Effect Of Sulfate Attack On Properties Of Different Cement-based Materials

Sulfate attack is a common phenomenon of durability degradation of cement-based materials.The mineral admixture with the pozzolanic effect can effectively improve the microstructure of the cement-based material,thereby improving the sulfate-resistant performance of the cement-based material.In this paper,the effects of fly ash,slag powder and phosphorus slag powder on the macro-mechanical properties,impermeability and frost resistance of cement-based materials under sulfate attack(soaking in 5%sodium sulfate solution)were studied.At the same time,microscopic testing methods were used to study the hydration process,hydration erosion product type,morphology and pore structure of cement-based materials mixed with different minerals.To some extent,it reveals the mechanism of sulfate resistance of three mineral admixtures.The main research contents of this paper are as follows:The effects of different mineral admixtures on the compressive strength,flexural strength,impermeability and frost resistance of cement mortar under sulfate attack conditions were studied.The results show:Due to the short age of sulfate attack,the influence of different mineral admixtures on the mechanical properties of cement mortar is not obvious;Fly ash and phosphorus slag powder can effectively improve the impermeability of cement mortar(electric flux method),and the slag powder does not improve the impermeability of cement mortar;Different mineral admixtures have different degrees of improvement in frost resistance of cement mortar,specifically phosphorus slag powder > slag powder > fly ash.The hydration heat test,X-ray diffraction test(XRD),thermogravimetric analysis(TG-DSC),scanning electron microscopy(SEM),and mercury intrusion structure analysis(MIP)were used to study the mixing of different minerals under sulfate attack conditions.The influence of the hydration history,hydration product type and microstructure of cement-based materials.The results show:Sulfate can increase the early hydration heat release rate and heat release of different cement-based materials;The main product of sulfate attack is gypsum,and the hydrated product has a loose microstructure.The content of calcium hydroxide(CH)in different cement-based materials is significantly reduced;As the age of erosion increases,the porosity of cement pastes of different cement-based materials tends to increase gradually.At 270 days of erosive age,the cement slurry porosity of fly ash,slag powder and phosphorus slag powder decreased by 4.24%,2.16% and 1.40%,respectively,compared with the blank sample.In the early stage of sulfate attack,the mechanical filling,impermeability and frost resistance of different cement-based materials can be improved due to the dense filling effect of the swelling product ettringite and the alkaline excitation of sulfate;In the later stage of sulfate attack,different cement-based materials are deteriorated to varying degrees due to the large amount of expansion of the expansive product gypsum.However,in general,the three mineral admixtures in a certain range can effectively improve the sulfate attack resistance of cement mortar.This paper summarizes the influence of different mineral admixtures on the sulfate resistance of cement-based materials,and provides a scientific basis for the rational use of mineral admixtures in the prevention and treatment of sulfate attack.