Good job — your sentence is almost perfect! Here are a few clear, simple ways to show it and explain why.
Corrected single sentence (keeps your style):
The drum of the concrete mixer spins and spins, making strong concrete for building.
Other simple options:
- The concrete mixer's drum spins and spins, making strong concrete.
- The drum of the concrete mixer spins and spins. It makes strong concrete for building.
Step-by-step explanation (easy to understand):
- Start with a capital letter: "The" is correct.
- "Spins and spins" is a good repetition to show something keeps turning — that is okay.
- The comma before "making" is correct because "making strong concrete for building" tells what happens when the drum spins.
- "Making" starts a short phrase that describes the result (the drum makes concrete).
- "For building" means the concrete is used to build things. If you want to be clearer you can say "for building houses" or "for construction."
If you want the sentence shorter and simpler, use: "The concrete mixer spins and makes strong concrete."
Nice sentence — just join the two lines into one sentence or make two short sentences as shown above.