RMC has also hired APCO UK on a project basis to handle public affairs work.
RMC UK comms manager Vicky Leonard said CPN would handle B2B and B2C campaigns.
In addition, CPN will work with the HR department to 'help to ensure we recruit high-quality employees,' according to Leonard.
The contract will involve work across all four RMC divisions: Aggregates, Building Products, Readymix and Cement.
CPN directors Keith Taylor and Simon Taylor lead the account team, with deputy chairman Rosemary Grogan leading the work at APCO.