO₄ at the same age.With the increase of the slag content and decrease of water-cement ratio,the variation range of MCL,Al[4]/Si and Ca/Si of C-?A?-S-H gel is reduced when MgSO₄ corrosion age lasts.Mixing with slag and reducing water-cement ratio will weaken the dealumination and decalcification effects of MgSO₄ corrosion with dry-wet circle on C-?A?-S-H gel,and improve the stability of C-?A?-S-H gel microstructure under MgSO₄ corrosion with dry-wet circle.The corrosive ion transport inhibitor is combined with Ca²⁺in the pore solution and form crystals that are insoluble in water and salt solutions,which clogs the pores of the paste,increases the pores of<10 and 50nm,reduces the pores of>100nm,inhibits the transport of Mg²⁺,SO₄^2-and Cl^-in the paste,the dealumination and decalcification effects on C-?A?-S-H and the transition of hydration products containing Al.By using corrosive ion transport inhibitor,slag and fly ash,the low temperature rise,crack resistance and corrosion resistance marine mass concrete were prepared,of which the amount of cementitious material is 400Kg/m³,cement content is 150Kg/m³,and the adiabatic temperature is only 41.3?,28d compressive strength is 47⁵5MPa,electric flux is less than 600C.It improves the durability of concrete and has been applied in Humen two bridge bearing platform mass concrete project.