The notice board by the Elan Valley dams mentioned "concrete with plums". I'd not heard of that before. Turns out it is plain concrete with large rocks placed in it. When water is added to cement, an exothermic reaction takes place that releases a lot of thermal energy. The amount released in something the size of a dam would be huge. If maybe 40% of the mix is rocks, then this reduces the amount of thermal energy released.
