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CLOSE THIS BOOKStabilizers and Mortars ( for compressed earth blocks) (GTZ, 1994, 20 p.)
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The term "mortar" is collectively used to refer to bonding mortar in masonry, interior plasters and external renderings of buildings.

A mortar is a mixture of a fine aggregate filler (eg sand) and a water activated binding medium (eg lime or cement, or a combination of these), possibly with an additive (such as a colouring agent, fibres, a lightweight aggregate, or a chemical composite). Water is added to the dry mortar mix, just enough to activate the binder(s) and make a workable paste, which hardens when the binder sets and dries, but is applied while it is plastic.

bonding mortar / plaster / render


aggregate + binder (+ additive) + water

Note: Earth is, in fact, a natural mortar. The clay portion acts as the binder and the inert components (silt, sand and gravel) as the aggregates.

Types of Mortar

The proportion of the different materials making up a mortar will vary depending on the use to which the mortar is to be put.

Bonding Mortars

In horizontal masonry joints, the function of the mortar is to

spread the vertical loads exerted in the wall, and

compensate for poor workmanship and any dimensional variations in the blocks, so that they can be laid in more or less horizonal and vertical alignment.

In horizontal and vertical joints, it serves to

fill the voids between blocks, preventing air and rainwater from passing through

At the foot of the wall, it has an additional function, which is to

prevent water from rising up from the soil by capillary action. In this event, it has to be waterproof and is referred to as an anti capillary barrier, or waterproof joint.

In general terms, mortar used for bonding ensures that

blocks are bonded together, and by friction gives the wall a certain resistance to eccentric or lateral forces.


Plasters are layers of mortar applied to interior walls and ceilings to cover up uneveness in the background, providing smooth hygienic, crack free surfaces.

A plaster is typically appllied in several coats: one thin coat (forming a key) to ensure that the plaster adheres to the surface, a main coat (the body of the plaster) to ensure that the surface is impermeable and to make it smooth, and a thin surface coat (finishing coat) for appearance and to achieve complete impermeability. Each of these coats has a specific function and a particular mortar should be selected to suit each one.


Renders are principally the same as plasters, but applied on external building surfaces, not only to protect them against weathering, rain penetration and mechanical damage, but also to cover up unpleasant and uneven surfaces, or as decoration.

The Need for Mortar in Building with Earth

The use of mortars for laying blocks and/or protecting walls is a significant cost factor, which needs to be considered carefully, before deciding whether or not it can be done without.

Bonding Mortars

The development of semi-industrial presses, allowing the manufacture of blocks of very accurate dimensions has led certain manufacturers to introduce building systems, which do not require the use of a masonry mortar, for instance, by using interlocking blocks, which can be laid dry, or blocks that can be bonded by an adhesive. The short-term economic advantage is attractive. But laying the blocks remains a manual operation and can, therefore, never be completely perfect. Using a mortar enables imperfections in laying to be rectified and allows each course to be level. In addition, the bond between the blocks created by the mortar is important to the final strength of the building; comparative tests have shown that the compressive strength and shearing strength of the construction can fall by 50%, if no mortar is used. Finally, a mortar will also ensure that the joint is windproof and that water cannot infiltrate between blocks. Hence, the use of mortar for laying blocks or bricks is generally recommended.

Plasters and Renders

A great many earth buildings, even in exposed areas, are never rendered. It is, therefore, wrong to think that the durability of a building is systematically linked to the presence of a render.

The first function of a plaster finish or rendering is to improve the appearance of a building. If rendering is to be avoided, the building must be attractive in itself This will only be achieved by the careful use of high quality materials and workmanship. Hence, in order to economize on plasters and renders, it is often necessary to invest in the training of manufacturing personnel and builders, end to encourage the development of their professional pride.

External rendering has an additional function, namely to protect the body of the wall against weathering. In order to avoid the need for rendering, either the facade must be sheltered from inclement weather, or the wall must be made resistant to weathering. The first of these measures implies that the problem has been addressed at the architectural design stage, through the use of overhanging roofs, organic or artificial barriers, and suitable orientation. The second presupposes that the building materials or the material making up the wall have been reinforced and made insensitive to the effect of water. Therefore, at the production stage, stabilization procedures are used, either throughout or solely for the surface of the wall. In practice, several solutions are often used in combination, making it possible either to eliminate the need for rendering or reduce the stresses it will have to withstand. This simplifies the formulation of the mortar mix and gives a greater guarantee of its viability over a significant period of time.

Main Characteristics of Mortars

The main characteristics of mortars are


workability, in the ease with which it can be applied;

adhesion to the blocks;

compatibility with the blocks, in terms of natural moisture permeability and mechanical cohesion;

impermeability, either only to prevent rainwater penetration or to provide complete waterproofness;

compressive strength;

resistance to shocks and impact;

capacity to withstand cracking caused by

- movements in the masonry structure,

- differential dimensional variations in the masonry structure due to rapid variations in temperature and humidity,

- internal stresses in the mortar as it dries and sets.

A mortar's capacity to withstand cracking is linked to its capacity to accept deformation, which is measured by its modulus of elasticity, (E). Elasticity is the capacity of a mortar to accept deformation without breaking. The more it can accept deformation, the lower is its modulus of elasticity.

Specific Characteristics of Earth Walls

Even a cursory understanding of the principles of preparing a masonry or plastering mortar, or protective coat (such as whitewash), clearly shows the importance of suiting the composition of the product to the nature and physical and mechanical characteristics of the material it is applied on. Most often, problems encountered with plasters on earth buildings are due to the composition of the mortar being unsuitable to the specific nature of the materials, which is either poorly understood or not taken into account.

The characteristics of earth as a building material, which need to be taken into account are essentially as follows:

Shrinkage during drying and settling

During drying out, the withdrawal of water often results in dimensional shrinkage (0.2 to 1 mm/m for compressed earth blocks); it is therefore preferable to wait for a month after completing the masonry work. By waiting still longer, other problems can also be avoided, especially those resulting from general movements in the building (mainly caused by settling), which are fairly frequent during the first year of the life of any built structure.

Dimensional variations

Earth is susceptible to significant thermal expansion (0.02 to 0.2% for stabilized compressed earth blocks, compared with 0.02 to 0.05% for concrete blocks and 0 to 0.02% for fired bricks). Swelling due to wetting will vary according to the mineralogical nature of the clay contained in the soil and the proportion of stabilizer, but can be quite significant. When subjected to a load, an earth wall changes shape perceptibly. Tests have shown that at the limit of its modulus of elasticity, an earth wall is 4 times more flexible (or less rigid) than a concrete block wall. The capacity of the mortar to accept deformation must, therefore, be at least as high as that of the blocks (ie it should have a low elasticity modulus).

Mechanical cohesion

Earth has only fairly low tensile strength. The shrinkage of a mortar capable of high mechanical performance, for example due to its rapid dilation in the event of a sudden thermal change, can lead to the surface layer becoming detached from the supporting structure.

Moisture permeability

Earth is a material which is easily penetrated by moisture. If the passage of water is blocked by a mortar, which will not even allow gases to pass through easily, this will lead to moisture accumulating up to the point of saturation, and thus result in gradual deterioration.


Blocks will display a tendency to absorb some of the water contained in the mortar. When a mortar has been prepared using a hydraulic binder, which requires a certain amount of water for hydration to take place, care must be taken that the wall does not dry out the mortar too early, and thus cause it to be poorly cured.

Surface state

The sides of compressed earth blocks are generally flat, but with a smooth surface. It is, therefore, unnecessary and not usually recommended to apply very thick plasters to achieve a plane final surface, but the plaster has to be made to adhere to the blocks, for example by using a scratch coat to create a rough surface.

General Principles for Earth Construction Mortars

Considering the basic characteristics of earth as a building material, the selection of a mortar must give preference mainly to those that ensure

the capability to accept deformation;
good moisture permeability;
mechanical performances, which are compatible with those of the blocks.

In the application stage, care should be taken to ensure that

the application surface is properly prepared;

there is a good bond between the mortar and the compressed earth blocks;

the mortar is not allowed to dry out too quickly during and after application;

extremehygro-thermal conditions or too rapid variations are avoided; (and in addition, for plasters and renderings)

building shrinkage and settling movements have come to an end.

Principal Conditions of Using Mortars

Depending on the intended use, the aim will be to seek to enhance one or the other quality of a mortar. These new qualities will, however, sometimes entail a mortar mix which breaks the general rules for the composition of a mortar, which are necessarily dictated by the characteristics of the soil. An ideal mortar with all the desired qualities is difficult to achieve. Compromise solutions will have to be sought, taking into account the envisaged use and the local climatic constraints.


A bonding mortar must have adequate compressive strength and be workable. Therefore, a lean hydraulic mortar, a sand mortar with high-calcium lime, or an earth mortar to which coarse sand and a stabilizer have been added, can all be used. A lean, unstabilized earth mortar can also be considered for well-sheltered walls. It should be noted that the masonry mortar should have the same compressive strength and resistance to erosion as the blocks. If the resistance of the mortar is less, erosion and infiltration of water will occur and the wall will deteriorate. Lime, which results in better workability and average mechanical strengths, is preferable to hydraulic binders. Only in the case of waterproof joints is it advisable to use a very compact mortar based on a hydraulic binder, with the addition of a high proportion of a water-repellent product wherever possible.

Mortars for Plasters and Renders

The composition of a plastering mortar will depend on whether the mortar has to be impermeable or not.


All that is necessary is good adherence and the ability to withstand cracking, as well as a pleasant appearance. Thus, very simple mortars, such as lean earth mortars with the addition of sand, lean mortars made with high calcium lime, gypsum plaster mortars, or lime and gypsum plaster mortars, are all suitable.


This must be both compact, in order to be impermeable, and must be able to withstand cracking - in other words, it must be supple. These qualities are very difficult to find united in a single mortar. That is why it is preferable to apply several layers and to change the composition of the mortar for each layer.

Main body of the rendering

The main layer should be fairly compact to ensure its impermeability and, therefore, should have a fairly high proportion of binder; at the same time, its mechanical performance must remain lower than that of the material it is applied on. This is why pure hydraulic binders, which would impart high mechanical strengths, are not recommended. Mortars comprising a combination of different binders are preferable (eg high-calcium lime and Portland cement, with the proportion of lime being higher). What is important is to ensure that the modulus of elasticity is below that of the earth block. The proportion of binder can be lowered, or alternatively, the proportion of binder can be maintained, whilst the density of the aggregates is lowered, ie with a lightweight aggregate. The thickness of the main layer should be sufficient to prevent the passage of water, but at the same time it should not be too thick (in too heavy for the application surface), especially when it is made with mortars based on high-calcium lime (in which case, the carbon dioxide of the atmosphere, which is essential for the hardening process, cannot easily penetrate the mortar layer and curing is delayed).

In the finishing coat, small cracks are likely to appear after the main body of the render has dried out. These can be filled with a fine and very supple coat. The proportion of binder used in this final coat will, therefore, be lower than that used for the main layer.

Finally, in the scratch coat or key, it is important to ensure that the plaster adheres to the wall. However, increasing the ability of the main layer of mortar to withstand cracking (by amending the proportion of binder), also reduces its ability to adhere. An intermediate scratch coat or key is, therefore, used very often. This does not contribute to impermeability, and cracks that may appear can be filled after the main layer and finishing coat have dried out. The scratch coat can, therefore, be made with a mortar containing a high proportion of hydraulic or mixed binder. The only constraints are that the passage of moisture should not be blocked and that excessive tensile stress should not be transmitted during drying out. This means that the coat must be thin and discontinuous to allow moisture to escape through the cracks and pores that appear during drying out. With application on surfaces that are brittle or damaged, the scratch coat should be replaced or complemented by the use of fixing points such as broken pottery or nails driven in at different angles over the whole surface.


To withstand impact and continuous abrasion, a "working surface" mortar for floors should contain a very high proportion of binder. This high binder content would result in a low capacity to accept deformation. An earth floor, however, is very susceptible to movement by swelling or settling. Therefore, it is highly contradictory to put a rigid working surface on an earth floor. A cement screed laid directly on an earth floor will crack and deteriorate very rapidly and is, therefore, not recommended. Instead it is better to place a dry stone pitching between the floor and the screed, or treat the floor directly by compaction (tamping), possibly using a stabilizer. In the latter case, regular maintenance must be ensured, or the surface must be protected by another kind of covering (ceramic tiles laid on a bed of sand, for example). Earth block pavements would need to be stabilized throughout, in order to be able to withstand normal stresses without a protective coating.


This type of mortar must have an even higher proportion of binder than the preceding type in order to be compact and to ensure that it is totally impermeable. A strong hydraulic binder would make it especially impermeable to the passage of moisture. However, its rigidity would not allow it to resist the inevitable movements in the earth-built structure and it would crack very quickly, allowing water to pass into the structure. This kind of mortar has, therefore, to be both very supple and impermeable at the same time, requirements which are very difficult to achieve in combination. The fact, that it is impossible to reliably protect a drain or cistern made of earth, explains why building them in earth is not recommended unless the question is thoroughly understood and unless certain fairly specialized products, natural or artificial, are available.

Equipment for Mixing, Applying and Testing Mortars

Equipment for mixing earth with stabilizers and other additives are described in the GATE Product Information portfolio Soil Preparation Equipment. The methods of applying earth construction mortars, plasters and renders are principally the same as for any other type of mortar. Hence the equipment required- eg trowel, splatterdash, float- is also the same. Similarly, the equipment for testing the quality of hardened earth construction mortars, plasters and renders - eg using a pocket penetrometer or pendulum scleroscope (as illustrated below) is the same as for all other mortars.