Kevin Lagan has agreed a deal to sell his materials business, Lagan Group, to Breedon Group for £455m on a cash and debt free basis.
Under the terms of the deal, Breedon - the UK's largest independent construction materials group - will buy Whitemountain, Lagan Cement, Lagan Brick, Lagan Asphalt and Welsh Slate.
Lagan Homes, one of the largest housebuilders in Northern Ireland, is not part of today's deal.
Breedon already has a relationship with Lagan Group and operates a joint venture with its Whitemountain business in Scotland.
The combination of Breedon and Lagan's materials businesses will create a workforce of about 3,000 and turnover of more than £900m.
Founded in 1960s on the outskirts of Belfast, the company's turnover has grown to £272m last year with combined earnings (EBITDA) of approximately £55m.
After the deal with Breedon, Kevin Lagan and his family intend to pursue a managed growth strategy with the Lagan Homes and FastHouse businesses, which are being retained.