Restoration & HeritageReturn
Natural cement render
At first sight, it may appear surprising that Roman cements can have been used for renders. Indeed, their quick setting means that this application is a daunting technological feat! And yet mineralogical analysis of render mortars over 100 years old have shown that it was possible, and even in several layers. Applied wet-on-wet, the different layers have excellent bonding between them. By way of example, the 19th century façades, known to be so richly decorated, turn out to have fairly small rendered surfaces.
In like for like restoration, Prompt natural cement can be used for both interior and exterior renders, or indeed in a mix with lime so as to reproduce the natural or Roman cement renders of the 19th and 20th centuries. The low dosage of these mortars gives good permeability to water vapour.
Preparing the surface: clean and dampened (controlling the absorption of the surface is very important).
Applying to edges: 1:2 sand ratio mortar (0/3 mm) to obtain a plastic consistence with a retarding agent of 0.5 to 1 capful per litre of Prompt natural cement (according to temperature).
Roughcast: 1:2 ratio sand mortar (0/5mm) for semi-liquid consistence without retarding agent. Cast and leave rough. Each layer must be applied wet-on-wet for better bonding.
Floating coat: 1:2.5 ratio sand mortar (0/3mm), with consistence adapted to trowel casting, retarding agent 0.5 to 1 capful of Tempo per litre of Prompt natural cement (according to temperature). Cast with a trowel. The Floating coat loses workability after 15 to 20 minutes (at 20°C), which is enough to apply several layers, up to the level of salient edges. It is possible to obtain additional thickness to 50 mm with several successive coats applied wet-on-wet. Level off and smooth with a wooden float as soon as workability declines. The surface must be left rough and not closed.
Finishing coat: 1:3 ratio sand mortar (0/1 mm) coloured with pigments or marble powder.
Incorporate a retarding agent of 0.5 to 1 capful of Tempo per litre of Prompt natural cement (according to temperature). This produces a very plastic consistency for a thickness of 3 mm. Level off and smooth with the float.
It is also possible to achieve a finish with a lime–Prompt natural cement mix, or with NHL alone.
For optimum bonding between each coat, it is necessary to work wet-on-wet.
In comparison with NHL at the same dosage ratio (1:3), Prompt natural cement renders have slightly better mechanical characteristics than an NHL 5. Shrinkage during drying will be less. However, where NHL 5 has a permeance of between 0.5 and 0.6 for NHL 5, this is between 0.4 and 0.5 for Prompt natural cement render.
In all render works that use lime, it is possible to facilitate application and improve the quality of the surface using Prompt natural cement.