Garnishing, plastering, rendering, monolayer, rendering, stucco... What is the difference between these coatings? What materials are used in their composition? With this post we try to shed some light on the different types of continuous coatings, focusing on those with a lime base due to their properties in the rehabilitation and restoration of historic buildings.

Lime, cement and gypsum are the most commonly used binders as a base for continuous coatings. These materials are derived from different minerals and therefore have different chemical compositions:

  • Lime: It comes from the calcination of limestone rock (calcium carbonate).
  • Cement: Mixture of limestone (calcium carbonate) and clay (silicates).
  • Gypsum: derived from calcium sulphate.

Mortars are obtained by adding water and aggregates in different proportions to the first two. Thus, we have lime mortars, cement mortars and bastard mortars (lime and cement). The aggregates used, their granulometry and the proportion used give rise to different types of mortars with different properties. Gypsum, on the other hand, can be used mixed exclusively with water.

Plasters and mortars are used as continuous coatings in construction. In this way, we have:

  • Garnish: Based on black plaster, its main use is as a first coat on interior walls. It has a less fine finish than plaster. 
  • Plaster: This is the finishing layer of white plaster placed on top of the trim. The plaster is usually painted over, although a varnish can also be applied directly, thus achieving a more natural appearance. 
  • Plastering: This is a coating formed by a layer of cement mortar or lime mortar with strong aggregate and applied directly to the wall. It is normally used in chambers and exteriors and can be painted directly or used as a base for a subsequent render finish.
  • Single-coat: Cementitious-based and mass-pigmented, it is usually used on exteriors as an alternative to rendering and painting.
  • Rendering: This consists of applying lime or cement mortar with a fine granulometry of aggregate on a previous rendering base. Depending on the aggregate used, the layers applied and the tools used, different finishes can be obtained (madrileña, martillina, rasqueta, finjido...). 
Marble aggregate with different granulometries and hammer finishing hammer
  • Stucco: It is composed of lime mortar, fine marble powder and pigments. It is applied in different layers (lean, repretar, plaster, ink...) on a smooth plaster base and allows a very fine finish and shiny aspects similar to polished marble. There are different types of stuccos, but in Spain the most common are the smooth washed stucco and the fire stucco. The latter requires first coats of smooth stucco with different granulometries, 3 coats of ink (soap water and lime paint) and application of the stucco iron.
Heater and stucco irons

At Antana we like lime. In addition to its intrinsic recyclability (it returns to its natural composition over time), its physical properties make it ideal for certain renovation interventions. In this post we focus on lime mortars and their applications.

Manufacturing process of air lime and hydraulic lime:

The manufacturing process of lime and its mortars is natural and simple:

  1. The starting point is limestone (CaCO3) that is as pure as possible (above 95%). The stones are burnt (calcined) above 900º and in this process CO2 is released and calcium oxide (CaO) appears. In the case of thicker stones, the combustion process will not reach the centre, so they will have to be "peeled" and the "bone" (the centre which remains as limestone) will have to be discarded.
  2. Calcium oxide (quicklime) is a very caustic product that reacts with water giving off heat and producing calcium hydroxide Ca(OH)2. This process is called "lime slaking" or lime hydration and can be carried out by spraying or immersion. The result of the spray process (minimum amount of water) is calcium hydroxide powder. The immersion process, on the other hand, results in lime putty or fatty lime with a higher water content, which gives it elasticity and homogeneity in the carbonation process that we will see later on. The fat lime is kept with a layer of water on top to prevent carbonation from starting. This fatty lime can stand for many years and, in fact, the passage of time improves its properties. In the past, in certain areas of Spain, when a child was born, lime was put out by immersion and kept protected with water to be used years later in the construction of the child's house.
  3. This lime is mixed with aggregates (usually marble) and water to produce lime mortar. It reacts with air (air lime) through a carbonation process, absorbing CO2 and giving rise, once again, to calcium carbonate (CaCO3). In this process the mortar hardens progressively, from the area in contact with the air inwards, which gives it its characteristic flexibility.

The main differences between hydraulic and aerial lime are:

  • Air lime: It comes from limestone (CaCO3) with a low percentage of impurities. Once it has dried, it hardens slowly by carbonation in contact with air. It is not recommended for use in humid environments or in interventions that require short-term resistance.
  • Hydraulic lime: Hydraulic lime has less lime, either because it comes from less pure stones, or because of the addition of other products (clays, pozzolans...). Hydraulic lime first sets by hydration of the clays in contact with water and then hardens by carbonation of the lime in contact with air. This double reaction gives it short-term resistance and the possibility of working in humid environments without losing the plasticity and flexibility of air lime.

Advantages of lime

The advantages of lime are manifold:

  • Sustainability and recyclability: In the calcination process, the temperature does not exceed 1000º and therefore not much CO2 is produced. On the other hand, in the carbonation process, the lime absorbs CO2 and returns to its original limestone composition. This process and the absence of additives make it a recyclable material.
  • Durability: Lime mortars last longer than cement mortars. Lime mortars deteriorate quickly for various reasons, whereas well manufactured and applied lime mortars can last for centuries.
  • Sanitary properties: As a caustic product, it prevents the proliferation of micro-organisms.
  • Ease of use: It has low shrinkage and good adhesion. It is plastic and easily manipulated, allowing a wide range of textures and chromatisms.
  • Insulation: As it hardens progressively, from the outside in, it is a "stratified" material, with layers of different hardness, which makes it a good thermal and acoustic insulator.
  • Water vapour permeability: It is porous, which allows the building to "breathe" and prevents condensation.
  • Compatibility: Lime mortars have low mechanical resistance, which makes them compatible with different substrates and therefore suitable for the rehabilitation and restoration of historic buildings.
  • Flexibility: The carbonation process from the outside to the inside makes it a flexible material, capable of absorbing small movements without cracking, thus preventing water ingress.

Uses of lime

Lime mortars are widely used in all types of construction work, but especially in rehabilitation and restoration of historic buildings. Some of the uses of lime in construction:

  • Exterior and interior coatings (plasters, stuccos, paints...)
  • Consolidation grouting
  • Stone and brick grouting
  • Tile, brick and stone laying
  • Regularisation of walls

At Antana we prescribe the use of lime and the recovery of industry and craftsmanship derived from its manufacture and application. It is a recyclable, durable, hygienic, breathable, flexible and easy to apply material. Its physical properties make it a material particularly suitable for rehabilitation and restoration.

This post has been written incollaboration with Julio Barbero, a company specialised in the manufacture and application of lime mortars.

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