The deal will allow CRH to significantly expand its business in the UK and globally.
As well as the Lafarge Tarmac business it will acquire production facilities in the Philippines, Canada, Brazil and several countries in Europe.
It also clears the way for a major shake-up in the global cement market, removing a significant condition to Lafarge and Holcim creating a world cement titan.
It should now be a rubber stamping exercise for the European Commission to give regulatory approval to cement the Franco-Swiss merger.
Lafarge Tarmac was put up for sale last summer, with the exception of its Cauldon cement works in Staffordshire, which is being kept, with a price tag of around £1.4bn.
In the UK, Holcim owns Aggregate Industries, which boasts a network of aggregate, asphalt, ready-mix and concrete products sites, but presently imports cement.
CRH will fund its part of the deal with cash, debt and a 9.99% equity placing
For the Irish materals giant, the asset purchase ranks as by far the biggest deal it has ever done. Since 2000 CRH has spent around £15bn on over 600 acquisitions but these have tended to be smaller.