Table of Contents Table of Contents
Previous Page  31 / 54 Next Page
Information
Show Menu
Previous Page 31 / 54 Next Page
Page Background

BCD SPECIAL REPORT ON

HERITAGE RETROFIT

FIRST ANNUAL EDITION

31

HOME AND DRY

Developing a non-hydraulic setting air lime for the insulation

and repair of traditional buildings

HARRY CURSHAM

A

LMOST ONE in four buildings

in the UK are traditionally

constructed using lime rather than

cement. If well maintained, solid walls of

brick or stone set in a lime mortar work

well, drying rapidly after a rain shower so

damp never penetrates far into the wall.

Condensation inside the building also

dries quickly, so the walls act as a buffer

for both humidity and heat, moderating

extremes in the building.

By the end of the first world war lime

technology had largely been abandoned

in favour of faster-setting mortars. It

was only in the late 20th century that

conservationists began to realise that

these cementitious mortars were actually

damaging traditionally constructed

buildings. Traditional mortars were softer

and tolerated the natural expansion and

contraction of solid masonry without

failing. And they were highly permeable.

It was discovered that problems occurred

when old masonry was repointed, as

this introduced just a thin layer of hard

cement at the surface alone. As the core

remained flexible, even modest thermal

movement could cause the surface to

spall, as pressure is exerted across the face

of the wall. Being relatively impermeable,

cement also prevented the mortar

from wicking moisture to the surface.

In particular, the cement renders used

tended to trap moisture, and if cracked,

more moisture is drawn in by capillarity,

making the walls cold and damp.

Over the course of 60 years or so prior

to the lime revival, traditional methods

of making and using mortars were

forgotten. Text book descriptions were

often ambiguous, and a new generation of

conservators had to rely on trial and error

and on the analysis of old mortars. Today,

new discoveries are still being made.

For the retrofit sector these

developments are important because

damp walls are known to leak up to

30 per cent more heat than dry walls,

and in some cases the actual figure can

be far higher. Simply by getting all our

traditionally constructed buildings up to

a sound condition would help reduce the

UK’s carbon emissions substantially.

The types of lime used generally fall

into two categories: non-hydraulic or ‘air’

limes which set very slowly by a chemical

reaction with carbon dioxide alone; and

hydraulic limes which stiffen more quickly

due to a partial crystallisation set. One

area of great interest is in the development

of non-hydraulic hot-mixed mortars,

because they seem to be producing

mortars which are much closer in nature

to those found historically and there is a

growing consensus that these mortars can

provide the best performance in use.

While their slow set means that

air limes require more care and skill in

use, they tend to be more permeable,

and some hydraulic limes have been

shown to become almost as hard and as

impermeable as cementitious mortars

when aged. Mortars made with air limes

also offer lower conductivity and are

therefore a good insulant, all of which

make these mortars ideally suitable

for older solid wall buildings. These

properties are also significantly enhanced

when used with appropriate aggregates,

and the final result is akin to most pre-

industrial mortars found in the UK and on

the continent.

This article looks at one proprietary

product which has recently been

developed. Although a form of non-

hydraulic calcium hydroxide, it is supplied

as a dry powder and when water is added

the mix stiffens to provide a preliminary

set without any addition of setting agent

or pozzolan. The set is referred to by

its manufacturer as a ‘Vivus’ set, which

results from the way that the quicklime

is manufactured and slaked. There are no

clay impurities in the limestone used to

make it, and none are added, so in essence

it remains a pure air-lime and within

the normal ‘lime cycle’. Carbonation is

a secondary setting process in that it

is unnecessary for construction work

to proceed, but adds strength in the

long term. Like a hydraulic lime, it

will continue to carbonate over the

following months and years, depending

on the depth of material. In the process

carbon dioxide is absorbed from the air,

completing the lime cycle (see Figure 1).

This non-hydraulic setting mortar is

able to perform in the application stage

A traditional roughcast lime render on a solid masonry wall