liu had spent most of his working career compressing industrial freight using hydraulic presses. in 1999, he was given some fly ash by a client, and decided to compress it "just to see what would come out." liu mixed the fly ash with water and applied 4,000 psi (28 mpa) of pressure. after two weeks, he found that the mixture had set into blocks with the strength of concrete. owing to the high concentration of calcium oxide in fly ash, the bricks can be described as "self-cementing".