Accelerated Carbonation of Steel Slag Compacts: Development of High-Strength Construction Materials

Abstract

Mineral carbonation involves the capture and storage of carbon dioxide in carbonate minerals. Mineral carbonation presents opportunities for the recycling of steel slags and other alkaline residues that are currently landfilled. The Carbstone process was initially developed to transform non-hydraulic steel slags [stainless steel (SS) slag and basic oxygen furnace (BOF) slags] in high-quality construction materials. The process makes use of accelerated mineral carbonation by treating different types of steel slags with CO2 at elevated pressure (up to 2 MPa) and temperatures (20-140°C). For SS slags, raising the temperature from 20 to 140°C had a positive effect on the CO2 uptake, strength development, and the environmental properties (i.e., leaching of Cr and Mo) of the carbonated slag compacts. For BOF slags, raising the temperature was not beneficial for the carbonation process. Elevated CO2 pressure and CO2 concentration of the feed gas had a positive effect on the CO2 uptake and strength development for both types of steel slags. In addition, the compaction force had a positive effect on the strength development. The carbonates that are produced in situ during the carbonation reaction act as a binder, cementing the slag particles together. The carbonated compacts (Carbstones) have technical properties that are equivalent to conventional concrete products. An additional advantage is that the carbonated materials sequester 100-150 g CO2/kg slag. The technology was developed on lab scale by the optimization of process parameters with regard to compressive strength development, CO2 uptake, and environmental properties of the carbonated construction materials. The Carbstone technology was validated using (semi-)industrial equipment and process conditions.

Carbonate-bonded construction materials from alkaline residues

“…By optimizing the particle packing and compacting pressure EAF steel slag compacts (300x100x50 mm) were produced with compressive strengths up to 134 MPa after carbonation and that complied to environmental leaching limit regulations [18]. The CO 2 uptake was 10-15 %.…”

“…Chemicals such as NaOH can be added to accelerate and enhance the process even further [7]. Most recent research developments in producing CO 2 uptake cements depart from calcium (and magnesium)-rich materials and relate to 1) accelerated curing of concrete [8][9][10], 2) carbonation of brines [11][12][13], 3) carbonation of hydrated lime [14,15] and 4) carbonation of calcium silicates [16][17][18][19].…”

Carbonation of BOF Slag in a Rotary Kiln Reactor in View of the Scale‐Up of the Wet Route Process

“…The lowest CO 2 uptake was obtained in test Z3, characterized by the lowest L/S ratio, suggesting that the high temperature achieved during this test (59.8 C) probably lead to an excessive drying of the solid material, hindering the wetroute carbonation reaction. The possible explanation is that less water was available for CO 2 dissolution when operating at higher temperature, but also that less CO 2 was dissolved in the liquid thin film due to the lower solubility of CO 2 with increasing temperature [26]. The average temperatures recorded during the tests, 41, 47, and 59.8 C, respectively, for Z1, Z2, and Z3 (see Figure 2), may explain the decreasing trend of the CO 2 uptakes measured for the carbonated slag, reported in Figure 4.…”

Assessing, Understanding and Unlocking Supplementary Cementitious Materials

“…Alkali-activation with alkalihydroxides or alkalisilicates has attracted most attention in this area [4], but may actually represent just one of multiple options. The use of tailored residues as SCM in calcium sulfoaluminate cements [50], as raw material for mechanically activated cements [51] or as precursor for carbonate-bonded materials [52] may well represent viable options depending on local availability of resources and infrastructure.…”