This sounds like the type of situation I find myself in every now and then... I love a good deal, love 'em maybe a bit too much. Which is how in the past I've ended up with about 30 calculators, 14 day packs, and a bunch of of scaffold planks I still have by the side of our house. All that stuff was too great a bargain to pass up, despite the fact I had no need for any of it. They were all solutions to problems I didn't have, answers in search of questions. I've found that getting something just because it's cheap or free (especially if it's free!) is not the ideal way to operate.
Ask yourself this: "What about this arrangement - other than it's cheap - appeals to me?" It sounds like you have a new space and want to maximize its utility and your enjoyment. And clearly you like the thought of granite. But it seems like real granite isn't the best material to use, even if its initial cost is nada, its installation, final look, utility and upkeep costs may far outweigh the "bargain" acquisition price.
You might want to consider an epoxy coating that looks like granite. Most epoxy dealers have a coating and flake combination or two that will give you the look of granite without the negatives of actual stone. Plus you'll have all the benefits of an epoxy coating - tough, durable, seamless, easy to clean , etc.
Click this link to see what another GarageJournal member did to get his "granite" floor.
You'll want to read his whole account because there are a lot of great tips there, but here's a quick preview: