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Lime–Prompt natural cement render on old masonry

Structures: 
Interior/exterior renders. Prompt natural cement is used in a mix with lime:

  • to restore natural or Roman cement renders from the 19th and 20th centuries;
  • to render old surfaces in general

Area of use: 
Old buildings, decoration, sea environments.


Benefits of using in a mixture with lime: 
In all rendering work using lime, it is possible to aid application and improve the quality of structures by using Prompt natural cement.
The presence of this binder in the render means:

  • the possibility of working in cold or wet weather;
  • thicker layers;
  • reduced time between layers;
  • security to structures in the face of immediate weather risks;
  • reduced shrinkage, improving the protective role of the render;
  • a light ochre colour suitable for façades;

Obtaining these qualities does not compromise lime mortars’ permeance characteristics (wall breathability).


Dosage and preparation: 
Natural lime–Prompt natural cement mortar mix; the most suitable mortars are those with the lowest doses: lean-mix mortar and medium-mix mortar.
DTU 26-1 (work with mortar renders) gives the following indications:

  NHL Lime CL Lime PNC
Roughcast
Overall dosage 400-450 kg/m3 
250-320   100-200
Floating coat 
Overall dosage 250-350 kg/m3 
200-280 200-250 50-150
50-150
Finishing layer 
Overall dose 200-300 kg/m3 
150-200 100-150 50-100
50-100

The doses are given in kg per m3 of dry sand. 
NHL = Natural Hydraulic Lime / CL or DL = Non-Hydraulic or calcic lime (CL)

Addition of Tempo is recommended to delay setting: 1 capful of Tempo per litre of Prompt natural cement.

Example of doses in volumes:

  NHL 2 or 3.5 PNC Sand
Roughcast - 1 2
Floating coat 3 1 10
Finishing 3 - 10

Apparent density of binders:

  • Prompt natural cement: 0.9 to 1
  • hydraulic lime: 0.8
  • non-hydraulic lime: 0.5

Advice: 
For uniformity of colour across the façade, the top coat of render should be with lime only.


NB (Prompt natural cement only renders): 
For small surfaces, it is possible to use Prompt natural only renders, as with “Roman cement” in the 19th century.
The doses indicated below deliver the same porosity. Doses are for information (in litres):

  Tempo PNC Sand Onset of setting (20°C)
Roughcast 0 1 2 (0/5 mm) 5-10 min
Floating coat 0.5 to 1 capful 1 2,5  (0/3 mm) 20-25 min
Finishing layer 0.5 to 1 capful 1 3 (0/2 mm) 20-25 min

For maximum bonding, it is important to apply the different layers wet-on-wet; the finishing coat may be lime only.
It should be noted that the permeance of the above mortars is between 0.4 and 0.5 g/m2. h.mmHg, after 6 months. (By way of comparison, for the same doses and over the same timeline, that of natural hydraulic lime NHL5 is between 0.5 and 0.6 g/m2. h.mmHg).


Recommendations for old buildings: 
For protected buildings under the historic monuments programme (MH) or protected sectors, please refer to the specific instructions of the departments concerned.

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