We are offering Gypsum for Fly Ash Bricks. fly ash brick (fab) is a building material, specifically masonry units, containing class c fly ash and water. compressed at 4,000 psi and cured for 24 hours in a 150 â°f (66 â°c) steam bath, then toughened with an air entrainment agent, the bricks last for more than 100 freeze-thaw cycles. owing to the high concentration of calcium oxide in class c fly ash, the brick is described as "self-cementing". the manufacturing method saves energy, reduces mercury pollution, and costs 20% less than traditional clay brick manufacturing