Study On The Durability Of Carbonated Steel Slag Product

Steel slag is a by-product of the steel manufacturing process.Carbonation curing can solve the problem of poor stability of steel slag and results in good mechanical behavior,but their durability characteristics are still unknown.This paper focuses on the durability of it and the following researches were carried out:（1）The behavior of carbonated steel slag products fully immersed in 3wt%Na₂SO₄solution for 120 days was studied,and it was compared with ordinary Portland cement（OPC）.The resistance ability of the specimens was evaluated by visual observation,corrosion resistance coefficient K and microstructural analysis.Carbonated steel slag products showed better sulfate resistance ability compared with OPC.The amount of expansive erosion products ettringite and gypsum formed in carbonated steel slag products was 8.4%and 6.1%less than that of the OPC specimens until 120 days.The small amounts of erosion products in CO₂ cured blended steel slag specimens resulted in a decrease of porosity,a denser microstructure and increment of strength.However,the accumulation of large amounts of erosion products in internal pores of OPC specimens led to an increase of porosity,a looser microstructure,and degradation.（2）The performances of carbonated steel slag products in hydrochloric acid（HCl）solutions with pH=1-5 for 56 days were studied and compared to OPC specimens.The compressive strength of specimens and pH of solutions were monitored at different ages.The microstructural modifications at pH=1 in 56 d were characterized.And the ease of minerals to react with HCl was analyzed by thermodynamics.The results revealed that carbonated steel slag products suffered more serious degradation than OPC.The alteration mechanisms of both included almost complete decalcification,dissolution of some crystallized or amorphous phases and the formation of silica gel.The thermodynamic analysis showed that ettringite had the highest potential to react with HCl while the CaCO₃ had the lowest ability among the mineral phases studied.（3）Compressive strength,open porosity and microstructure of carbonated steel slag specimen,secondary carbonation specimen,post-carbonation water-curing specimen and post-water-curing secondary carbonation specimen were tested and compared.The results showed that during the carbonation process,the negative effect of the formation of silicon gel from C-S-H and the positive effect of the generation of CaCO₃ from Ca（OH）₂ or other minerals could overlap.Direct secondary carbonation had a greater positive effect,while after water curing,the negative effect was greater due to the lack of Ca（OH）₂.Therefore,it could be considered that under normal conditions carbonated steel slag products had good resistance to carbonation,but not after water curing.（4）The destruction process of carbonated steel slag mortar under rapid freeze-thaw cycles was studied,and the OPC mortar was compared with it.The appearance of mortar was recorded every 25 cycles,at the same time the flexural strength,compressive strength and weight were tested.The results showed that carbonated steel slag mortar deteriorated worsely than OPC mortar and the fracture of it occurred first.The strength losses were positively correlated with the mass loss.The destruction of mortar under rapid freeze-thaw was caused by the repeated actions of stress caused by the difference of volume between mortar and ice in box and the temperature difference between the inside and outside of mortar together.