An introduction to lime - The Hempcrete Topic: Designing and Building with Hemp-lime - page 60

These pozzolans all contain a form of very finely divided clay, which combine with some
of the free limes present in air limes to produce a hydraulic effect similar to that achieved in
naturally occurring hydraulic limes. Examples of pozzolans include certain volcanic ashes
(such as pozzolana, found in Pozzuoli, near Naples in Italy, which gave its name to this
type of additive), crushed clay brick dust or pulverized fly ash (ash produced by power
stations that burn pulverised coal).
The use of pozzolans to create artificial hydraulic limes continues today, especially in the
UK, where there has always been a drive to achieve a more hydraulic set than is achievable
with locally occurring materials.
Pozzolans can be added to air limes by the builder at the point of mixing or using the mor-
tar or render, and are sold in their raw state from traditional building suppliers for this pur-
pose. In addition, some companies have produced ready-mixed artificial hydraulic limes
from dry hydrate of lime (sold as 'hydrated lime') and pozzolans. Such limes are known
as 'formulated hydraulic limes' (usually called either FL or HL - to distinguish them from
the 'natural' NHL), and are usually sold through specialist building material suppliers.
Portland cement
The increased interest from the 1750s onwards in the properties of different building limes
led to many artificial limes being synthesized and patented in an effort to create mortars
with particular desirable characteristics.
The most significant development was the patenting of Portland cement, by Joseph Asp-
din, a Leeds bricklayer, in 1824. Portland cement is formed by the burning of specific
quantities of limestone and clay-containing materials at very high temperatures. Since it
has many of the same setting properties as natural cements, it took a long time for Portland
cement to replace lime in the construction industry, owing to its initial high cost and the
fact that limes had always been available locally. But its use really took off from the early
twentieth century, and by the post-war period of the 1950s and 1960s, ordinary Portland
cement had all but replaced hydraulic lime for mortars, renders and engineering applica-
tions in UK construction.
The main advantage of Portland cement over most building limes was its hard and predict-
able set, giving mortars with greater structural performance and strength in tension com-
pared with the softer-setting lime. Also, cement's shorter curing time meant that builders
could go on working into the colder months; masonry construction had traditionally been