Effect of Different CO2 Treatments on the Metal Leaching in Steel Slag Binders

Carbonation is an effective method to promote the quality of the steel slag binder. In this article, two carbonation approaches, namely hot-stage carbonation and accelerated carbonation, were employed to leach the metals, and the influence mechanism on the metal sequential leachability of the binders composed of 80 wt% of EAF slag incorporating 20 wt% of Portland cement (PC) was revealed. The carbonate products, microstructures, and chemical states were investigated, and the results indicated that chromium, vanadium, and titanium gradually transformed into inactive phases after two carbonation approaches, while zinc appeared the opposite trend. The sequential leachability of chromium declined with the increase of the carbonation efficiency, in which the exchangeable chromium decreased from 1.99 mg/kg in the A2A binder to below the detection limit in the A2C binder and C2C binder. Hot-stage carbonation treatment facilitated particle agglomeration, minerals remodeling, and calcite formation. The carbonation curing of the steel slag paste resulted in the formation of amorphous CaCO3, calcite crystalline and Si-bearing hydrates that covered the pores of the matrix, and silicate structure with a higher disorder. The hot-stage carbonation and accelerated carbonation curing methods were adopted to jointly prevent the leaching of harmful metals and facilitate promising high-volume steel slag-based binders with structural densification and CO2 storage.

Experimental investigation on the mechanical and chemical properties of lightweight aggregate concrete with CO2 curing

In the cement and concrete industry, enormous amounts of Carbon Dioxide (CO2) are emitted during their production processes. Carbon dioxide emission significantly contributes to the global climate change, which has been one of the biggest challenges of our times. Some novel solutions have been proposed for CO2 capture and storage, as well as reducing CO2 emission in concrete production. Carbonation curing is an effective alternative for conventional water curing for concrete. It can store CO2 in the hardened concrete and meanwhile improve early mechanical properties of concrete. Partial replacement of cement with fly ash shows environmental benefits, such as reducing greenhouse gas emissions and industrial waste destined for landfills. There has been some previous research studying on the effect of carbonation curing on normal Portland concrete in the past decade. Nevertheless, few studies have focused on the CO2 curing for lightweight aggregate concrete (LWAC). In this paper, the influence of early carbonation curing on LWAC is studied. LWAC specimens with two different water-to-cement ratios are cast and cured for a series of experimental investigations. The mechanical and chemical properties including the 1-day compressive strength, 28-day compressive strength, flexural strength, CO2 uptake, heat development, and pH level are investigated. Specimens with ordinary Portland cement are also tested as references in terms of compressive strength and CO2 uptake.

Carbon-Negative Cement Manufacturing from Seawater-Derived Magnesium Feedstocks

This study describes and demonstrates a carbon-negative process for manufacturing cement from widely abundant seawater-derived magnesium (Mg) feedstocks. In contrast to conventional Portland cement, which starts with carbon-containing limestone as the source material, the proposed process uses membrane-free electrolyzers to facilitate the conversion of carbon-free magnesium ions (Mg2+) in seawater into magnesium hydroxide (Mg(OH)2) precursors for the production of Mg-based cement. After a low-temperature carbonation curing step converts Mg(OH)2 into magnesium carbonates through reaction with carbon dioxide (CO2), the resulting Mg-based binders can exhibit compressive strength comparable to that achieved by Portland cement after curing for only two days. Although the proposed “cement-from-seawater” process requires similar energy use per ton of cement as existing processes, its potential to achieve a carbon-negative footprint makes it highly attractive to decarbonize one of the most carbon intensive industries.