Clogged drain & drywall

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 2" cast iron pipe.
It is not in the sink trap. It is in a 2" cast iron pipe that services only that sink and the former location of the W/D and a vent for the basement 1/2 bath. Where the drain goes into the floor is right next to where the water main comes up.
What options do I have to unclog it ?
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