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Rilem TC 203-RHM: Repair mortars for historic masonry.Testing of hardened mortars, a process of questioningand interpreting
TC 203-RHM
Published online: 19 December 2008
� RILEM 2008
Abstract This paper presents an approach to the use
and interpretation of tests on mortar samples when
restoring historic masonry. It is largely based on the
work performed by the former RILEM technical
committee 167-COM, Characterisation of old mortars,
closed in 2003, and the ongoing committee 203-RHM,
Repair mortars for historic masonry. The focus of the
present paper is on the decision process: what to test
and how to interpret the test results.
Keywords Mortars � Render � Plaster �Masonry � Testing � Restoration
1 Introduction
Testing of historical mortars is often performed as
part of restoration programmes for historic masonry.
The usefulness of these tests is sometimes ques-
tioned. A prerequisite for useful test results is that
tests be based on clearly identified questions and a
preliminary understanding of how the results will
help define the requirements for the repair mortar. It
requires a good understanding of the relation between
observation and problems formulated on site, and the
work performed in the laboratory. It is crucial to
identify the proper preliminary tests to be performed.
But also at a later stage it is necessary to have a
correct interpretation of the test results in relation to
the situation on site. This paper aims at giving some
guidance in this process. The main focus is on the
choice of laboratory test methods and on the inter-
pretation of the test results. How to perform the tests
has been comprehensively described elsewhere [1–3].
The sampling and field description is a crucial step in
order to achieve the objectives mentioned above. A
systematic approach to sampling has been described
previously [4]. The present text is a contribution from
RILEM technical committee 203-RHM Repair Mor-
tars for Historic Masonry. The target groups for this
paper are both the people performing the analysis and
those who use the results.
The aim of an analysis may be to document a
building of great historic value before the restoration.
Such an investigation may include the history of the
TC Membership:
Chairman: Caspar Groot, The Netherlands.
Secretary: John Hughes, Scotland.
Members: Koen van Balen, Belgium; Beril Bicer-Simsir, USA;
Luigia Binda, Italy; Christine Blauer, Switzerland; Jan Elsen,
Belgium; Eric Hansen, USA; Rob van Hees, The Netherlands;
Fernando Henriques; Portugal; Eleni-Eva Toumbakari, Greece;
Thorborg von Konow, Finland; Jan Erik Lindqvist, Sweden;
Paul Maurenbrecher, Canada; Bernhard Middendorf, Germany;
Ioanna Papayanni, Greece; Stefan Simon, Germany;
Maria Subercaseaux, Canada; Cristina Tedeschi; Margaret
Thompson, USA; Jan Valek, Czech Republic;
Maria Rosa Valluzi, Italy; Yves Vanhellemont, Belgium;
Rosario Veiga, Portugal; Alf Waldum, Norway.
TC 203-RHM (Jan Erik Lindqvist) (&)
Swedish Cement and Concrete Research Institute,
Stockholm, Sweden
e-mail: JanErik.Lindqvist@CBI.SE
Materials and Structures (2009) 42:853–865
DOI 10.1617/s11527-008-9455-x
building, its chronology and the history of techniques.
What type of mortar was used originally and in
subsequent building stages? It may include studying
how the mortars were applied. Furthermore questions
to be solved through analysis may relate to various
aspects of compatibility including the aesthetic
expression of the building. The aim of the analysis
programme may also be to provide a basis for the
choice of repair materials and repair techniques,
mainly to ensure compatibility with the existing
structure.
An early step in such a process is to identify the
questions that a test and analysis program should
answer. It is recommended as a first stage to make
investigations on site through observation and non-
destructive testing. It is only when the questions are
clearly identified that it is possible to select labora-
tory test methods and develop a sampling procedure
that is relevant to the problem and the analysis.
Several of the methods used for testing of histor-
ical masonry and mortars are not standardised and
information about them is spread over several
publications. The report published by RILEM com-
mittee 167-COM [5] is an important source of
information about how to perform the tests and
analyses mentioned in this paper. It also provides
guidance on approaches to interpreting the damage
observed on site. Older standards and books related to
building materials are other possible sources of
information. The focus in this paper is on the test
methods most commonly applied. There are also very
specific methods that may be applied in research
projects but these are mainly outside the scope of the
present paper.
2 Testing of hardened mortars
2.1 Type of binder
2.1.1 Questions
A first question is what type of binder was used in
the existing mortar? Common binders and binder
components are lime, hydraulic lime, cement,
pozzolans and clay. Gypsum is common in plasters
and decorations but has also been used for external
joints in specific locations [6]. Pozzolans and clays
are described in Sect. 2.2 but they could, depending
on the character of the mortar, also have been
treated in this chapter. The amount of binder and
aggregate in the existing mortar, and the mix
proportions are treated in Sect. 2.5. The chemical
composition and the structure of the binder will
show the proportion of pure lime, which hardens
through reaction with carbon dioxide in the atmo-
sphere, and hydraulic components, which harden
through reaction with water. A mortar with a pure
lime binder has different properties from one using a
hydraulic binder. The degree of hydraulicity of the
binder has an important effect on the properties of
the mortar. Another question may concern the origin
of the limestone used for the production of the
binder [7].
2.1.2 When is this important?
From the ancient times to the early nineteenth century
in Europe, mortars were generally pure lime, subhy-
draulic or pozzolanic. Pure lime is produced from
pure limestone while the subhydraulic mortar is
produced from a limestone containing a small amount
of clay and other siliceous minerals. Dolomitic
(magnesium–calcium) lime mortars are common in
some areas [8]. Their properties are different from
calcium lime mortars [9]. The calcium–magnesium
carbonates may be transformed to a harmful salt [10].
If the mortar is from about 1850 or later, it may
contain a hydraulic binder [11, 12]. Cast decorations
from this time are often made of natural cement or
similar binders [13]. Natural cement is a strongly
hydraulic binder produced from argillaceous lime-
stones. It is important to understand the hydraulic
properties of the mortars because a large number of
buildings in city and town centres are made with
hydraulic mortars. If there are doubts about the type
of binder this should be determined.
2.1.3 How is the analysis performed?
The type of binder may be determined through
microscopical analysis applying thin section tech-
niques or through the analysis of acid soluble
chemical components, mainly calcium and silica. It
is also recommended to check for acid soluble
alumina, iron and magnesium when assessing the
hydraulic properties of the mortars.
854 Materials and Structures (2009) 42:853–865
2.1.4 What information is obtained?
Generally, the microscopical analysis shows what
gives the hydraulic effect while the chemical analysis
gives information about the strength of the hydraulic
effect.
Microscopy: Assessment of the type of binder
can show if it is a pure lime mortar or if it has
hydraulic properties, and, if the latter, whether it is
natural hydraulic or Portland cement based. The
presence of slag, brick particles, volcanic ashes or
other pozzolanic materials can also affect the
hydraulic properties as they can react with lime
in the presence of water. A microscopical analysis
can provide quantitative data for cement mortars
but there are no microscopical methods for quan-
tifying the hydraulicity of natural hydraulic mortars.
A test report can provide information on the type of
binder and a quantification of the amount of
cement, if present, and a very approximate assess-
ment of how strong the hydraulic properties are of
a natural hydraulic mortar. Gypsum mortars may be
identified using microchemistry in electron
microscopy.
Chemical analysis: Wet chemical methods can be
used to identify some binder types such as lime and
gypsum. It is possible to analyse the content of
components that influence the hydraulic properties.
The results can be used to estimate the hydraulicity
of the mortar. One method of doing this is the
cementation index (CI) developed by Eckel [14]
according to the formula below. Usually, only acid
soluble calcium and silica are analysed. From this it
is possible to calculate an approximate CI. Binders
can be divided into pure lime with a CI \ 0.15,
subhydraulic with a CI of 0.15–0.30 [15], feebly
hydraulic with a CI of 0.3–0.5, moderately hydraulic
with a CI of 0.5–0.7, and eminently hydraulic with a
CI of 0.7–1.1. A binder with a CI of 1.0 is
comparable to Portland cement. It is important to
keep in mind that the results give no information on
the type of binder or type of pozzolanic material. If
the aim of the test is to determine the hydraulic
properties of the mortar a calculation of the CI is
recommended.
CI ¼ 2:8� SiO2 þ 1:1� Al2O3 þ 0:7� Fe2O3
CaOþ 1:4�MgO
2.1.5 Sampling
A microscopical analysis requires a sample size of
preferably 3 by 5 cm although smaller samples are
possible. For the chemical analysis of acid soluble
components it is possible to use a representative
sample composed of several small pieces. Samples
that are characteristic of damaged mortar may be
taken if that is the purpose of the test but in other
cases the samples must come from undisturbed and
unweathered locations.
2.2 Additives
Chemical admixtures, added in very small amounts
and that require a different set of analyses, are treated
in Sect. 2.4.
2.2.1 Questions
Several different types of additives may have been
added to the mortar mix: these may be pozzolans
such as slag, brick, burned shale or volcanic ash or
they may be non-pozzolanic materials that essentially
do not react with the lime and are inert such as
unburned clay, charcoal or hair.
2.2.2 When is this important?
Pozzolanic additives have been used since ancient
times. They can react with lime and water and
thereby give the mortar hydraulic properties. In
countries with volcanic activity, such as Italy, Greece
and Portugal (Azores) volcanic ash was used as a
natural pozzolan. Another example is trass that was
used for masonry canals and harbours in the Neth-
erlands (Table 1). Mortars with burned alum shale
were used for similar purposes in Sweden in the
eighteenth century. Brick dust derived from bricks
burned at low temperature also has a weak pozzolanic
effect. Some pozzolans gives a colour to the mortar
while others do not.
2.2.3 How is the analysis performed?
Some additives are easily identified through visual
inspection such as straw, hair and coarser coal
Materials and Structures (2009) 42:853–865 855
particles. Others such as brick dust, slag and burned
shale may be identified by the naked eye or by using a
hand lens. Identification with better certainty or a
quantitative analysis requires further analysis
(Table 2). Clay mortars can also be identified through
visual examination although further analyses may be
necessary in order to make a definite identification.
X-ray diffraction (XRD) of the acid insoluble residue
can be used to identify the clay, which often has been
partly decomposed in the high pH environment of the
mortar prior to carbonation. Gypsum mortars and
pure clay mortars can be identified using XRD.
Volcanic ash may have a high pozzolanicity, which
means that it reacts strongly and almost entirely, and
is therefore difficult to identify. XRD is a useful
method for the identification of volcanic material as
most of them contain zeolitic minerals. However, it is
an advantage if the presence of additives can, as far
as possible, be identified on site and that further
analysis is based on these observations.
2.2.4 What information is obtained?
The presence of pozzolan gives a mortar with lime
the same properties as a hydraulic mortar. Analysis of
acid soluble components can be interpreted in
accordance with analysis of mortars based on
hydraulic binders. A strong pozzolanic mortar is
analogous to a strong hydraulic mortar. Strong
pozzolanic mortars were some times referred to as
cements in older literature.
2.2.5 Sampling
For microscopical and chemical analyses of acid
soluble components, the sampling is similar to that
used for the analysis of binders. For scanning electron
microscopical analysis, the samples can be similar to
those for optical microscopy but they are generally
smaller, commonly about 1–15 cm2.
Table 1 Example of
different types of additivesMaterial Properties Colour
Brick Colour and weakly pozzolanic Pink or reddish colour
Burned shale Pozzolanic Dark shade of lilac
Dutch trass Pozzolanic Grey or brown
Volcanic ashes Pozzolanic
Slag Pozzolanic Pink or reddish
Clay Weak mortars Grey or yellowish
Fibres straw Reinforcement during drying shrinkage
Hair Reinforcement during drying shrinkage
increased elasticity
Seen on fracture surfaces
Coal particles Probably contaminants Black particles
Table 2 Example of analytical methods for identification and quantification of additives
Material Method Complementary methods
Brick Optical microscopy Micro chemistry in electron microscopy
Burned shale Optical microscopy Acid soluble components
Dutch trass Optical microscopy Acid soluble components
Volcanic ashes, Dutch trass, pozzolana, Santorin earth Optical microscopy XRD Acid soluble components
Slag Optical microscopy Micro chemistry in electron microscopy,
Acid soluble components
Clay XRD, acid insoluble residue Optical microscopy
Fibres, straw, hair Visual assessment Acid insoluble residue
Coal particles Visual assessment Acid insoluble residue
856 Materials and Structures (2009) 42:853–865
2.3 Aggregate
2.3.1 Questions
Aggregate is defined by the rock from which it is
derived and its mineralogical composition, particle
shape and size distribution. Also the spatial distribu-
tion, orientation and heterogeneity can provide
information on how the mortar was worked. Infor-
mation about the aggregate can be significant both
from a technical and an historic point of view. For
example, it has certain importance to know if well-
graded sand has been used or if it is possible to trace
the aggregate back to a local source. Determination of
the amount of sand in the mix is described under mix
proportions (Sect. 2.5).
2.3.2 When is this important?
The mineralogical composition, size distribution and
grain shape can be used to identify the origin of the
aggregate. The size distribution is mostly given as a
grain size distribution curve but can also be given as
an index [16]. The size distribution has an influence
on the technical properties of the mortar. In aggregate
rich mortars with well-graded aggregate, the fine
aggregate will fill the voids between the coarser
aggregate and form a densely packed and interacting
structure. In binder rich mortars, since the sand
particles do not directly interact, the size distribution
has less influence. A high content of fines may give a
better workability to the fresh mortar but it results in
a lower strength mortar when hardened. A coarser
sand grading will counteract shrinkage of the mortar.
The shape of the aggregate particles influences the
workability of the mortar, for example a flaky
material gives a stiffer mix.
If a local sand is being considered for use on site
it is important to assess if it is suitable and is not
likely to cause problems by having ingredients
reducing the freeze-thaw resistance, or causing
discolouring and surface damage. The colour and
size of the sand particles may also be important if it
is desirable to have the same aesthetic appearance as
the previous mortar. The mineralogical composition
is of importance when performing an analysis of
acid soluble components. An example is limestone
aggregate that is acid soluble and will contribute
calcium in the chemical analysis. It may also be an
aggregate of local character, such as limonite-
sandstone, that may seriously influence the results
of a chemical analysis.
2.3.3 How is the analysis performed?
The mineralogical composition of the aggregate
may be determined through petrographic analysis
using an optical microscope and thin section
technique [1, 17]. The grain shape and grain size
distribution may be assessed using optical micros-
copy and computerised image analysis [18, 19]. The
size distribution of the aggregate may be assessed
through sieving of the acid insoluble residue. For
friable samples, a thermal pre-treatment at 400�C
can be performed. The residue contains fine mate-
rials that are not derived from the aggregate and
lacks calcite grains and other minerals, for example
dolomite and gypsum, which are dissolved in the
acid treatment.
2.3.4 What information is obtained?
The mineralogical composition, size and shape
distribution indicates the origin of the sand. Rounded
sand is likely to be of fluvial origin while sand with
sharp particles is likely to be of terrestrial origin such
as till or erosion materials. A flaky aggregate gives a
stiffer mortar. If the mortar will be pumped a rounded
aggregate is to be preferred. The maximum particle
size should be no greater than 1/3–1/2 of the
thickness of the render or the mortar joint. Aggregate
with a high content of fines may result in a mortar
with low frost resistance [20].
The mineralogical composition of the aggregate as
well as the shape and grain size distribution may be
compared with nearby deposits. Comparison can be
made between different mortars at the same project in
order to see if they have the same origin. Well graded
sand indicates that it has been chosen with care.
Sand used as aggregate shall be free of organic or
inorganic constituents that may cause discolouring
such as iron sulphides, sulphates or iron hydroxides.
Also avoid loose shale or clay particles that can
change their volume during wetting and drying
cycles. They may cause surface damage. If the sand
particles are weak and porous they may have low
frost resistance.
Materials and Structures (2009) 42:853–865 857
2.3.5 Sampling
Sampling is similar to that used for the microscopical
analysis of binders. If the size distribution of the
aggregate is analysed by sieving of the acid insoluble
remains, then the sample can consist of several
smaller pieces.
2.4 Admixtures
2.4.1 Questions
Admixtures are added in small quantities in order to
improve properties of the mortar such as workability
during mixing, and improved frost resistance by
introducing air voids in the mortar. Historically the
most common admixtures are based on proteins.
2.4.2 When is this important?
Analysis of older mortars will give an insight into the
historical techniques used to improve properties of
the mortar. If air voids were formed through the
addition of organic admixtures it was done in order to
change the technical properties of the mortar.
2.4.3 How is the analysis performed?
One analytical approach is the Kjeldahl analysis [21]
where the protein is transformed into ammoniac,
which is analysed. There are also methods based on a
colour change when treated with a mixture of
ninhydrin and alcohol. There are furthermore meth-
ods based on immunological methods, such as
ELISA, which can give very specific information on
which substances were added [22]. Generally the
identification of ancient organic admixtures, usually
present in small quantities, is very difficult.
2.4.4 What information is obtained?
An analysis may indicate if proteins have been used.
There is a risk that protein from plants, algae and
bacteria have contaminated the mortar or that the
protein originally present in the mortar has disap-
peared. The chemical analysis may be combined with
a microscopical analysis of the microstructure in
order to see if it is consistent with the use of
admixtures. A high content of small and rounded air
voids would indicate the use of proteins.
2.4.5 Sampling
The sample may be in several small pieces. Avoid
areas with plants or dirt and areas exposed to
moisture.
2.5 Mix proportions
2.5.1 Questions
What are the mix proportions of the original mortar?
2.5.2 When is this important?
For documentation of the original mortar and to
provide a basis for developing requirements for the
repair mortars.
2.5.3 How is the analysis performed?
For pure lime mortars and subhydraulic lime mortars
[15], the amount of binder can be determined through
chemical analysis of the acid soluble calcium and
silica, the same methods used for determining the
type of binder. A limitation is mortars with acid
soluble aggregates, mainly carbonates. This can,
however, to some extent be corrected for [3]. For
pure lime mortars, determining the loss on ignition
may be sufficient. The mix proportions may also be
determined through microscopical analysis of thin
sections where the volume proportions of paste,
aggregate and other materials are quantified using
point counting.
2.5.4 What information is obtained?
For pure lime mortars and subhydraulic lime mortars
with a low content of silica and alumina it is possible
to calculate the mix proportions from the loss on
ignition. For a better characterisation, it is recom-
mended to also determine the acid soluble calcium
and silica. For hydraulic and pozzolanic mortars, the
acid soluble alumina, iron and magnesium should
also be determined. The results from the point
counting by optical microscope can be used to
calculate the mix proportions. These results are often
858 Materials and Structures (2009) 42:853–865
reported as the volume portion of aggregate and paste
in the analysed sample but this is not the same as the
mix proportion in the mortar mix! The mix propor-
tion can, however, be calculated from these values
[23, 24]. When interpreting the results, it should be
kept in mind that chemical reactions with other
materials in the mortar or the environment, as well as
deterioration processes, may significantly change the
original composition over the years.
There is a high variability in the binder to aggregate
ratio. In historical mortars, the ratio is often higher than
in modern mortars. A binder/aggregate ratio of 1:1 by
volume or higher is not uncommon. A new mortar with
the same mix proportions may be less durable. This
may be due to different techniques of mixing and
application of the historical mortars, which were
probably better adapted to ancient materials than those
used today. Another reason may be that what comes out
from the analysis as a binder may, in the traditional way
of working, not have been a binder. One example is that
when lime is identified we do not know how much of
the lime used was still uncarbonated at the moment the
mortar was prepared. Stated in another way, we do not
know the purity of the materials used in ancient times.
2.5.5 Sampling
A microscopical analysis requires a sample with a
size of preferably 3 by 5 cm although smaller
samples are possible. For a bedding or pointing
mortar it is best, if possible, to include the mortar
itself and the part of each of the adjacent bricks or
stones. For chemical analysis of acid soluble com-
ponents, it is possible to use a representative sample
composed of several small pieces.
2.6 Mechanical properties: strength and modulus
of elasticity
2.6.1 Questions
The question may concern the mechanical properties
of the mortar or the surface strength. Another concern
is often the adhesion of the mortar to its substrate.
2.6.2 When is this important?
The mechanical strength of the mortar is of impor-
tance for the interaction between the mortar and the
substrate. A repair mortar with too high a strength or
elastic modulus may cause damage to the stones or
bricks in the masonry [25]. The adhesion to the
substrate shows if the mortar has sufficient interaction
with the substrate. It is mainly the type of binder that
determines the adhesion but several other factors
influence the adhesion as well, such as plasticity of
the fresh mortar, suction of the substrate, curing
conditions and workmanship. Mortar adhesion over
the entire contact surface in a homogeneous way is
more important than a strong adhesion, which may
lead to damage of the substrate. An increased
porosity gives lower strength. Large air voids or
cracks are more crucial than several small pores
especially for the tensile strength. A large maximum
aggregate size lowers the strength but here the mortar
strength also depends on the adhesion between
aggregate and the binder.
2.6.3 How is the analysis performed?
Adhesion of renders and plasters to the substrate may
be tested by drilling a circular groove through the
mortar down to the substrate. For testing of the
pullout strength of the surface layer, a depth of only a
few millimetres is drilled. The diameter of the
circular groove can be 80 mm. The test is performed
using equipment for a pull-out test. The moisture
content of the mortar during the test is of importance
because a dry mortar has a higher strength than a wet
one. Testing the bond strength of repointing mortar
using the bond wrench method is described in [26].
Compressive strength and indirect tensile strength
of mortar can be assessed using cubes or cylinders. It
may be difficult to obtain samples large enough for
the test. Some methods for testing irregular, friable
samples have been studied, generally with resource to
confinement mortars, and are described in published
works [27]. The Schmidt hammer, drilling resistance
and ultrasonic velocity are other methods that can
give indirect measures of the strength.
2.6.4 What information is obtained?
Renders are normally non-loadbearing, and the
strength of masonry is not directly proportional to
the strength of the bedding mortar. The strength of
the mortar itself is thus, in most cases, not critical. It
may, however, be of importance for the compatibility
Materials and Structures (2009) 42:853–865 859
between different mortar layers. For example, the
strength of the mortar in a render with several coats
should increase inwards (lowest strength on the
exterior). The mechanical strength in combination
with knowledge about the type of binder can provide
an important indication on functional properties. The
mechanical strength is also used to assess the state of
conservation of mortars. In fact, a very low com-
pressive strength usually shows loss of cohesion as a
result of damage mechanism [28].
When testing repair mortars of pure lime or lime
with pozzolan it is important to let the mortar cure a
sufficiently long time to allow it to carbonate and
allow pozzolanic reactions to take place. This is about
12 months for lime mortars and at least 90 days for
pozzolanic mortars. As an example, the compressive
strength for a lime mortar was found to be about 0.2–
0.6 MPa after 28 days and 1–1.7 MPa after 1 year
[29].
The elastic modulus of a material is the relation
between the applied stress and the elastic deforma-
tion. Important to understand as well is the plastic
(irreversible) deformation of the material. A stiffer
material has a higher modulus. An elastic modulus of
a repair mortar that is higher than that of the existing
mortars and masonry elements may cause cracking
and spalling. Weak limestones and sandstones, and
even weak granites, have a lower elastic modulus
than a cement mortar. A very low modulus of a
mortar may indicate low durability of the mortar
itself while if it is too high it is likely to cause
damage to the masonry. It must be stressed that the
strength is not an important criteria for durability,
apart from special cases such as wet environments.
We are, however, often too blinded by the use of high
strength as a criterion of durability which in many
cases has caused damage to historic masonry struc-
tures [30]. For example, a very low elastic modulus
may be necessary for very weak substrates, such as
earth walls.
2.6.5 Sampling
A suitable sample size for testing the compressive
strength of historic mortars is 25 9 25 9 25 mm.
Pointing and bedding mortars may be just 10 mm
thick while for plasters and renders single layers may
be thinner than 15 mm. A method suitable for testing
these mortars is described in [31]. The Brazilian
tensile test may be performed on cylindrical samples
with a minimum diameter of 25 mm or a prism with a
section of 20 by 20 mm. The size of the samples
should preferably be at least three times the maxi-
mum aggregate particle size.
2.7 Porosity
2.7.1 Questions
The questions may be related to the total pore
volume, the interconnected pore volume (open
porosity), the pore size distribution and the air void
structure of the mortar. In order to design compatible
repair mortars, information about the porosity of the
surrounding stone or brick could also be very useful.
2.7.2 When is this important?
The porosity and type of binder determines the
strength and moisture properties of the paste in the
mortar and to a major extent also determines
functional properties such as frost resistance. It is
important for assessing the compatibility between the
original and repair mortars. The test data can be used
to determine an appropriate pore size distribution and
total porosity for the repair mortar. It should,
however, be pointed out that it is not simple to
transfer the results from an analysis of porosity to
recommended properties for a repair mortar.
2.7.3 How is the analysis performed?
The most common analyses are those based on water,
mercury or gas penetrating the mortar and filling the
voids [32]. The porosity measured this way is called
open porosity. A straightforward method to measure
water absorption is to let the sample absorb water
through capillary suction [33]. The amount of water
absorbed under vacuum gives a better indication of
the total porosity open to water. Mortars with a coarse
porosity have a faster water uptake than mortars with
fine pores. A recording of the rate of water uptake in
a test can give information about the size distribution
of the pores in the mortar. Microscopical methods
give information also about the closed porosity. This
is usually performed on ground sections or thin
sections. It is possible to use manual or automatic
methods based on image analysis. The microscopical
860 Materials and Structures (2009) 42:853–865
methods for the analysis of the porosity are mainly
used for assessment of the freeze-thaw durability of
the mortar. The assessment is based on total porosity,
spacing factor and pore size distribution.
2.7.4 What information is obtained?
An increase in porosity reduces the strength of the
mortar. In a very porous mortar, the high air content
in the contact surface with the substrate can be
responsible for a reduction in adhesion. The distri-
bution of fine and large pores, and their
interconnection, influences the frost resistance.
In a mortar with fine pores, the damp front will rise
higher through capillary transport. But the rate of
transport is more rapid in coarse pores. In the diagram
for capillary suction in Fig. 1, a mortar with coarse
pores has a steeper initial slope than the one with fine
pores. The total amount of water absorbed gives the
open porosity accessible to capillary suction. This is
less than the total open porosity unless the test is
continued for a long time. Vacuum saturation gives a
better indication of the total open porosity.
The pore structure of the new mortar should be
adapted to the old mortar, in order to stop as much as
possible further weathering of the old mortar. The
size distribution of the pores influences the water
transport between mortar layers with different
porosity, and between the mortar and the masonry
units. Water transport goes from the coarser pores to
the finer pores. This has significance if there is risk
for salt or frost damage. Avoid placing a dense mortar
over a more porous mortar. A plaster with fine
porosity on coarse bricks may lead to salt deposition
in the plaster while a plaster with coarse pores on a
finely porous substrate may lead to salt deposition in
the substrate [35]. If water repellents have been used,
porosity may not be the governing factor for moisture
transport [36].
2.7.5 Sampling
The test for capillary suction requires a fairly large
sample, about 2 9 5 9 5 cm. Microscopical methods
require a sample covering a surface of about 5 by
3 cm. Gas adsorption and mercury porosimetry are
performed on samples with a diameter of a few
millimetres.
2.8 Lime wash, paints and pigmented mortars
2.8.1 Questions
What types of pigments were used on renders? A
special case is iron vitriol (iron sulphate), which gives
a hard and dense surface. It is also possible to analyse
the binder in the paint on the render surface. In
addition to this, the number of paint layers, and their
thickness and variation in composition can be of
interest.
2.8.2 When is this important?
Iron vitriol gives a hard and dense surface, which
makes it difficult to get good adhesion when it is used
as a substrate for a new mortar. The type of binder in
the paint defines the type of paint.
2.8.3 How is the analysis performed?
Pigments may be analysed in a scanning electron
microscope equipped for micro-chemical analysis
(SEM/EDS), or the pigment is compared to reference
materials using an optical microscope. Analysis of
the binder may be performed using infrared spec-
troscopy (FTIR). It is also possible to evaluate the
type of binder in a paint layer on site using reagents
0
1
2
3
4
5
6
7
8
9
10
0 2 4 6 8 10
Abs
orbe
d w
ater
(kg
/m2)
Square root time h
Fig. 1 The amount of water absorbed over time by capillary
suction in three medieval mortar samples from the Saxtorp
church in southern Sweden [34]. The initial slope of the curves
marked with squares and diamonds is steeper than for the curve
marked with circles because the pores are coarser (faster water
uptake). The final uptake of water gives an indication of the
pore volume (in this case, lowest in the sample with the finer
pores). The samples were about one square decimetre in size
but samples as small as a few square centimetres could also be
suitable
Materials and Structures (2009) 42:853–865 861
such as hydrochloric acid which dissolves lime and
cement based paints but not silica and organic paints;
ethanol dissolves latex paints while ethyl acetate
dissolves organic binders.
2.8.4 What information is obtained?
If the existing mortar or paint is pigmented with iron
vitriol, it has to be removed before a new mortar is
applied. An organic paint also has to be removed
before a new mortar layer is applied.
2.8.5 Sampling
Analysis of pigments and binders using SEM/EDS
can be performed on millimetre sized flakes while
FTIR usually requires slightly larger samples.
2.9 Salt and moisture
2.9.1 Questions
What type of salt is present in the mortar and in what
concentration? What is the moisture source and what
is the salt source [37]? This may be ground water,
material in the masonry such as sulphate containing
bricks, or the formation of ettringite and thaumasite
through leaching of cement based repairs or sulphates
contained in the mortar [38]. Air pollution and sea
spray may cause deposition of salts on exposed
surfaces. Salts may come from ongoing or previous
activities in the building. For example, a tannery may
have a high salt content in the walls. It may also be of
importance to identify were in the construction the
salts are deposited.
2.9.2 When is this important?
Salt may be transported dissolved in water and
deposited where the liquid is supersaturated [39,
40]. This often happens where the water transport
mechanism changes from liquid capillary transport
to vapour transport, as the salt cannot be transported
in a gas phase. Salt deposition may occur as
efflorescence on the surface of the mortar in the
form of individual salt crystals or as a crust. Salt
deposition on the surface does not damage the
mortar. The deposition may also occur as subflores-
cence (or crypto-florescence) directly below the
surface. This may lead to damage of the mortar
surface which may be very serious if the surface is
of high value. Salts may also be deposited in the
masonry; this occurs mainly at the top level of
rising damp. With rising damp a certain pattern is
often observed on the wall surface where the salts
deposit along an evaporation front on the surface.
Bulging of a wall occurs when the bedding mortar
expands due to the formation of swelling com-
pounds or from frost damage. Therefore, it can be of
great importance to determine the salt profiles in
order to document the variation of salt concentration
with depth in the masonry. The moisture source is
important in order to understand the cause of the
salt damage and to plan adequate repair measures.
Salts may also be deposited directly were they
are formed. This could be iron sulphides near
oxidizing pyrite grains (iron sulphide). This may,
in case it occurs near the surface, lead to discol-
ouring and pitting of the surface. The relative
humidity in the surrounding atmosphere may change
the water content and then also the volume of salts
containing crystal water such as epsomite and
gypsum. These volume changes may lead to dam-
age. A change in relative humidity may, however,
also affect salt with no water in its crystal structure
through dissolution, re-precipitation and re-
crystallisation.
Examples of different types of salt and their source
are given in Table 3.
2.9.3 How is the analysis performed?
Analysis of the moisture content of powder samples
is performed by the gravimetric method (weighing,
drying and re-weighing of the sample) [41]. By
assessing the hygroscopic moisture uptake of the
same samples, a sound indication may be obtained of
the presence of soluble salts. Further analysis of
water-soluble salts is performed on salts leached from
ground powder samples placed in water [42]. The
amount of salt is determined thorough chemical
analysis. This method can give information about the
amount of water-soluble chlorides, sulphates and
nitrides. Determination of the type of salt or mineral
is mainly done using XRD or micro chemical analysis
in a scanning electron microscope (SEM/EDS). The
insoluble salts, such as carbonates are mostly iden-
tified using XRD.
862 Materials and Structures (2009) 42:853–865
2.9.4 What information is obtained?
A moisture profile over the height and depth of a wall
can show the source of moisture. The type of salt
gives an indication about the source of salt and the
tendency to cause damage. A salt profile shows where
the salts are deposited and thus also what type of
damage can be expected. It indicates, together with
the moisture profile, the type of process that causes
the damage.
2.9.5 Sampling
If the purpose is to analyse water-soluble salts at
different levels in a profile the sample may be one
piece, for instance a drill core that is divided in the
Table 3 Example of different types of salts
Salt Chemistry Example of source, comments
Carbonates
Calcite CaCO3 Leached from mortars in moist environments
Vaterite CaCO3 Leached from mortars in moist environments, mainly hydraulic
mortars
Magnesite MgCO3 From dolomitic lime mortars
Thermonatrite Na2CO3 � H2O From alkaline building materials
Nesquehonite MgCO3 � 3H2O From dolomitic lime mortars
Trona Na3H(CO3)2 � 2H2O From alkaline building materials
Artinite MgCO3 Mg(OH)2 � 3H2O May form from burned dolomite
Nahcolite NaHCO3 From alkaline building materials
Kalicinite KHCO3 From alkaline building materials
Sulphates
Gypsum CaSO4 � 2H2O Polluted air, sulphate containing bricks, groundwater, sulphur in
the aggregate Dehydrate to hemihydrate and anhydrite
Syngenite K2Ca(SO4)2 � 2H2O High potassium content
Thenardite Na2SO4 From reaction of alkaline building materials with autochthonous
salts
Epsomite MgSO4 � 7H2O Dehydrate to hexahydrite, starkeyite and kieserite; from
groundwater in dolomite areas, from dolomitic lime mortars
Melanterite FeSO4 � 7H2O Oxidation of pyrite and in vitriol
Mirabilite Na2SO4 � 10H2O From reaction of alkaline building materials with autochthonous
salts
Glauberite Na2Ca(SO4)2
Ettringite Ca6Al2(SO4)3(OH)12 � 26H2O From cement repairs
Thaumasite Ca3Si(OH)6(CO3)(SO4) � 12 H2O From cement repairs
Chlorides
Halite NaCl From ground water, sea water and sea spray, deicing salts and salt
containing aggregate
Sylvite KCl
Calciumoxychloride CaCl2(OH)6 � 13H2O Deicing salts
Magnesiumoxychloride Mg2Cl(OH)3 � 4H2O
Oxalates
Whewellite Ca(C2O4) � H2O From conservation treatment
May also come from dolomite or from biological growth
Weddelite Ca(C2O4) � 2H2O
Materials and Structures (2009) 42:853–865 863
laboratory. Alternatively samples can be taken at
different levels on site. The sample may be in several
small pieces or as a powder if the purpose is to take a
general sample for analysis of water-soluble salts.
The sample size should be a few grams or more; in
order to assess moisture content and hygroscopic
behaviour about 10 g is necessary. For lime wash and
mural paint, the sample is generally smaller. For
XRD, the sample can be in powder form or in one
piece. It is possible to analyse samples much smaller
than 1 g but a few grams is preferable. When
identifying the type of salt with XRD or SEM/EDS
it is preferable to sample and analyse individual
crystals. A general sample, if possible, should be big
enough to be representative of the sampled mortar.
Acknowledgements Jan Erik Lindqvist, from the SwedishCement and Concrete Research Institute and Paul
Maurenbrecher, from the Institute for Research in Constructionof the National Research Counsel Canada, took the lead in
preparing this paper.
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