RECOMMENDATIONS FOR DEVELOPMENT OF A STABILIZATION
MORTAR FOR THE PRAETORIUM RECONSTRUCTION/CONSOLIDATION
Edith Dunn
Although most structures at Umm el-Jimal appear
to have been constructed with "dry-laid" masonry, mortars were used
selectively in certain areas during the Byzantine and Druze periods.
Since early masonry mortar at Umm el-Jimal appears to have included
a mud mortar, this would be the preferable choice for use as a pointing
material. It is the most visually compatible and is consistent with
historic techniques. The pointing should be done first to prevent
any leakage of the grout during installation. However, due to the
advanced state of deterioration, a slightly stronger formulation
is recommended for the grouting of wall cavities.
The grout itself may be developed from any number
of formulations which should be tested before installation for pH,
soluble salts and compressive strength. The compressive strength
should be significantly lower than that of the masonry material
itself, but strong enough to impart structural stability to the
wall. Long term mechanical and chemical stability is extremely important
and should be considered carefully when choosing a grout. It is
especially important to avoid sulphates and soluble salts.
Grout leakage will be unsightly where white cement
is used. Therefore, if inert colorants are available, the grout
should be tinted dark gray to blend with the basalt.
The following are some possible approaches for the formulation of
a compatible restoration gout at Umm el-Jimal.
1) Cement/Lime mixture - Must use a specially
formulated low alkali, low sulphate cement. Use 1 part
hydrated lime [slaked lime, Ca(OH)2] plus 4 parts specified
cement. If fillers are used use only inert (chemically
non-reactive) materials. Do not use fillers which contain sulphates
or which continue to expand over time.
2) "Hydraulic Lime" mixture - Must use a specially
formulated low alkali, low sulphate cement. Add calcium carbonate
(ground limestone which has not been burned) to cement mixture
as required to achieve compressive strength similar to cement
above.
3) Pozzalonic Mortar, Type "A" - Develop special
mortar using lime and ground pumice. This type of mortar may already
have a precedent at Umm el-Jimal in antiquity. Pumice generally
has good reactive qualities and this type of formulation is preferred
over Type "B" pozzalonic mortar (below). Since reactivity of different
basalt and pumice varies, this formulation will require testing
to determine precise recipe proportions. The addition of coarsely
ground pumice as an inert aggregate will make the mortar lighter
due to the large number of air pockets contained within the pumice.
4) Pozzalonic Mortar, Type "B" - Develop
special mortar using lime and finely ground terra cotta bricks
or pottery. Note that the terra cotta used in this type of formulation
is less reactive than pumice. Since reactivity of different terra
cotta materials varies, this formulation would require testing
to determine precise recipe proportions.
For Further Reference:
Ashurst,
John & Nicola. Practical Building Conservation, Volumes
1-5, Halsted Press, NY, 1988.
Hill, Neville, ed. Lime and Other Alternative
Cements, Intermediate
Technology Publications, London, 1992.
The Smeaton Report, English
Heritage, 1995.
Torraca,
Giorgio, Porous Building Materials: Materials Science for
Architectural Conservation ICCROM,
Rome, 1982.
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