Properties And Affecting Factors Of Carbonated Steel Slag Brick

The cement industry is one of the major point sources of CO₂emission. Atpresent, the utilization of industrial wastes, as many as possible, to reduce theproduction and appliance of clinker is the most widely used method for the silicateindustries to save energy and reduce CO₂emission. With alkali-activation, largeamount of industrial wastes can be fully used, while the alkali added during thesynthetic process would bring about durability problems of cementitious materials.Accelerated carbonation, a new method to produce carbonated materials, is poorlydeveloped due to the limited resources of raw materials.With industrial wastes （steel slag, slag and fly ash, etc.） as raw materials,Na₂CO₃as activator, standard sand as aggregate, the carbonating abilities andaffecting factors of industrial wastes were evaluated. Then carbonated steel slag bricksbased on cooperation of alkali and CO₂were prepared, the properties and affectingfactors were studied likewise.The carbonating abilities and affecting factors of industrial wastes werediscussed. The results showed that the carbonating abilities of industrial wastes, whichwere efficiently increased when Na₂CO₃was added as activator, were dominated bythe content and formation of Ca. Therefore, the steel slag containing higher content off-CaO and calcium silicate （C2S） had the highest carbonating ability.The effects of mix proportion, carbonation and processing parameter onproperties of carbonated steel slag brick were discussed respectively, and theoptimized mix proportion, carbonation and processing parameters were obtained: thesteel slag adobes, prepared with the mix proportion of “1640kg/m³steel slag（with aspecific surface area of434m2/kg）+410kg/m³aggregates+13.12kg/m³Na₂CO₃” anda constant W/C ratio of0.07, were settled at ambient temperature （25±2℃） for2.0h,and then moved into the accelerated carbonation equipments to get acceleratedcarbonation （T=60℃, RH=50%, P=0.25MPa, ρCO₂=100%by volume） for120min. Inthe conditions mentioned above, carbonated steel slag bricks （40×40×160mm） withcompressive strength of35.0MPa, in which the CO₂sequestration of steel slagreached10.19%, were successfully prepared.The effects of CO₂on the products and microstructure of carbonated steel slagbrick were studied by XRD, IR, TG-DSC, MIP and ESEM, and the mechanisms were discussed likewise. The results showed that the C-S-H gel resulting from alkaliactivation of steel slag and Na₂CO₃provided the initial performances of adobe, whilethe “filling effect” of CaCO3produced during accelerated carbonation, dominated theoptimizations of pore structure and strength of carbonated steel slag brick. In addition,the cooperation of alkali and CO₂were observed——During alkali activation of steelslag and Na₂CO₃, C-S-H gel and CaCO3were produced respectively. The formeroffered carbonation sources and the latter played a role of “crystal nucleus”, whichresulted into the promotion of carbonation; then the adobe became further compactedwith the CaCO3and SiO2gel attributed to the reaction of C-S-H gel and CO₂, andtherefore the properties of carbonated steel slag brick were optimized.