drywall & clogged drain

We had our house remodeled 18 months ago. The drywallers may have used the utility sink for more than just water. The sink drain was clogged after words so I successfully snaked it out. Now after some grouting it is clogged again. When I used it to clean up after thinset and grout I would only wash off the tools after washing the heavy stuff outdoors. I ran the water much.
Anyway, is there a known way of removing the blockage ? It is a 3" cast iron pipe.
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Steve wrote:

The clog is probably in the trap. The plaster and sand would settle there. Try removing the trap -- it may be easier to just replace it rather than clean it.
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To really stop up a trap, I once ran crab leg shells through the disposer. They don't float. They got no farther than the trap and it locked up solid.

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