The fire at Fegan's Quarry on the Madden Road in Keady happened in the early hours of Tuesday.
Police have said they are working to establish a motive for the incident.
They are appealing for anyone who noticed any suspicious activity in the area, or who has any other information, to contact them.
A fire which destroyed more than 300 cars at a breaker's yard in County Armagh is being treated as suspicious, police have said.