MEXICO CITY, July 11 (Reuters) - Mexican cement maker Cemex said Monday it had completed the acquisition of a 53% stake in ProStein, a German company that makes aggregates, or grained particulate used in construction.

The company's aggregates sales in Germany are expected to double with the acquisition, Cemex said in a statement, and increase capacity to serve major cities in Poland and the Czech Republic as well as Berlin.

With the acquisition, Cemex gains access to ProStein's six active hardstone plants and six construction, demolition and excavation waste recovery sites, it said.

"This investment allows us to have a sustainable supply of high-quality aggregates in a major metropolitan center for years to come," said Sergio Menendez, president of CEMEX Europe, Middle East, Africa and Asia.

The current reserves should last for at least the next 25 years, taking sales growth into account, Cemex said. (Reporting by Kylie Madry in Mexico City Editing by Mark Potter)