New Delhi: Adani Group on Tuesday said it has agreed to acquire CK Birla group firm Orient Cement Ltd in a deal valued at Rs 8,100 crore, as billionaire Gautam Adani-run conglomerate snaps up smaller rivals to challenge India's top cement maker UltraTech. Ambuja Cement, India's No.2 cement maker, will buy 46.8 per cent of its founders including chairman CK Birla, and certain public shareholders for Rs 3,791 crore. This would trigger an open offer for an additional 26 per cent stake in Orient, according to a company statement. Orient, which has two cement plants in the south and one in western India, will be Ambuja's second acquisition this year. It will add 8.5 million tonnes to take Ambuja's operational capacity to 97.4 million tonnes.