Carbonate sand and gravel dredged for cement production from the Nukubuco barrier reef at Laucala Bay near Suva, Fiji, have magnesium contents close to the acceptable limits for cement manufacture. The sediments are mainly fragments of coral, calcareous algae, molluscs, and foraminifera, with bulk MgO contents averaging 2.9 percent (usual range 2.5–4.5 percent). MgO comes partly from the foraminifer Marginopora vertebralis and rare echinoderm fragments, but mainly from calcareous red algae. Selectively dredging areas with lower magnesium content is difficult because of the unpredictable distribution of high-magnesium clasts; however, the magnesium content of the sediment can be reduced after dredging in two ways. Because larger clasts have higher MgO content, removal of grain sizes coarser than 1 mm (0.0 ϕ) by sieving lowered the MgO level by up to 0.2 percentage points (from 3.5 percent to 3.3 percent on average), but this gave inconsistent results and led to the loss of 25 to 50 percent of the raw material. Abrasion of larger clasts by tumbling to remove encrusting red algae rapidly and consistently lowered MgO values by 0.6 percentage points (from average 4.1 percent to average 3.5 percent), with loss of only 3 to 4 percent of the initial raw material to produce magnesium-rich lime residues. This efficiency is caused by the selective removal of outer, algal-encrusted and high-magnesium layers of the clasts, which are already weakened by natural processes of bio-erosion. Results should be widely applicable wherever tropical carbonates are used for cement manufacture. The technique should be readily scalable to commercial levels and has the potential to greatly reduce the amounts of reef material dredged.

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