Heritage

Stone restoration and repair: the use of St. Astier Lithomex mortars

By Ugo Spano, St. Astier Limes

Today all of us are charged with ensuring the survival of our historical and vernacular built heritage through careful and considered conservation, restoration and renovation.

Much of the masonry replaced in the last thirty years has resulted in the partial of complete removal of predominantly structurally sound masonry that has suffered from little more than face weathering.

This superficial weathering is often through neglect and lack of maintenance of basic building details such as gutters and down pipes. Where replacement stone has been used in these circumstances, buildings have been left with thin skins of new stone that seldom match the true characteristics of the original; many of the thin skins are backed up in concrete, brick or other inappropriate materials, or with significant voids behind the new veneers. Therefore, a highly invasive intervention is carried out in what was an otherwise stable structure.

In most cases, the mortar used for repairs, restoration or conservation contains cement. In these buildings, the use of cement mortars promotes condensation build up, salt reactions, and generally the long term survival of the building.

In general, surface repairs are required to provide a good visual match for the existing masonry. The physical properties of the masonry may vary from one part of the building to the next and the ability to vary the properties of the repair mortar is an advantage.

For surface repairs to be successful, the materials employed need to have certain characteristics. They should:

  • Have characteristics similar to the host masonry
  • Have adequate bond strength
  • Have a good modulus of elasticity
  • Be vapour permeable
  • Be durable
  • Be capable of being dressed with similar tools to the original masonry
  • Remain workable to allow details to be fashioned
  • Match in colour and texture
  • Adsorb water sufficiently in wetting and drying periods to match adjacent masonry.
  • They must always be reversible

They should not:

  • Become significantly stronger than the surrounding masonry
  • Have a significantly lower rate of absorption or adsorption than surrounding units
  • Create a barrier to diffusion

St. Astier natural hydraulic lime mortars provide a valid alternative to cement mortars or cement/lime mixes and, if applied correctly, they will have a much longer life. Environmentally, they offer far greater efficiency than cementicious mortars by reabsorbing CO2  and not compromising the recyclability of the building materials.

Lithomex is a very cost effective, versatile mortar widely used for repair, restoration and stone simulation. Stone colours are precisely matched. Tonalities and surface finishes are easily achieved by trained masons.

The UK is one of the largest users of Lithomex, as colour matching services and training are available.   St. Astier Limes specialists, when required, actively work with the specifier, giving technical advice to establish the most appropriate mortar for the proposed intervention.

If mortars are not dosed or mixed properly, if they are not cured and protected correctly, if the application surfaces are not properly prepared, if suction is not controlled, if salt movements are not taken into consideration and so forth, the result will be a failure.

For more information, clarification or for any query, please contact: technical@stastier.co.uk

 

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