Only Celfix mortar can be used with Celcon Blocks for the thin joint system. No other mortar will be guaranteed as part of the system. Celfix mortar comes in this standard 25kg bag and the most important thing is to get the right proportion of water. You must use 4.5 litres of water for each bag. LightweightConcreteBlocks, AircreteBlocks. H+HUKLtdCelcon