A new type of mineral
carbonation process for concrete sludge,
a waste of fresh concrete under hydration, was developed, and the
carbonation performances of the process were examined by laboratory-scale
experiments. The process is composed of two steps; filtration of concrete
sludge and bubbling of CO
2
into the filtrate to form calcium
carbonate. Model concrete sludge, a mixture of cement and water, was
filtered through a cellulose filter after hydration for 24 h to obtain
a solution containing dissolved calcium ions. Then, the model flue
gas containing CO
2
(10%) was bubbled through the filtrate
solution, and calcium carbonate was precipitated by the carbonation
reaction. About 3% of calcium in the concrete sludge could be extracted
into the filtrate in a single filtration step, and more than 95% of
dissolved calcium was recovered as calcium carbonate by the bubbling
of CO
2
. The obtained calcium carbonate was calcite with
a high purity (>95%) and 5–10 μm. The solid residue
(concrete
sludge) after filtration was mixed with fresh water and filtered through
a cellulose filter. Then, the model flue gas was bubbled into the
filtrate solution for carbonation. This filtration–bubbling
step was repeated 5 times, and 10.8% of calcium in the feed cement
was extracted into the filtrates in total. More than 95% of the extracted
calcium could be recovered as calcium carbonate with high purity (>97%),
and the overall conversion of calcium in the feed cement to calcium
carbonate was 10.1%. The purity of calcium carbonate and the calcium
conversion were much higher than those for the direct bubbling method,
where the model flue gas is bubbled into concrete sludge.