MICROCAL boards are large in size and possess high mechanical strength in combination with good thermal stability and low thermal conductivity. The boards are surface-active and absorb water but give this up again extremely rapidly. Following drying out, the properties of the boards are unchanged. For work in combination with refractory concretes, a hydrophobic sealing primer, that can be applied in our works, is recom- mended. In this way time-consuming work with barrier film can be dispensed with.