RILEM TC 203-RHM Repair Mortars for Historic Masonry

RILEM TECHNICAL COMMITTEE

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: [email protected]

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.


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.


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.


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


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).


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.


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.


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.


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.


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.


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.


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?


For documentation of the original mortar and to

provide a basis for developing requirements for the

repair mortars.


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.


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


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.


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.


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.


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


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.


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.


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


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.


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.


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.


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


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.


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.


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.


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.



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


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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RILEM TC 203-RHM Repair Mortars for Historic Masonry