MUMBAI, Dec. 23 -- After spending nearly $10 billion over three years to build India's second-largest cement business, the Adani Group is moving to simplify what has become a sprawling corporate structure.

The board of Ambuja Cements Ltd, part of billionaire Gautam Adani's conglomerate, late on Monday approved a plan to merge ACC Ltd and Orient Cement Ltd into Ambuja. Ambuja currently holds 50.05% in ACC and 72.66% in Orient. The cement maker estimates that it will take about 12 months to secure approvals from shareholders, creditors and regulators and complete the transactions.

Shares of Ambuja Cements rose 2% on Tuesday following news of the merger, while the benchmark Sensex was largely flat.

The announcement comes a year after Ambu...