Accelerated Carbonation of Stainless Steel Slag Compacts at Low pCO2 Pressure – Microstructure and Strength Development As a Function of Temperature and CO2 Content

Steel slag is a mineral byproduct generated during steel production. Steel slags are generally rich in Ca and Mg which make them suitable for mineral carbonation. Through mineral carbonation the slags can be transformed into marketable construction materials, without the use of cement or other binders. While several studies focused on the paramters that influence CO₂-uptake, less is known about the parameters that affect the strength development through carbonation.Therefore, the main objective of this study was to explore the factors that affect the strength development of the slag compacts through carbonation. In this study compacts of 30 to 50 MPa strength were made by mineral carbonation of compacted steel slag in a climate chamber or autoclave under mild operating conditions (i.e. 10 to 60 °C, 1.5 bar P_total , 5 to 100 % CO₂). The role of the parameters that are known to play an important part in the kinetics of the carbonation reactions, was examined in detail. For a given compaction pressure and gas pressure the parameters evaluated were: moisture content, CO₂content, temperature and exposure time.

We discovered that not only the degree of conversion of the slag particles but also the microstructure of the carbonation products is important for the compressive strength of the compacts. The study also revealed that a high initial conversion rate can be counter productive with regard to development of the final compressive strength. At a temperature of 60 °C the initial conversion rate was very high but the compressive strength was lower than expected based on the CO₂ up-take. Carbonation at 60 °C resulted in the precipitation of very porous carbonate textures. During carbonation at lower temperatures the carbonates are more dense and typically precipitate at the slag grain contacts, where carbonate precipitation results in better strength development. After carbonation at low temperatures large intergranular pores are still present in the compacts that are not observed in the compacts carbonated at 60 °C.

In case of carbon sequestration the general aim is to find the conditions in which the CO₂-uptake rate and degree is optimized. When applying carbon sequestration to produce construction materials (with a low or even negative CO₂ footprint), the compressive strength of the material is generally a key parameter. When high strength compacts are desired, it is important to optimize the experimental conditions that favour the crystallisation of calcium carbonate at the grain contacts of the slag particles in the compacts to enhance the compressive strength, more so than finding the optimum conditions for a maximum CO₂-uptake in the compacts.