Formulating mortars for use in restoration practice

The principal uses of Roman cement mortars in the field of restoration are for the production of cast decorative elements and renders. The formulation of these mortars differs in terms of mix proportions, workability and workable life. A typical specification for a “cast” mortar is 1:0.5 to 1:1 by volume, flow of 19.5 cm and a workable life in the range 15 – 30 minutes; a render mortar is typically 1:1.5 to 1:2.5 by volume, flow of 15.5 cm and a workable life of between 1 – 2 hours. Whilst the former workable life can be obtained by the use of chemical retarders the prolonged life required for renders is generally not possible without the excessive use of chemicals which impairs the performance of the hardened mortar and an alternative pre-hydration technique has been developed.