Influence of the combination of Roman cement and lime as the binder phase in render mortars for restoration

•The addition of lime (NHL5 or CL90) does not retard Roman cement hydration.•Retardation of Roman cement is essential in hybrid mortars.•The cement can be retarded by means of a pre-hydration technique (DARC).•Lime permits fine control of mortar (workability, workable life, strength, WAC).

It is known that lime was added to historic Roman cement render mortars. The focus of this work is the influence of the combination of NHL5 and CL90 with Roman cement in mortars for restoration; however, the results indicate a wider potential for render applications in general. It is shown that simply adding lime to Roman cement does not retard its hydration and yields mortars where the binding action of the cement is compromised by the mixing process. If the cement is retarded by means of a pre-hydration process, hybrid mortars can be produced with improved workability and workable life as well as permitting the fine control of strength and moisture transport.