Carbonation of cementitious wasteforms under supercritical and high pressure subcritical conditions

Abstract

A comparison of cement wasteform carbonation under subcritical and supercritical CO2 conditions has been conducted, as well as leach tests on the carbonated products. Cement samples were prepared with water containing As, Cs, Sr, Ni and Cl, and dried under vacuum prior to exposure to CO2 gas at sub and supercritical conditions. Carbonation under supercritical conditions (50 degrees C, 1200 psi) showed similar CO2 mass uptake as subcritical carbonation (25 degrees C, 800 psi). The apparent depth of carbonation from microscopic examination, however, appeared greater with the supercritical treatment. Microscopic examination also revealed a dark intermediate zone, likely a chemical transition area, between the uncarbonated and carbonated material. Results from experiments where the initial water to cement ratio (w/c) and curing time were varied indicate that high w/c ratios and short curing times promote greater carbonation. In addition, the pore network in specimens with low w/c ratios (0.45) completely filled with water during carbonation. This caused pore water containing soluble contaminants to exude from the specimens. This does not occur at a w/c of 0.60 because the larger pore network after the drying step easily accommodates all the water produced during the reaction. No measurable arsenic or nickel were released in leach tests conducted on both carbonated and uncarbonated specimens. Calcium, cesium and strontium leachability were reduced by carbonation while the leachability of chloride and nitrate were increased with carbonation. Samples carbonated under supercritical pressure conditions showed similar leaching characteristics as samples carbonated under subcritical conditions.

Little evidence that lowering the pH of concrete supports greater biodiversity on tropical and temperate seawalls

“…In this experiment, a chamber was created using a large cooler box and dry ice, thus incubating the concrete tiles in quasi-saturated CO 2 at atmospheric pressure and room temperature. This method is readily implemented, without requiring specialized equipment for supercritical and high-pressure CO 2 (Venhuis & Reardon 2003).…”

Stabilization/solidification of an alkyd paint waste by carbonation of waste-lime based formulations

“…The waste/binder ratio was selected to be constant and equal to 1.3 based on previous work where this ratio produced an uncarbonated product with the lower contaminants release in leaching tests [17][18][19]. The amount of added water was changed over a range representative of waste carbonation processes from 0.2 to 0.6 (w/w) water to solid ratio [26,27]. Table 2 reports the studied formulations and treatment conditions to obtain the carbonated products.…”

“…Previous waste carbonation studies [21,25,26] show that the water to solid ratio (W/S) is crucial for carbonation to proceed at an optimum rate when lime, different cement types and pozzolanic admixtures (fuel ash, blast furnace slag, metal hydroxide filter cakes) are used. Different water contents have been used in the accelerated carbonation of wastes; values of the water/solids ratio in the range of 0.05-0.6 (w/w) [26,27] have been proposed for carbonation in the thin water film near the gas-solid interface, being necessary in each particular case for the optimization of this operational parameter. In this work, the accelerated carbonated process is used for paint waste treatment of S/S with lime and mixtures of lime, Portland cement and fly-ash.…”

Effect of limestone fillers on microstructure and permeability due to carbonation of cement pastes under controlled CO 2 pressure conditions

“…Recently, carbonation is applied during S/ S of heavy metals to improve immobilization. By inducing a slight alkali environment, the solubility of many toxic heavy metals is minimized [6,7]. Furthermore, CO 2 sequestration in cement-based materials seems to be one of the promising methods to cut down CO 2 emissions from industrial sources [8].…”