Mortar is a homogeneous mixture formed by combining sand, water and an inorganic binder, which may be lime, gypsum or cement.
This variability in the binder is what gives rise to the classification of mortars according to their composition, and this will be discussed in detail later on.
Composition of mortar
In basic terms, the composition is defined by three elements, although it is worth noting that additives can be included, which give the mortar specific properties, resulting in a wider variety of mortar types.
The key characteristics of each component are explained below:
Conglomerant
This is the key component, as it is responsible for creating a complete bond between the sand and the water, producing a homogeneous and stable substance.
Commonly, its structure is composed of very fine particles such as a powder, or it may have a pasty consistency structure.
The most commonly used binder is cement, followed by lime and gypsum. When any of these substances reacts with water (hydration), it produces a mortar mixture in a plastic state.
This will subsequently become rigid, due to the fact that the paste has been lodged in the interstitial spaces of the sand.
Statistically, the amount of binder is directly proportional to the shrinkage of the mortar.
Sand
Unlike the previous one, this component sand, makes up the largest proportion of the mixture, ranging from 40% to 80%.
It is a mineral and granular material, which can be limestone or siliceous in nature and whose maximum size must not exceed 4 mm.
This component is the cornerstone of mortars, the basis on which the hydration of the agglutinant is carried out, as it is an inert material, unable to react chemically with other compounds.
The sand controls shrinkage and contributes to mechanical strength.
Water
This component is responsible for creating the chemical environment necessary for the hydration of the binder; as such, it is the primary factor determining the workability of the mortar.
The amount of water in the mix varies depending on temperature conditions, the binder, the amount of sand and the desired consistency.
In addition, this amount is inversely proportional to the mechanical strength of the mortar, but is directly proportional to its shrinkage and porosity.
The mixing water must have a minimum pH of 5, as well as low concentrations of sulphate and chloride ions, carbohydrates and organic compounds.
Additives
Additives may be organic or inorganic in nature, and they alter the properties of the mortar’s plastic state. They may be liquid compounds (emulsions) or powders.
Most of them are rheological or setting modifiers. And their proportion in the mortar mixture depends on the amount of cement; on their weight.
Complementarily, there are some compounds called additions, which configure the characteristics of the hardened state of the mixture.
There are even pigments and fibres that alter other aspects of traditional mortar.
Types of mortars
The most common classification of mortars is based on the type of binder.
The main types of mortar are as follows:
Cement Mortars
These are mortars with the typical composition of sand and water, with cement acting as a binder. They are characterised by their high compressive strength.
Their workability therefore depends on the proportion of cement and sand; for this reason, sands containing traces of silt and clay are often used to improve this aspect of the mixture, although this does affect its strength.
Lime mortars
This is traditional mortar, consisting of sand, water and lime, which may be air-setting or hydraulic. Air-setting lime is the most commonly used, with either white lime or grey (dolomitic) lime being employed.
Sand plays a very important role in these mixtures, as it must help to control cracks caused by the contraction of the mortar during the carbonation reaction.
Bastard mortars
They are so called because they are produced by combining two binders. The most commonly used combination is that of lime and cement.
It is probably the most efficient type of mortar, due to the fact that it acquires a suitable consistency for workability. In addition, it has a good water retention and a high resistance value.
However, the magnitudes of each characteristic depend on the numerical dependencies of the components.
Uses of mortar
Generally speaking, mortars can serve a structural purpose, meaning they can be used in the construction of structural elements, or they can perform masonry functions, acting as a binder or filler.
Furthermore, mortars can be used as coatings, such as render, plaster or screed.
Taking into consideration the aforementioned, we have a further classification of mortars, this time according to use:
- Bonding mortars: due to the specific requirements of the construction system of which they form part, they must possess special properties, such as considerable strength to absorb stresses.
- Filling mortars: ideal for masonry, these mortars are typically used to fill the spaces between bricks in walls, for example.
One requirement is that they must develop a good resistance, comparatively similar to the previous mortars.
- Coating or rendering mortars: their function is purely aesthetic, ensuring a uniform and smooth finish on the surface of the structure in question.
Strength plays almost no role; consistency is the most important factor.