English bricklayer and inventor who patented Portland Cement. Concrete, a mixture of cement, gravel, and sand, had existed since the time of the Romans. However, the cement portion was often of uneven quality. Aspdin discovered that the key to making hydraulic cement (cement that would harden when mixed with water) was roasting a mixture of clay and limestone powder in a furnace. Mixing this cement with sand, gravel, and water allowed the lime and clay to form a kind of gel that coated the sand and gravel, cementing them together when it dried. The hardened mixture reminded Aspdin of the prized building rock quarried on the Isle of Portland, thus the term "Portland Cement." Modern concrete contains only about 11 percent cement by weight, sand and gravel constituting over 65 percent of the weight and water most of the rest.