Agriculture Reference
In-Depth Information
As discussed earlier, feebly hydraulic limes of varying strengths were once
widely available in the Britain. Formerly termed 'greystone', 'grey chalk',
'grey lime' or 'stone' lime, they were preferred for setting most gauged work;
except for carved work. Some types of feebly hydraulic lime putties slaked from
grey chalk quicklime, like Totternhoe, were capable of being kept for several
weeks below a thin film of water, as the internal set was both slow and weaker
compared to others. The compressive strengths of these grey limes ranged from
a minimum of 0.7 N/mm
2
up to a maximum of 2.0 N/mm
2
(Holmes, 2006, 18).
When needed, being water retentive, the stiffened putty would re-work back to
a workable condition, needing only a small amount of clean 'potable' water and
an appropriate gauging of silver sand to be added and then mixed to the con-
sistency appropriate for setting that particular element of gauged work. By the
1970s hydraulic limes had to conform to BS 890:1972, and writing about their
manufacture during the late 1980s Michael Wingate (1988, 11) stated that:
…production in the UK seems to have collapsed because of the failure to agree
British Standards to cover such wide-ranging and variable materials.
At the time of that statement Totternhoe in Bedfordshire was the only site in
Britain that was producing grey chalk, or feebly hydraulic lime, but ceased
that line of production in 1993; this was not only due to the increasing use of
Portland Cement but also ironically because of an over-emphasis on the use
of pure non-hydraulic limes in conservation and repair works. In the years
that have followed the use of hydraulic limes has revived and enjoy recogni-
tion; though unfortunately it has been mainly through imported limes rather
than utilising domestic products. In 2002 the Standard, BS 890 was withdrawn.
Today all hydraulic limes are required to conform to BS EN 459, current from
2002, and to have the coding (NHL), which stands for Natural Hydraulic Lime,
denoting they have fired to the correct temperature and are not contaminated
in any way, and so classed a true hydraulic lime. They are classified in ascend-
ing order of strength, as NHL2, NHL3.5 and NHL5 respectively, the numbers
representing a compressive strength in N/mm². There is a common miscon-
ception that these are broadly equivalent to the traditional classifications of
feebly, moderately and eminently hydraulic limes. In fact the new standard BS
EN 459 gives NHL 2 a compressive strength of between 2 and 7 N/mm². The
withdrawn BS 890 gave the compressive strength for grey chalk limes as 0.7
N/mm² to 2 N/mm². This was the true traditionally feebly hydraulic lime. The
new NHL 2 category is equivalent to the traditional moderately hydraulic lime
and can be stronger than this. Traditional grey chalk limes, like Totternhoe
(Bedfordshire), are therefore not at present in this new Standard because they
were generally of a higher free lime content than NHL 2 currently manufac-
tured by most producers, and took longer to set. There is a great need to revive
