The novelty/innovation of the proposed work is as follows. Supercritical carbon dioxide (SC-CO 2 )-based extrusion and molding technology can be used to produce significantly improved (in terms of strength/unit weight, durability, hardness and chemical resistance) cement-based products. SC-CO 2 can rapidly convert the calcium hydroxide in cured cement to calcium carbonate, which increases the density and unconfined compressive strength in the treated region. In cured concrete, this treated

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... n is typically a several-mm thick layer (generally <-5mm, unless treatment time is excessive). However, we have found that by treating the entire cement matrix with SC-CO 2 as part of the curing process, we can carbonate it rapidly, regardless of the thickness. By "rapidly" we mean simultaneous carbonation/curing in < 5 ks even for large cement forms, compared to typical carbonation times of several days or even years at low pressures. Carbonation changes the pH in the treated region from -13 to -8, almost exactly compatible with seawater. Therefore the leaching rates from these cements is reduced. These cement improvements are directed to the development of strong but thin artificial reefs, to which can be attached microalgae used for the enhanced fixation of CO 2 . It is shown below that attached microalgae, as algal beds or reefs, are more efficient for CO 2 fixation by a factor of 20, compared to the open ocean on an area basis. We have performed preliminary tests of the pH-neutral cements of our invention for attachment of microalgae populations. We have found pH-neutral materials which attach microalgae readily. These include silica-enriched (pozzolanic) cements, blast-furnace slags and fly ash, which are also silica-rich. We have already developed technology to simultaneously foam, carbonate and cure the cements; this foaming process further increases cement surface areas for microalgae attachment, in some cases to >10 m 2 /g internal surface area. This project involves a team of researchers with backgrounds in cement technology, supercritical fluid technology, materials science, oceanography, and wetland biogeochemistry. iv