How much grout? Clumped up? Floor scraper or floor polisher with honing heads will take it down even enough. ..if its just a little then there are hand held honing stones. ..larger area may take the dual headed floor polisher. ..any acid or abrasive water blasting that will take the grout away shoukd also have the same effect on the concrete as well and if they used the same grout to fill cracks the n you dont want to compromise that.good luck and welcome to pwi there are sure to be others to add in there xperiences definitely fill in your signature youll get more responses