Hi Michael,
Good thinking in your proactive approach to utilising available resources for your project. Take a moment to explore the photo galleries and interactives provided. In the photo galleries of Waste management section (you can find the galleries towards the bottom of the page), you can see that there are some building material that are unused and often considered waste. Materials such as concrete blocks, steel bars, and Corrugated iron can be seen in the pictures. You are allowed to make an informed assumption based on what you see.
Hope that helps,
Cheers,
Sai