BlackRock has agreed to buy private-equity firm Global Infrastructure Partners for roughly $12.5 billion in cash and stock, a significant push into private-market investments for the world’s largest asset manager.

New York-based GIP owns and operates energy, transportation, and water and waste companies, including a stake in London’s Gatwick Airport. The acquisition would be BlackRock’s largest since it bought Barclays’s asset management business in 2009.