Carbonation-Activated Steel Slag Binder for Cement Replacement and Carbon Utilization

Steel slag is the by-product of steel making process. Because of its chemical composition and high temperature process, steel slag is carbon dioxide reactive. This property can be utilized to develop steel slag binder to replace Portland cement in making precast building products. This paper summarizes a recent study on the carbonation activation of steel slag for cementing binder application. Different types of steel slags are examined for their capacity to develop strength. They include converter slag (EAF slag, BOF slag, KOBM slag) and ladle slag. The slags are ground to a size close to cement powder and compaction formed into a precast product. The slag compacts are then carbonated in a pressure chamber at ambient temperature with a gas pressure of 1-2 bar for different duration. The strength gain and carbon uptake are evaluated. It was found that BOF and KOBM slags are more carbon reactive than EAF slag. The former can develop a strength of 40-50 MPa in two hours reaction while the latter can reach only 20-30 MPa. If iron content can be reduced below 5%, the reactivity can be further enhanced. A compressive strength of 80 MPa and a carbon uptake of 13% have been achieved through carbonation activation of a de-ironized KOBM slag in just two hours. The dominant parameter for carbon-activated strength gain is the calcium silicate phases with any polymorphs. The strength gain is attributed to the formation of calcium-silicate-hydrates coupled with calcium carbonates through the carbon activation of calcium silicates. The carbon reactivity of ladle slag is also investigated. For as-received ladle slag with low free lime content, the carbon activation can produce a strength of about 35 MPa after 24 hours reaction.  High free lime ladle slag cannot be used for activation. It experiences significant cracking by the extreme heat due to the carbonation of CaO. Heat treatment is necessary to convert free lime into calcium silicates. The treated ladle slag can show a strength of 26 MPa after carbonation of 24 hours. It is conclusive that not all as-received steel lag can be activated to gain strength. The free lime content has to be low and calcium silica content has to be high.

Two building products are successfully manufactured using activated slag as primary binder: slag-bond wallboards and slag-bond construction blocks. For wallboard application, slag is mixed with sawdust and cast into panels of 12mm thick. Carbonation activation can produce a strength comparable to commercial cement-bond fibreboard. In construction block application, slag is mixed with lightweight aggregates to make masonry units. While the strength of slag block can be made equivalent to cement block, the freeze-thaw durability of slag block is much improved due to the precipitation of carbonates on surface.

It is promising to use activated steel slag to replace Portland cement in making precast building products. Since carbonation activation is a CO2 uptake process, gaseous carbon dioxide can be converted into solid carbonates and stored in building products. The final products can be made carbon-negative since it preserves natural resources, recycles industry wastes and serves as carbon sinks for emission reduction.