Mud as a mortar
Practical Action
Masonry structures
Adobe blocks, used in wall construction, are usually bonded with a mud-based mortar, but
mud mortars can also bond walls of field or cut stones, compressed earth blocks and fired
clay bricks.
Mud mortars are also used for arches, vaults and domes built with these materials and the
fact that they can be built without any form of shuttering is an acknowledgement of the
adhesive strength of mud mortar
Wattle and daub
In wattle and daub construction mud mortar is used to fill in a secondary framework
supported by the primary structure of a building. This framework can be single skin - the
mortar simply applied to the panels of interlocking wood, cane or bamboo strips; or double
skin, with the mortar sandwiched in between two panels.
Plasters and renders
Mud-based mortars can be applied to masonry, monolithic walls and wattle and daub, and
even to flat roofs, vaults and domes. They function as a waterproofing coat and also improve
the appearance of a building. External renders are liable to wear away at a rate depending on
the harshness of the exposure conditions. They require regular maintenance and periodic
repair, although if well-protected they can last a very long time indeed.
Figure 2: The good cohesion of mud mortar allows construction of vaults and
cupolas without shuttering. Photo: CRATerra-EAG.
Limitations of mud mortars
Mud mortars have a rather low tensile strength and are subject to shrinkage. With an
increasing proportion of clay in a soil, as well as clays with high plasticity, the tensile
strength will increase but with the penalty of increased shrinkage. Above a particular clay
content shrinkage effects will cause large numbers of microcracks in the mortar which will
reduce its bond strength and therefore the strength of the whole structure, and possibly
cause visible cracks to appear in the structure several days after construction.
If using a soil of high shrinkage for building it is usually possible to take account of the
effects of shrinkage in the building design and, indeed, this has traditionally been done
empirically based on experience in areas where earth construction is practised. Recent
trends to reduce construction costs by reducing the bulk of structures, labour requirements
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