I thought I would mention papercrete to you, its a mixture of portland cement and paper. Commonly used paper is newspaper torn into shreds and soaked in water over night, You take the paper and squeeze out the excess water and mix equal amounts of white portland cement and the paper using a drill with a mixer attached adding a little water until it becomes soupy. This stuff is hard like cement but about 40% lighter and can be tinted with cement tints which come in black and green and red and blue and yellow and brown. Newspaper makes a speckled grey-white, looking much like granite. You can take the mix and place it on a plastic sheet and form it any shape you prefer and then cover it over with plastic to allow it to slow cure. The weight would be far less than any natural rock and very strong, you could leave a lower area in the center to place your trees and even form a ridge around the edges if you choose too. Its much like clay before it sets hard. I know there are several videos on youtube that show it being used to make pots and one guy who makes slabs for building use.
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