Hydraulic lime: an introduction
Practical Action
with significant variation in properties such as strength and setting time, even with a high
level of quality control during production. Allowance for such a variation should be made in
using the lime.
Firing the raw materials
Heating of argillaceous limestones is done in order to:
drive off carbon dioxide gas to produce quicklime, as in the production of ordinary
lime,
promote a chemical reaction between the quicklime and the minerals in the clay
component to yield calcium silicate compounds.
The latter reaction begins if firing is done at temperatures between 50 and 100°C higher
than when firing ordinary limestone. Kiln temperatures for production of hydraulic lime
can be as high as 1200°C.
Most designs of kilns for the production of ordinary lime should also be suitable for producing
hydraulic lime. However, burning times will be shorter for hydraulic lime because less carbon
dioxide needs to be driven off. When using wood as fuel some softwoods may not have
sufficient calorific content to reach the temperatures required. In this case, the choice would
be to use another fuel such as coal, use, wood from a different species of tree, or convert the
available wood to charcoal.
Hydraulic limes of different properties can be produced by burning the kiln to different
temperatures. Hence, it is not possible to give specific information on quantities of fuel
required or on firing times. These values will also depend on the type of raw materials which
are used and the performance of the kiln and, for a particular project, can be determined only
by trials and from experience.
Hydrating Hydraulic Limes
Sufficient water should be used to hydrate the available quicklime but not cause calcium
silicate components to start to set. Ideally the hydration process should just convert the
quicklime lumps to a fine dry powder. Hydration can be done by hand on a firm level surface
or in a mechanical hydrator. The hydration reaction is generally less violent and a lot slower
than with ordinary quicklime and, in some cases, hot water needs to be used or even, with
highly hydraulic lime, grinding of the quicklime before hydration.
With the quicklime containing between 10 and 15 per cent free or uncombined lime, most
of the product can be converted to a powder on hydration. However, there may be some
clinker-like lumps, known as grappiers, left particularly if the limestone has been fired
close to the upper end of the temperature range for hydraulic lime and the raw material has
a high clay content. These need to be removed on a screen and ground down separately.
The product can then be used as a cement on its own (known as natural cement or Roman
cement), in which case it will have properties approaching those of Portland cement, or it
can be blended back with the hydraulic lime to give it slightly higher strength and a shorter
setting time. This blending process allows the producer to meet strength standards even
with a variable raw material. Hydraulic lime must be stored in a dry place and preferably in
sealed bags if it is not going to be used straight away.
In production of Portland cement all of the clinker produced at the kiln stage is ground to a
fine powder in expensive ball or tube mills. In contrast, hydraulic lime requires no mill or,
possibly only a small mill for grinding down grappiers, or quicklime for hydration. Because of
these and other less complex equipment requirements, hydraulic lime is much more suitable
for small-scale production than Portland cement.
Using hydraulic lime
The main use of hydraulic lime is as a mortar. However, it can also be used as a render or
plaster, in floors and ceilings, as a cement in making blocks and in unreinforced lime
2