Cement

According to the newly-regenerated Romana II, cement was the combination of limestone and clay with water. Concrete was made with gravel and cement. The Fourth Doctor essentially agreed, and noted that it was the indication of civilisation on the mysterious planet on which they'd landed. (TV: Destiny of the Daleks) He and Romana I had earlier buried a nuclear reactor inside the mountain of T'ai Shan beneath hundreds of tonnes of cement in August 1937. (PROSE: The Shadow of Weng-Chiang)