Agriculture Reference
In-Depth Information
recommend the use of an hydraulic lime and well-graded aggregate, again no
larger than one third of the joint size, to a ratio that would be subject to the sup-
plier, but probably one part binder to two parts aggregate.
Feebly hydraulic lime traditionally obtained from Grey chalk was the preferred
binder for gauged work because it had very good workability and the strength
of final set was not too strong for the rubbing bricks. Like the other and
stronger classes of hydraulic limes, it hardens by both an internal set (due to
reactive minerals within the grey chalk) as well as through long-term carbon-
ation. Grey chalk limes are not currently available in the UK but fortunately
at the time of writing, Singleton Birch (Single Birch Limited, Melton Ross
Quarries, North Lincolnshire; e-mail: sales@singletonbirch.co.uk) are trialling
production of the first true traditional feebly hydraulic building lime since
Totternhoe ceased manufacture of their grey chalk in 1993.
Specifying any mortar must always be based on suitability of purpose, the
brick type, nature and performance of the construction and degree of expos-
ure, so that it is of sufficient strength, but no stronger - a mortar should never
exceed the strength of the brick it binds - and can perform satisfactorily. In the
absence of hydraulic grey chalk limes within the present range of modern NHL
products one would only use the NHL2 as a hydraulic binder for gauged work,
the other two remaining classes being far too strong. The range of strength for
NHL 2 under BS EN 459 at present is broad and the majority of the strength
requirement overlaps with NHL 3.5. It is therefore necessary to obtain test
results and guarantees of the final strength from the manufacturer before
deciding whether the lime is suitable for gauged work. Fortunately, following
many years of lobbying by The Building Limes Forum (Edinburgh; e-mail:
admin@buildinglimesforum.org.uk <mailto:admin@buildinglimesforum.org.
uk>; www.buildinglimesforum.org.uk) and the efforts of the British Standards
Committee on Building Limes, the next revision to BS EN 459, shortly to be
published, will include a new category of NHL 1. This will be a new truly feebly
hydraulic lime classification which will enable the traditional grey chalk and
feebly hydraulic limes to be unambiguously specified once again. As this lime
comes bagged, powdered, hydrated, to create a putty it has to be slowly added
to a container partially filled with clean water and thoroughly whisked to the
consistency of thick cream, then left to 'fatten up' for a maximum of 24 hours.
After this period it would then be re-worked, or 'knocked-up', back to that
condition and silver sand added ready for use; substantiated in the technical
literature of hydraulic lime producers such as St. Astier. Any left over at the
end of a working day must be discarded and new putty made.
Pure or high-calcium lime, however, as it has no internal set, is always best
left for a longer period of storage, under a film of water - termed limewater -to
prevent carbonation; the longer the better as it matures and increases in bulk
