(433d) CO2 Fixation through Carbonation of Waste Cement and Concrete

CO₂ capturing and fixation processes with waste concrete via mineral carbonation reactions have been developed. The waste concrete is either waste concrete generated from demolished concrete buildings (waste concrete, hereafter) or concrete sludge. Waste concrete is composed of aggregates and waste (hydrated) cement, and waste cement powder of which the diameter is under about 100 μm would be separated in the recycling process of aggregates. The concrete sludge is waste fresh concrete in the form of slurry generated at the concrete using sites. Both concrete wastes are alkaline containing calcium that could be used as a capturing and fixation material for CO₂ via carbonation reaction. Several options are considered as the CO₂ fixation process with these waste concrete, and process feasibility has been evaluated based on the laboratory experimental studies. The followings are the options examined in this study.

(1) Direct fixation of CO₂ with concrete sludge

     This option is a direct fixation process of CO₂ with waste concrete sludge. Flue gas is introduced to the slurry of concrete sludge, and CO₂ will react with hydrating cement components, mainly calcium hydroxide, to form calcium carbonate.  The effect of hydration reaction time, gas flow rate, water content, CO₂ concentrations on the CO₂ fixation performances were examined in a small-scale laboratory experiments.

(2) Indirect multistep fixation of CO₂ with waste cement powder

      This option is four-step process composed of (i) dissolution of calcium ions into an acid, (ii) CO₂ capture with an alkaline solution, (iii) formation of calcium carbonate by mixing the above two solutions, and (iv) regeneration of the acid and the alkaline solutions via electrodialysis. Parameters affecting the process performances were examined for each step in laboratory experimental studies. 

Based on the experimental studies, the feasibilities of these processes have been evaluated in views of the process power consumption as well as the cost.