An introduction to lime - The Hempcrete Topic: Designing and Building with Hemp-lime - page 57

'Natural hydraulic lime' (NHL), produced from naturally occurring impure chalk and lime-
stone (in contrast to 'formulated hydraulic limes' - see overleaf), is sold in a range of
strengths, for example feebly hydraulic (NHL 1, NHL 2), moderately hydraulic (NHL 3.5)
and eminently hydraulic (NHL 5). These are sold in specialist builders' merchants in the
UK, and more recently NHL 3.5 is also sometimes stocked, or at least available to order, in
general builders' merchants. Hydraulic limes are easier to use than air limes for those who
have experience of using Portland cement, since their preparation and application is more
akin to that of cement.
Natural cements
In limestone in which a very high level of impurities is present, there is sometimes not even
enough free lime to enable the lumps of burnt lime to break down on slaking. These types
of hydraulic lime are instead ground to a fine powder, to which water is added to form a
mortar. They set very quickly and give a very high strength on the addition of water, which
has led to their description as 'natural cements'. In fact, natural cements are so quick to set
that a retardant chemical is often added at the point of mixing, to give a longer working
time.
Such extremely hydraulic limes were used by the Romans in many of their large-scale en-
gineering works, and one particular naturally occurring cement was patented and marketed
as 'Roman cement' by James Parker from the end of the eighteenth century. No seams of
stone suitable for the production of natural cements exist within the UK; the nearest loca-
tion in which they are produced is southern France.
One natural cement on the market today, which is now sold as a hempcrete binder, is
Vicat's Prompt Natural Cement (see Chapter 6, page 80 ). Prompt is supplied with a re-
tardant, Tempo (powdered citric acid), which is added on mixing to delay the setting and
increase working time.
Other than the difference in production process (grinding rather than slaking) and the ex-
tremely fast and hard set, natural cements are largely the same as other hydraulic limes.
They retain the important property of vapour permeability, but they are hydrophobic - they
repel liquid water.