Complex sentence: He rediscovered the use of "hydraulic lime," a form of concrete used in Roman times that will set under water, and developed a technique of securing the granite blocks together using dovetail joints and marble dowels.
Split into simpler sentences: He rediscovered the use of "hydraulic lime," a form of concrete used in Roman times. The technique allowed concrete to set under water, as Smeaton put granite blocks together using dovetail joints and marble dowels.