Analysis of Mortar
The Past 20 Years
Where historic buildings
are concerned, repairs should be carried out using materials and techniques
which match those used originally as closely as possible. There are three
main reasons for doing this: firstly, repair materials which match the
originals will provide some continuity with the past, keeping intervention
to the minimum; secondly, by matching the original materials and techniques
closely, the repair materials will age in a similar way as the original;
and finally, modern materials and techniques introduced in past repair
work have often proved to be incompatible with the original, causing accelerated
deterioration of building fabric. Changes are usually made only where
the existing materials have been shown to be inappropriate.
In recent decades,
specialists involved in the conservation and repair of historic buildings
have become increasingly concerned by the damage caused by the use of
certain types of mortar on historic brick and stonework, and by the use
of ordinary Portland cement in particular. This has sparked renewed interest
in traditional lime mortars, which are more compatible with old buildings.
Some analysis of the existing mortar is now standard practice, if only
to match aggregates. However, a better understanding of historic mortars
is clearly required, not least because there have been some cases where
modern lime renders have failed and, at the opposite extreme, there is
now concern as some types of modern hydraulic lime mortars continue to
gain strength as they age.
parallel with the conservation industry, mortar analysis has progressed
dramatically over the last 20 years. The procedures pioneered by Ian Constantinides
and others, based as so much was (and still is), on John and Nicola Ashurst's
excellent English Heritage Technical Handbooks have been replaced by various
more sophisticated techniques.
individuals, companies, laboratories, and universities are now offering
the service, and much research is being undertaken. Analysis is carried
out with differing goals, from a simple colour match to ensure a sympathetic
'matching' repair mix, investigative assessments and performance evaluations,
to academic studies that attempt to determine the precise reasons for
the remarkable durability of ancient mortars.
analytical techniques are being employed, each with particular strengths
and limitations. The established techniques are summarised in the following
TECHNIQUES OF ANALYSIS
techniques - Photography; visual inspection; and touch and feel This group requires
no samples, and allows consideration in context, especially with reference
to performance and decay. Photography is useful for documentation
Light microscopy - Binocular microscopy;
polarised light microscopy; thin sections The binocular microscope
equipped with incident and transmitted tungsten light, with the potential
for observation of samples in cross-polars, allows determination of
the mineralogy and distribution of components and their interrelationship.
Wet chemical analysis - The constituents
of a mortar can be determined using various chemical tests after dissolution
in both acid and alkali. Measurement of soluble silica provides data
on the hydraulic property of the mortar, and this considered with
the calcium result enables the proportion of hydraulic components
to be estimated. A calcium result, reported as calcium oxide, does
not mean that calcium oxide is present. Calcium can be present as
calcium carbonate, calcium sulphate, calcium hydroxide, calcium silicate
hydrates for example, or in combinations of these compounds.
Instrumental techniques for the analysis
of component materials - Scanning electron microscopy (SEM);
electron microscopy with x-ray analysis (SEM/EDX); and x-ray diffraction
(XRD) SEM high resolution images of the surface of samples with magnification
of up to 100,000x show the structure of the mortar. SEM/EDX allows
elemental analysis of samples and is used for characterisation of
morphologies and textural and compositional interrelationships of
mortar components. XRD allows analysis of crystalline materials including
binder phases; belite and alite; and crystallised alteration products.
Data from the testing of complex materials is difficult to interpret,
and a highly skilled analyst is crucial.
Instrumental techniques for the analysis
of organic materials - Gas chromatography with mass spectrometry;
ion, liquid and thin layer chromatography These techniques are used
(although rarely in mortars) for the identification of oils, resins,
Instrumental methods for characterisation
of organic and inorganic materials - Thermal analysis (DTA,
TGA, DSC) and infra-red spectroscopy (FTIR) Thermal analysis can be
carried out on very small samples and can positively identify the
composition of certain components, including calcium carbonate, calcium
hydroxide, calcium sulphate, calcium silicate hydrates, and depending
upon the constituents remaining after ageing, complex calcium silicate
and aluminate hydrates.
Physical testing (for durability
assessment) Tests that determine pore structure such as water or gas
permeability, freeze thaw resistance, porosity and pore size distribution
provide data on durability.
Mechanical testing (for performance
assessment) Testing for compressive, tensile or flexural strength
on prepared samples will determine suitability for different applications.
Dating technology - Radiocarbon
dating This technique has recently been used to date mortars to an
accuracy of about 30 years.
important, but often underestimated first step is to ensure that any sample
taken for examination is representative of the mortar to be analysed.
Far too often the method of sampling influences the result, and taking
small or insufficient samples can lead to poor assessments or diagnosis.
The original 'kitchen sink' tests are no longer considered to be of any
use for much more than insoluble aggregate type matching. Generally these
tests consisted of dissolving a sample in dilute acid to separate the
acid-soluble from the insoluble. The soluble proportion is (often incorrectly)
assumed to be the binder, (as it so often includes soluble limestone aggregate
and calcareous clays), and the binder is assumed to be carbonated lime
(which of course it often isn't). In some cases the hydraulic component
is being assessed on the insoluble 'fines' proportion. This is meaningless,
as in the vast majority of cases where the proportion of 'fines' is high,
the mortar includes unwashed clayey aggregates.
more chemical tests undertaken, the better the overall understanding of
the mortar, and the higher the confidence in interpreting the data on
minimum requirement is to carry out recognised standard tests for sulphate
(to determine whether the binder is, or contains, gypsum), calcium, soluble
silica, and insoluble content. The soluble silica test is critical for
assessing the hydraulic proportion as it determines the combined percentage
of calcium silicates, calcium silicate hydrates and hydrated silica gel
present. It is the amount of this material which determines the hydraulicity
of binder, whether it be Portland cement, Roman cement, hydraulic lime
(of any grade) or an added pozzolan. In fact, as this result is so critical,
the soluble silica test should be carried out to a high degree of confidence
and calibrated using two distinct methods, the BS technique listed in
BS4551, and the colorimetric method. In common with much in the analysis
of mortars, great caution must be exercised, as some granite aggregates
will release soluble silica and this could be taken as part of the hydraulic
chemical tests can also help; a magnesium test for example determines
whether the lime was dolomitic.
properties, such as porosity, should be determined. All reliable data
indicates a complex interrelationship between porosity, permeability,
pore size distribution and durability.
meaningful mortar analysis should consist not only of chemical tests properly
conducted, but these tests should be augmented by one or more collaborative
instrumental techniques such as DTA, microscopy, XRD, or SEM.
of the most effective collaborative techniques is DTA (differential thermal
analysis). This technique is particularly useful in determining the calcium
compounds present. It positively determines calcium sulphate, calcium
hydroxide (lime), calcium carbonate, often distinguishing between carbonated
lime and calcareous aggregate, and calcium silicate hydrates. In older
mortars, DTA cannot distinguish between hydrated alite (C3S)
and hydrated belite (C2S) as these are essentially
the same, and so on its own cannot be used to determine whether the binder
is cement or hydraulic lime. However, positive DTA identification of calcium
silicate hydrates and calcium carbonate, and a porosity test, considered
with the quantitative soluble silica test results combine in making an
informed assessment of binder type. Only by determining whether unhydrated
C3S is present can Portland cement be confirmed,
although there is an indicative calcium oxide : soluble silica ratio.
The experience of the person interpreting the results is almost as important
as the results themselves. In one recent project the same mortar was tested
by four laboratories, and each interpreted differently. Conclusions ranged
from a cement/lime blend, a hydraulic lime (possibly in the form of a
natural cement), a hydraulic lime/non-hydraulic lime blend, to a 'hydraulic
or cementitious' binder.
another occasion, a fresh lime putty plaster sample had been tested because
gypsum gauging was suspected. The testing laboratory, using XRD found
no gypsum but identified C3A, a constituent of cement,
and some hydraulic limes. Cement or hydraulic lime gauging was immediately
identified ignoring the facts that the wet putty plaster was not setting
and that calcium silicate hydrates had not been identified. This resulted
in a high level meeting of client, architects, analysts, main contractor,
and plastering sub-contractor, most of us flown in at great expense. DTA
was able to prove conclusively that hydrated calcium aluminates were not
present. The outcome was that the XRD reflection had been incorrectly
identified as C3A, and it was in fact a constituent
of the complex igneous aggregate.
area most underestimated is how mortars age and the complex chemical reactions
and changes that occur with time. The fact that the binder in an aged
sample is now principally calcium carbonate does not indicate that it
was lime originally, as much of the hydrated hydraulic compounds in cement
and hydraulic lime will themselves react with CO2 (carbon dioxide), and carbonate. To complicate further, lime still present
in a 40-year old sample for example, does not necessarily indicate a lime
mortar originally, as lime is a reaction product of the hydration of C3S
and C2S. Indeed, the presence of lime in such a
sample is more likely to indicate a cement mortar as its non-permeable
nature would have impeded or prevented the access of CO2,
and carbonation of the reaction product has therefore not taken place.
analysis is now a very sophisticated business. However, any examination
that measures only part of the components, characteristics or properties
of a mortar and their relationship with durability or performance must
be viewed with caution. As with everything, a little knowledge can be
a dangerous thing.
Portland cement OPC is prepared by intimately mixing limestone and clay, burning
them at above clinkering temperature ( >1,260 degrees C), and grinding
the resulting clinker. The compounds present are formed by the interaction
during burning of the lime, silica, alumina and ferric oxide compounds.
The principal setting compounds in OPC are tricalcium silicate (C3S),
dicalcium silicate (C2S), tricalcium aluminate
(C3A), and tetracalcium aluminoferrite (C4AF).
These compounds are present in known controlled proportions. In
the 20th century, the desire for a higher strength product led to
increased C3S and reduced C2S
proportions. The setting process is the hydration of these four
compounds, but it is the C3S
that has all the essential properties of OPC.
lime The essential difference between modern hydraulic
limes and OPC is that hydraulic lime should not contain C3S
and should contain lime. Various types are available and they
are produced in various grades. Limestone containing clay and/or
silica is burnt in a kiln at below clinkering temperature (<1,200
degrees C) and the resultant product is hydrated with only sufficient
water to convert the calcium oxide to calcium hydroxide, but
not to hydrate the C2S, which in any case
is slow to hydrate. The setting process is a combination of
the hydration of C2S and carbonation of
the lime. In most hydraulic limes, a proportion of uncombined
reactive silica and alumina is also present, and these will
react with lime in the mortar to also produce calcium silicate
hydrates and calcium aluminate hydrates. In the United Kingdom
hydraulic limes are classed as:
lime Pozzolanas are defined as materials which though
not cementitious in themselves, contain constituents which will
combine with lime at ordinary temperatures in the presence of water
to form stable insoluble compounds possessing cementing properties.
Natural pozzolanas are mainly materials of volcanic origin, for
example Rhenish trass and Santorin earth. Artificial pozzolanas
are mainly products obtained by the heat treatment of natural materials,
such as china clay. The precise reasons for pozzolanic properties
is still a subject of controversy and various explanations have
been advanced to explain the reaction. Pozzolanas do in fact vary,
and there is probably more than one explanation. Indeed more recent
volcanic ashes of similar composition from Vesuvius for example
are (at best) of very low reactivity. It is generally accepted however
that two reactive products, a silica glass and fine grained clay
minerals, activated by heat, are of paramount importance. These
materials can combine with lime to produce calcium silicate hydrates,
calcium aluminate hydrates and calcium alumino-silicate hydrates.
lime Limes (>95% calcium hydroxide) made by hydrating
or 'slaking' the quicklime of relatively pure limestones which
set by 'carbonation', a reaction with atmospheric carbon dioxide
to form calcium carbonate. Two forms are available:
putty Ordinary (non-hydraulic) lime produced by slaking quicklime in an
excess of water to form a putty. Lime putty is matured for
several months in pits or under a thin film of water to
prevent carbonation, and during this process the portlandite
(lime) crystals change shape, becoming smaller and flatter,
thus aiding workability. It is used for the production of
lime plasters, mortars, and limewash. Also known as 'air'
limes, 'fat' limes and 'high calcium' limes.
hydrated lime Ordinary (non-hydraulic) lime produced as a dry powder by
hydrating the quicklime with sufficient water only to convert
calcium oxide to calcium hydroxide. Also known as 'bagged'
article is reproduced from Historic Churches, 2002
PETER ELLIS trained as a conservator of paintings in London, but has worked with older buildings for most of his career. For the last ten years he has managed Rose of Jericho, analysts and manufacturers of materials for the conservation and repair of historic buildings.
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