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The invention provides a plurality of polymeric particles embedded with alkaline-earth metal oxide. The gas-filled polymeric microelements have a shell and a density of 5 g/liter to 200 g/liter. The shell has an outer surface and a diameter of 5 μm to 200 μm with the outer surface of the shell of the gas-filled polymeric particles having alkaline-earth metal oxide-containing particles embedded in the polymer. The alkaline-earth metal oxide-containing particles have an average particle size of 0.01 to 3 μm distributed within each of the polymeric microelements to coat less than 50 percent of the outer surface of the polymeric microelements.