Synthesis and Characterisation of Reactive Silica Residues from Mineral Carbonation Process

The primary challenges for commercial exploitation of mineral carbonation processes are related to the high capital costs and energy demand, additional CO₂ emissions and the generation and accumulation of by-products. The development of an effective utilization route for the by-products of this process could offset these obstacles, rendering the technology more economically attractive. One of the advantages of mineral carbonation (utilising heat activated serpentine as magnesium source) is that one of the products of the process is amorphous silica, which may serve as a pozzolanic cement replacement material. Amorphous silica, when added to cement, reacts with Ca(OH)₂ and produces C-S-H which increases the compressive strength of concrete. The aim of this work is to synthesise amorphous silica-rich materials from the mineral carbonation process, to characterise and evaluate the pozzolanic activity of these materials by determining the extent of silicon extraction in alkaline solution with pH = 13, conditions similar to those encountered in cement paste.

As starting material, raw lizardite (<75 µm) was collected from the Great Serpentine Belt in NSW, Australia and prior to reaction was heat activated at 630 °C for 4 hrs in a stainless steel rotary kiln. In the dissolution stage (magnesium leaching) of a two stage mineral carbonation process, heat activated lizardite was dissolved in water saturated with CO₂ at 6.5 bar CO₂, 45 °C, agitated at 800 rpm mixing for 2 hrs to extract Mg and producing a silica-enriched residue. For further magnesium extraction studies, the solid residue produced at this stage was separated and wet milled in a two litre stainless steel ball mill and then re-dissolved under the same conditions. The residue from this stage was rinsed with distilled water, dried at 110 °C, 24 hr and named “silica enriched residue” (SER). A part of SER sample was treated by 2M nitric acid at room temperature, in a reactor being mixed at 600 rpm for 7 hrs to remove residual magnesium and enriching the residual material to produce a reactive amorphous silica product. It was then repeatedly rinsed with distilled water and dried at 110 °C for 24 hr and named “acid treated silica enriched residue” (ATSER). Synthesised materials were then tested for pozzolanic activity by dissolving them in a highly alkaline solution of 0.2 M NaOH (pH=13) to determine the rate of silicon extraction in environment similar to cement paste.

XRF and XRD analysis indicated that an amorphous silica phase comprised 40 and 66 wt% of heat activated lizardite and SER materials, respectively. Our work also showed that reactive amorphous silica material (ATSER), with 90 wt% purity, could be achieved through acid treatment of SER sample. Dissolving synthesised materials in high alkaline solution with pH similar to cement paste for 28 days showed that all synthesised materials have some pozzolanic activities. ATSER showed the fastest rate of silicon dissolution even higher than silica fume (the best commercial cement additive), followed by SER and heat activated lizardite all having faster rate compare to Portland cement as standard.