Brian Morton discusses the different types

of lime mortar and the do’s and don’ts

of using them successfully

A CONSERVATION ENGINEER’S

VIEW ON THE USE OF

LIME MORTAR

I have attempted to summarise my

view at the request of various conser-

vation professionals, following the

presentation of a paper to the National

Trust following difficulties with the

use of hydrated lime mortars which

were referred to me. In doing so I have

used the commonly used names for

the various types of lime for conven-

ience only; they may not be chemi-

cally correct.

HYDRATED LIME (Fat Lime delivered

to site in powdered form rather like

cement)

It is accepted that hydrated lime mor-

tars have been used down the centu-

des normally to a 1:3 mix. The ques-

tion asked is why failures have oc-

curred in recent times. The problem,

in my opinion, is that the modem hy-

drated limes are produced to the cur-

rent British Standard and are too pure.

The sands used today are also well

washed and contain no impurities of

significance. The result of this is that,

whereas the traditionally specified

mortars contained a natural pozzolana

in the form of clay or silt in the sand or

lime which gave it a chemical set, this

does not apply with the normal pure

hydrated limes provided by suppliers.

The carbonation process (air drying)

that occurs with our modern hydrated

lime means that the set is considerably

delayed, causing the problems that

have been experienced.

HYDRATED LIME (Fat Lime delivered

to site as a putty)

The comments above, in principle,

apply to the use of putty, but the differ-

ence, which is quite significant, is that

the carbonation process starts by the

oxygen in the water (immediately wa-

ter is added to the mix) and thus it can

set off fairly quickly in brickwork when

used. Even these putties have some

problems due to their purity. Experi-

mentation is necessary.

HYDRATED LIME (Fat Lime slaked on

site)

Lime can be delivered to site in lump

form and can be slaked on site by

mixing with water and left to stand for

at least two weeks after the slaking

process is finished. It can then be used

in the normal 1:3 mortar mix. Again, as

with the putty mix, this will start its

carbonation process before being used

and so may set off quite quickly. These

putties tend to contain impurities and

thus are generally better than putty

delivered to site in a tub.

HYDRAULIC LIME

Hydraulic limes are available from UK

sources in limited quantities. They are

also available from Europe but there

are some doubts about the qualities of

French hydraulic limes which, in some

cases, apparently, have some cement

added. Generally, the hydraulic limes

from Italian sources come from an-

cient pits and are considered to be

reliable. Care is required in using these

hydraulic limes because if Eminently

Hydraulic the chemical set can be very

rapid producing potential shrinkage

and problems associated with too

strong a mix. The suppliers’ instruc-

tions should be followed.

POZZOLANA

In order for there to be a chemical set,

rather than an air set, the lime mortar

needs a setting agent. Historically, the

Romans certainly used a pozzolanic

additive to give the mortar a set but

also, through the centuries, brick dust,

selected stone dust, ash and the natu-

ral impurities in the lime and sand have

given mortars the chemical set re-

quired.

The use of what is loosely called a

pozzuolan to any lime mortar mix is

historically correct and we know that

these mortars work well giving the

necessary flexibility to our historic

buildings in combination with gener-

ally soft construction. English Heritage

has promoted tests of the use of

pozzolanic additives to lime mortars

and the Building Research Establish-

ment are carrying out welcome tests

on these mortars.

Brick and stone dust are readily

available and it is suggested that if it is

proposed to use these additives tests

are carried out on these mortars to

establish their practical properties for

particular situations. Sample mortar

mixes can be made up on site to satisfy

the requirements of the architect and

the bricklayer carrying out the work.

There are further comments related to

the sand in these mortar mixes under a

separate heading, but it is important

that the bricklayer gets the correct

workability with the mortar that he

uses.

PORTLAND CEMENT, LIME, SAND

MIXES

Portland Cement used simplywith sand

has proved to be a disaster. The ce-

ment sand mortars, particularly the

stronger mixes such as 1 (cement) to 3

(sand) are not porous. They shrink as

they set and thermally they move at a

different rate to the soft bricks and

mortars used in historic buildings. They

further cause erosion of soft stones

and brickwork where cement sand

pointing has been used.

There is abasis forusingthese harder

mortars as rendering and jointing

material for granite or impervious

stone. Experience in Jersey suggests

that finely jointed granite walls work

well with cement mortars.

Weak properly gauged Portland

Cement lime sand mixes have similar

strength characteristics to the 1:3 lime:

sand mortar mixes. These gauged

mortar mixes 1 (cement): 1 (lime): 6

(sand) 1:2:9 and 1:3:12 have been

thoroughly tested and are acceptable

under the British Standards. I accept

that mixes with less cement in than

those specified above are probably

unacceptable related to the

cementatious portion of the mix not

being adequate to cover all the grains

of sand. It wifi be realised that the

gauged mixes are 1:3 of cementatious

material related to the sand content in

all these mixes.

The use of 1:3:12 cement:lime:sand,

weak mix, is directly comparable with

1:3 lime:sand mix, in strength terms,

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