Study On The Carbonation Mechanism Of Steel Slag Products Modified By Zeolite

Steel slag is an industrial by-product of the steel manufacturing process.Carbonation technology could modify issues of poor stability,dissolution of heavy metal ions and high alkalinity in steel slag matrix,effectively.However,the carbonation degree of steel slag product is still in low-grade.To reduce the consumption of cement and inhibit the alkali aggregate reaction,zeolite,as a supplementary cementitious material,has been widely used in cement and concrete industry.But the concrete products with zeolite showed poor capacity of carbonation resistance.Steel slag,similar to cement,have same mineral components partially,such as larnite(C₂S),brownmillerite(C₄AF),etc.It is worth exploring the effect of pozzolanic reaction on modifying the carbonation degree in steel slag matrix containing zeolite.According to this idea,the following study were carried out in this paper:(1)In this paper,a novel method,zeolite partially replacing steel slag(SZ),was proposed to further improve the compressive strength and carbonation degree of steel slag compacts by accelerated carbonation curing.In terms of the compressive strength and carbonation degree,the optimum content of steel slag substituted by zeolite was 5 wt.%and15 wt.%,respectively.After 1 d of hydration and 2 h of carbonation reaction,the compressive strength of the CSZ5 compacts was 14%higher than that of the CSZ0 compacts.The carbonation degree of the CSZ15 compacts was 10.2%higher than that of the CSZ0 compacts.Extra C-S-H gel was produced by pozzolanic reaction and enhanced the solubility of calcium,improved the carbonation degree in SZ matrix.The increment of Ca CO₃ could reduce the porosity of CSZ specimens further to modify the development of compressive strength in CSZ specimens.However,with the increment in zeolite content,the Ca/Si ratio of C-S-H gel produced by pozzolanic reaction would reduce,then aragonite and vaterite amount would increase to affect the strength development in CSZ specimens,so there is no linear relationship between compressive strength and carbonation degree.(2)It is concluded that C-S-H gel,produced by pozzolanic reaction,is beneficial to enhance the compressive strength and carbonation degree of CSZ specimens.In order to improve the C-S-H content before carbonation curing in SZ matrix,different prehydration times(1,3,7,14,196 d)were designed to investigate the effect of different prehydration times on the strength and carbonation degree in CSZ specimens.The results show that the optimum curing process was 1 day hydration and then 2 h carbonation,considering the carbonation degree and strength of SZ specimens both.the contribution ratio of carbonation curing for strength decreased gradually.After 1,3,7,196d of prehydration,the optimal zeolite contents for compressive strength and carbonation degree in CSZ specimens were 5wt.%and 15wt.%,respectively.However,after 14d of prehydration,the compressive strength and carbonation degree of CSZ15 specimens were the optimal,because the crystallized zeolite further participated in the pozzolanic reaction to generate AFmc mineral,which has good carbonation reactivity,improved the carbonation degree and pore structure in CSZ specimens,and enhanced the strength of CSZ15-14d further.The pozzolanic reaction could be divided into two-step.First,the active Si O₂ reacted with Ca(OH)₂ in the early hydration.Second,the skeleton structure of zeolite deteriorated in an alkaline environment and then participated in pozzolanic reaction.(3)It was concluded that the optimal prehydration time before carbonation was 1d,however zeolite in crystalline did not participate in the pozzolanic reaction at early hydration process,which affected the subsequent strength development in CSZ specimens.High-temperature calcination could destroy the tetrahedral structure in clinoptilolite and enhance the content of active Si O₂ in zeolite,to improve the pozzolanic reactivity and reduce the porosity of zeolite.Zeolites(5wt.%),after high-temperature calcination(0,200,400,600,800?),were added into the steel slag matrix for 1d hydration and then 2h carbonation curing.The compressive strength,porosity and microstructure in CSZ5 specimens were compared and the results presented as follow.The tetrahedral structure of zeolite in crystalline was relatively stable when the calcination temperature lower than 600?,and then the strcture of zeolite was destroyed partially at 600?and damaged totally at 800?.Compressive strength of CSZ5-800 was optimal,and the porosity in CSZ5-800 was 21.9%lower than that of CSZ5-25.The decrement of porosity in CSZ5-800 made the strength develop significantly.FTIR and NMR results confirmed that the content of C-S-H gel in CHZ speimens increased when the pretreated temperature in zeolite increased.After carbonation curing,Ca²⁺ion from C-S-H gel would transfer to Ca CO₃,the content of Si gel gradually enhanced and the chain length increased.Meanwhile,with the increment of modification temperature in zeolite,the content of AFmc minerals generated by pozzolanic increased,which was beneficial to improve the strength and carbonation degree in CSZ specimens.