Refractory castables consist of suitably graded aggregates and hydraulic cements in proportions formulated to achieve certain desired properties for the particular end use. They require only the addition of clean water either by hand or mechanical mixing or during pneumatic gunning to form a heat resistant concrete piece or structure. A medium temperature ultra light weight insulating refractory castable comprised of hydraulic setting aluminous cements and insulating aggregates suitable for back-up application. A medium temperature extra light weight insulating refractory castable comprised of hydraulic setting aluminous cements and insulating aggregates suitable for back-up application. A medium temperature light weight insulating refractory castable comprised of hydraulic setting aluminous cements and insulating aggregates suitable for hot face application. A high temperature medium weight refractory castable comprised of hydraulic setting high alumina cements and insulating aggregates suitable for hot face application.