Alternatives to Portland cement
Practical Action
The technical and economic advantages of alternative cements are not lost on architects and
engineers from developed countries. Increasingly, architects, are becoming aware of the
brittleness associated with Portland cement mortars, for example, and are now specifying
blended lime/Portland cement mortars instead. As well as re-discovering the ‘lost arts’ of
using alternative binders, recent research has enabled the properties of alternative binders to
be thoroughly investigated and catalogued. A body of experience has built up on the
appropriate application of traditional binders such as clay, lime and pozzolanas, not only in
the repair and conservation of historic buildings, monuments and structures but also in
adventurous and innovative new build applications.
In some developing countries traditional binders are still slighted, probably because they
might be associated with poverty or considered to be low status materials. Their performance
and technical specifications might, completely unjustifiably, also be considered inferior to
Portland cement, they might not be widely produced or available, or the skills to produce and
use them might well have disappeared. A good case can be made for disseminating the
developed country experience to the South more widely. This would increase interest and
awareness of alternative binders, allow producers and users to gain skills and confidence and
determine the rightful place of alternative binders in appropriate building for sustainable
development.
Types of alternative cements
Lime
There are two forms of lime: quicklime and hydrated lime.
Quicklime is produced by heating rock or stone containing calcium carbonate (limestone,
marble, chalk, shells, etc.) to a temperature of around 1000°C for several hours in a process
known as ‘calcining’ or sometimes simply ‘burning’. It is an unstable and slightly hazardous
product and therefore is normally ‘hydrated’ or ‘slaked’, by adding water, becoming not only
more stable but also easier and safer to handle.
To produce dry powdered hydrated lime just sufficient water is added for the quicklime lumps
to break down to a fine powder. This material would have a ‘shelf life’ of only a number of
weeks, depending on storage conditions. ‘Old’ hydrated lime would have partially carbonated
and become a less effective binder.
However, if quicklime is hydrated with a large excess of water and well agitated, it forms a
milky suspension known as milk of lime. Allowing the solids to settle, and drawing off the
excess water, forms a paste-like residue, termed lime putty, which is the form of lime which
can be used in building applications to best effect. This will keep almost indefinitely and, in
fact, improves with age. In most countries, though, lime is most widely available as a powder,
due to its widespread utilisation in process and treatment industries rather than in
construction. Lime putty, which needs a stiff bag or container for transportation, is more
rarely produced.
Limes with high calcium content, often called ‘fat’ or ‘white’ limes are desirable for most
industries, although the construction industry can use limes containing impurities. For
instance, limestones containing a proportion of clay are often seen as an advantage in
building as they produce hydraulic limes which will set under water and will produce stronger
mortars.
In the construction industry, lime, in its hydrated or putty form, is mixed with aggregate and
water to produce concrete or mortar in the usual manner. Lime putties generally produce
mortars and renders of excellent quality and consistency.
Plain lime-sand mortars are quite weak; any early adhesive strength results from drying out,
and longer term hardening occurs through the action of the air’s carbon dioxide on the lime.
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