the past 3 patios i have done i used a base of compacted crushed limestone (12-15 inches thick) and a layer of sand (1-2 inches thick) i was at a nursery today to get a price on materials i.e crushed limestone and sand. and was told that they always use slag it is finely crush stone i think, i am not sure if they use a top layer of sand with it or not but they said you only need 4-5 inches compacted for your base and it will be as strong as concrete, was wondering if everyone is using SLAG or are some people still using the crushed lime stone. i cant belive that 4-5 inches would be very solid ????