Made from cast concrete. Portland cement is the binder. Water needed for the hydration process. Aggregates vary from shingle for high-density frost-resistant products to lightweight aggregates (e.g., exfoliated clay nodules and pulverized fuel ash). Recycled materials, such as post-consumer glass, slag cement, or recycled aggregate, can also be incorporated into the composition of concrete blocks. Some blocks are solid, while others are hollow (core may be filled with foam for insulation).