Influence of decorative finishing solutions on the water transport

phenomena

Diogo Bernardo

Highlights

Suitability of five painting solutions for old buildings is evaluated

Limewashes are the more compatible solutions with the old buildings

Incorporation of an organic addition introduces positive modifications in the limewashes

There is a high heterogeneity between different silicate paints

Water paints could behave properly in old buildings in specific conditions

ABSTRACT

Paint coatings of renders have important protection functions of the substrates. Furthermore, they

constitute the border of interaction between the facades of the buildings and the environment, having

a great importance in this interchange. Hence, it is of extremely importance to understand in which

ways these finishing layers influence the behavior of the materials that they cover. In order to reach

a better understanding of how paints influence the water transport properties and the salt decay

processes, a variety of tests were carried out on paint and unpainted specimens, using five different

paint systems: one limewash, one modified limewash with an organic additions, two silicate paints

and one water paint, all tested on lime-based mortars and blended substitution mortars of lime and

hydraulic lime.

Keywords: Paint Coatings; Limewash; Silicate Paint; Water Paint; Renders; Old Buildings; Porous

Materials.

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1. INTRODUCTION

Paint coatings of renders have not only functions of decoration but also protection, since they are the

interface of interaction between the facades and the environment, conditioning it. Hence, it is of

extremely importance to understand in which ways these finishing layers influence the behavior of

the materials that they cover.

Since renders and their decorative finishing are seen as sacrificial layers, their reparation or

substitution is quite frequently in interventions of rehabilitation and conservation. However, it is often

verified that the dampness or salt decay problems that led to some restoration interventions are often

worsened since there is a lack of understanding from the technicians involved, of the mechanisms

responsible for the appearance of such anomalies, and most important, the level of influence that the

interaction between the paint coatings and the substrate has in the overall behavior of the collective

(Gonçalves et al., 2003; Gonçalves et al., 2008).

Old buildings are usually built of thick solid masonry walls of very porous materials that are in direct

contact with the soil, which originates a high content of moisture and soluble salts penetrating in the

building through the foundations by capillarity. For this reason, it’s imperative that the paint systems

adopted do not have a hindering effect on drying of the walls, presenting a vapor permeability

compatible with the substrate.

One of the most significant and pernicious degradation processes of the exterior coatings is the salt

crystallization pathology, either at the surface as efflorescence, or within the pores, as subflorescence

(Iglesia et al., 1997; Lubelli et al., 2006; Groot et al., 2009). Salt decay is a pathology that arises not

only aesthetical problems, but also worsens the hygienic conditions of the divisions, reduces the

durability of the materials, leads to loss of material and, on edge, it could originate serious structural

problems, leading frequently to high-cost repair operations. Apart of being efflorescence or

subflorescence, salts could also aggravate original dampness problems due to their hygroscopic

behavior, and affect the underlying drying process, since the formation of salt crystals blocks the

material pores and disrupts the material, altering the vapor flow (Gonçalves, 2000; Gonçalves et al,

2014).

Considering that the paint coatings are the most affected materials, it is necessary to study in which

ways these materials influence the transport and salt crystallization processes, but also understand

which mechanisms are behind this occurrences.

There is a strong connection between the vapor permeability of the paint and the drying capacity of

the substrate. For this reason, the influence of the coatings in the drying process is usually accessed

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indirectly from the vapor permeability (Brito, 2009). However, this evaluation could not be made

separately regarding the materials, the substrate and the paint have to be studied together, since one

influences the other, and vice-versa (Oliveira, 1996).

Indeed, it is not unusual the specification of paint systems that are not compatible with the specific

functioning of old buildings (Freitas, 1997). Furthermore, the large variety of solutions available in the

paint market that are too generically specified for use in old buildings, contributes to hinder the

decision of choosing the correct paint systems. The selection of paint coatings must be made

ensuring that the chosen solution is durable and compatible with the characteristics of the substrate

on which it will be applied. This involves an analysis that goes beyond the simple physical

characterization of the paints.

Due to their polymeric nature, the so called “water paints” impose an almost impermeable barrier to

the water vapor flow, preventing the “breathing” of the walls (Freitas, 1997). The usage of these

paints, not only precludes the normal functioning of the old buildings, but also enhances the fast

deterioration of the substrate, originating an increasing of the height of capillary rise and longer wet

periods, resulting in an accelerated degradation (Brito and Gonçalves, 2013).

It is generally accepted that the limewashes and the silicate paints are the most appropriate for old

buildings (Gonçalves, 2000; Ribeiro and Marques, 2002; Tavares, 2002; for example). Recently,

there has been emerging in the paint market, with growing expression, new paints such as Silicone

paints with polysiloxane resins (in addition to an acrylic or styrene-acrylic resin) and Hydro-pliolite

systems with a hydro-pliolite resin as binder, also specified by the manufactures for use in old

buildings (Karaglou et al., 2013).

In order to reach a better understanding of how paints influence the water transport properties and

the salt decay processes, a variety of tests were carried out on paint and unpainted specimens, using

five different paint systems.

2. EXPERIMENTAL WORK

2.1. MATERIALS

2.1.1. PAINT SYSTEMS

The aim of this experimental work was to study the influence of paint coatings specified by the

manufactures for use in old buildings. Despite being generally accepted as compatible finishing

solutions for old buildings, silicate paints often originate problems after their application (Brito, 2009).

For this reason, two different silicate paints were chosen for testing:

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SA – A silicate paint from a well-known historical brand, deeply connected with development

of this paints, with great expression in the European market. It is described in its technical

sheet as a silicate paint for organic, mineral and mixed substrates;

SB – A silicate paint from a small Portuguese company, cheaper and less known than the

previous paint. It is described in its technical sheet as a system with a primary and an

emulsion of potassium silicate as principal binder.

Furthermore, limewashes are doubtless one of the most compatible decorative finishing solutions

with the lime-based mortars used in the renders of the old buildings (Tavares, 2011). Nevertheless,

there are still some ambiguities regarding the use of organic additives to enhance the performance

of this paints. Two different limewashes were tested:

Ca – A simple pigmented limewash. This paint was provided for the execution of this research

by the technicians responsible for the painting stage of the rehabilitation work conducted in

the Portuguese national monument Palace of Queluz;

CaA – Modified limewash with a mixture of 5% (weight) of Adical, an addition based on

synthetic resins compatible with an high alkaline pH, described as a product responsible for

extending the life-cycle of limewashes, in terms of weather resistance, adherence, and water

absorption.

Finally, a widely used water paint was also considered, serving as reference. It represents the

common practice of using this kind of paints in old buildings, even though they are not specified for

that purpose. It is defined by the manufacture as follows:

TA – “Water paint ultra matt, based on a styrene-acrylic dispersion, for application on exterior

and interior walls”.

2.1.2. SUBSTRATE MATERIALS AND PREPARATION OF THE TEST SPECIMENS

Two different mortars were prepared to simulate the traditional renders used in old buildings, using

hydrated lime powder (LC 80 according to EN 459-1 (2002)) from Calcidrata and natural hydraulic

lime (NHL 3.5 according to EN 459-1 (2002) from Secil Martingança, all available as commercial

products.

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It was used a fine aggregate, with particle size ranging mainly between 0.3 and 1.1 mm, that is

commonly used in Portugal in the formulation of mortars for renders. The aggregates were dried at

100 ± 5ºC for 12h before the preparation of the mortars.

It was intended to replicate the mortars used in the execution of renders of the old buildings that are

mainly lime-based. It was prepared an aerial lime mortar, Mortar C, with a B/ag (binder/aggregate)

ratio of 1:3 by volume, because it’s a very common formulation used in lime mortars [Agostinho, 2008;

Tavares et al., 2002; Gonçalves e Aguiar, 2002]. To achieve a more precise mixing process, the ratio

by volume was converted in to weight, resulting in a 1:7,48 ratio. The water amount used for the

mixing was the necessary to obtain a consistency of 150 mm, measured by the flow table test (1996),

which gave good workability.

A blended mortar of aerial lime and natural hydraulic lime was also produced, Mortar CH, substituting

35% of the aerial lime weight by natural hydraulic lime. This percentage of substitution was chosen

considering the results of Silva et al. (2014). These authors studied the influence of the natural

hydraulic lime content in the blended mortars of lime and natural hydraulic lime and concluded that

this mix only originates an increasing of the resistance of the mortars in early age, for substitution

values superiors to 25%, while maintaining the overall characteristics of the aerial mortars. The water

amount used in this mortar was defined by the same criteria of the mortar C. Table 1 presents the

formulations of the test mortars.

Table 1 - Formulations of the test mortars

Mortar Aerial Lime Natural Hydraulic Lime Aggregate Consistency [mm] w/b

Aerial Lime C 1 0 7.48 150 1,25

Blended CH 1 0.54 7.48 150 1,23

Mortars were produced based on the procedures established in EN 196-1 (1996) molded in adapted

casts, allowing the use of the compaction table and the production of 150x50x50 mm3 prisms. After

the curing period (1 month at 20 ± 2ºC and 65 ± 5% RH plus 3 months at a carbonation chamber with

5% of CO2 concentration, 21 ± 2ºC and 65 ± 5% RH for the mortar C and 4 months at 20 ± 2ºC and

65 ± 5% RH for mortar CH), the paint coatings were applied and the prisms were cut into 45x50x50

mm3 specimens, ready to be tested.

The application of the paint coatings was made by brush in the top surface of the mortar cubes,

following the instructions of the manufactures in terms of substrate preparation, number of paint layers

and dilution. The commercial paints were easily applied, since they presented a good flow and

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substrate fixation. As for the limewashes, the simple limewash was more fluid than the modified on,

but the later presented a better fixation to the substrate. In both limewashes it was found difficult to

homogeneously disperse the pigments.

2.2. INFLUENCE OF PAINT COATINGS ON WATER TRANSPORT PROPERTIES

The study of the influence of the paint coatings in mortars water transport properties included the

determination of the water absorption by capillarity, water vapor permeability and the drying kinetics,

after a curing time of 150 days, in painted and unpainted specimens. All the tests were performed on

specimens with their lateral surfaces sealed with an epoxy resin and previously dried under 60ºC

inside an oven until constant mass was obtained. Each test was carried in 3 unpainted and painted

with each solution specimens, in a total of 18 specimens for each mortar. For the water absorption

by capillarity and the drying kinetics tests, it were used the 45x50x50 mm3 specimens, and for water

vapor permeability tests it were used slabs of 40 mm side and thickness between the 12.5 and 20

mm, cut from prismatic specimens. The referred slabs were only painted after their cut.

The presented results correspond to the average value of the tested specimens.

The influence of the paint coatings in mortars water absorption by capillarity was evaluated based on

the EN 1015-8 (1999) procedures. The specimens were vertically positioned inside a container, with

the coated surface facing down, placed over small glass rods in such a way that they were 5 mm

immersed in water. The container with the specimens inside was covered during the test with a plastic

lid to reduce the evaporation of the water. The specimens were periodically weighed until their weight

reached an asymptotic value. Water absorption by capillarity curves were obtained by plotting the

mass of water absorbed per unit of area against the square root of time. The coefficient of water

absorption (CWA), Kg.m-2.s-0.5, corresponds to the slop of the initial linear segment of the curves.

The evaluation of the water vapor permeability of the painted and unpainted specimens was carried

following the procedures adopted by Ferreira Pinto (2002), which are based on the RILEM Test No.

II.2 (1980) guidelines that describe the dry cup test.

The water vapor diffusion coefficient, π (kg.m-1.h-1.Pa-1), expresses the quantity of water vapor that

permeates through a unitary thickness of the tested mortars when a gradient in the partial pressure

of water vapor is established between two of their surfaces (Silva et al., 2014). In isothermal

conditions, this phenomenon can be expressed by the following equation:

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Where J – flow of water vapor that crosses the specimen (kg.m-1.h-1); e – thickness of the specimen;

p – partial pressures of the water vapor on the level of the exterior surfaces of the sample (Pa).

The partial pressure of water vapor of an atmosphere (Pa) at a given temperature (T) and relative

humidity, RH (%) is calculated from the following expression:

𝑃 =

𝑃𝑠𝑎𝑡𝑅𝐻

100

(2)

Where Psat is the saturation pressure at given temperature and RH. The pressure gradient responsible

for the percolation of the water vapor was obtained by introducing calcium chloride, which originates

a RH of 0%, in the interior of the individual measuring cells and a solution of sodium chloride, which

originates a RH of 75% at 25ºC, in the interior of the container where the measuring cells were

positioned. After closing the lid of the container, it is expected a constant relative humidity atmosphere

to be established.

Separating the two different atmospheres (0% RH in the interior of the measuring cells and 75% on

the exterior) were the testing specimens attached to the measuring cells with a mastic cord, which

sealed the border between the specimens and the cells, ensuring that the percolation of the water

vapor only occurred through the specimens.

Using a wire passing through a small hole in the lid of the container, it was possible to weight the

measuring cells suspending them to a scale. Immediately after this process, the hole in the lid was

sealed with a plug. The specimens were periodically weighed until a steady vapor flow was reached.

This method allows to draw a graphic of weight variation against time. After a steady vapor flow is

reached, moment when the points in the graphic define a straight line, it is possible to obtain the flow

of water vapor that crosses the specimen (J), given by the quotient of the quantity of water that

crosses the specimen per unit of surface and time.

The drying tests followed the RILEM Test No.II.5 (1980) guidelines. This procedure consists in

soaking the specimens and then letting them dry, in controlled conditions of temperature and RH,

while their mass is monitored to determine the loss of moisture content, which can be expressed in

𝐽 =𝜋

𝑒× (𝑝1 − 𝑝2) (1)

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an “evaporation curve” which opposes the moisture content against time. The test procedure was as

follows. After the annotation of the dry mass, the specimens were soaked through an absorption of

water by capillarity for 24h followed by an immersion in water for another 24h. In addiction of the

sealing of the 4 lateral surfaces, the base of the specimens was isolated with a polyethylene film,

guaranteeing that the evaporation process only occurred through the painted surface. The specimens

were periodically weight until the occurrence of a constant flow.

One way of systematize the information obtained in the evaporation curves is through the drying index

(DI), which is calculated by the integral of the curves normalized on the basis of a maximum water

content (w0) and the interval of time required for drying (tf), according to:

𝐼. 𝑆 = ∫

𝑓(𝑤𝑖) × 𝑑𝑡

𝑤0 × 𝑡𝑓

𝑡𝑓

𝑡0

(3)

2.3. INFLUENCE OF PAINT COATINGS ON SALT DECAY PHENOMENA

The influence of the paint coatings on the salt decay phenomena of renders was studied following

the experimental methodology proposed by Algarvio (2010), which is based on the procedure

described by Charles Selwitz e Eric Dowhne (2002).

Tests were carried in 3 (45x50x50 mm3) of unpainted and painted with each solution specimens, in a

total of 18 specimens for each mortar. The sealing of the lateral surfaces and the drying of the

specimens were made as previously described. This procedure implicates the previous

characterization of the mortars water absorption by capillarity, namely, of the times that the capillarity

rise reaches:

T1 – half of the specimens height;

T2 – three quarters of the specimens height;

T3 – specimens’ height.

This procedure includes two phases, as described by Algarvio (2010): phase A and phase B. During

phase A, the specimens absorb by capillarity a saline solution of sodium chlorite with a concentration

of 15%. In phase B, the specimens absorb distilled water by capillarity with the objective of mobilizing

the salts over the outline of the specimens. This two phases are described in 4 stages as follows:

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Phase A – Salinization:

1. Absorption of a saline solution of sodium chlorite with a concentration of 15% by capillarity,

during T1. 7 days drying.

Phase B – Mobilization:

2. Absorb of distilled water by capillarity during T3. 7 days drying;

3. Absorb of distilled water by capillarity during T2. 7 days drying;

4. Absorb of distilled water by capillarity during T1. 7 days drying;

As previously described, in addiction of the sealing of the 4 lateral surfaces, the base of the specimens

was isolated with a polyethylene film, guaranteeing that the evaporation process only occurred

through the painted surface.

The monitoring of the degradation due to salt decay was made by evaluation of the specimens’ weight

at the end of each stage and by visual observation that was compiled into a photograph record.

At the end of the described procedure, the formed efflorescence were removed by brush (and a small

quantity of water) and a second capillarity test was conducted, to evaluate the influence of the

executed crystallization test on the water absorption properties of the mortars.

3. RESULTS AND DISCUSSION

3.1. INFLUENCE OF PAINT COATINGS ON WATER ABSORPTION BY CAPILLARITY

The curves of water absorption by capillarity of the tested unpainted mortars are presented in Figure

1. Both mortars present curves with typical layouts of materials with relatively homogeneous porous

dimensions, connected and well distributed. The two curves have a similar slope of the initial section,

however, the mortar C stabilizes for a higher quantity of water, 8.77 kg.m-2 for mortar C and 8.01

kg.m-2 for mortar CH.

In Figure 2 e Figure 3, are presented the capillarity curves of the painted specimens, for each mortar.

Using this results, it were determined the coefficients of water absorption (CWA), for each case. This

results are presented in Figure 4 and Figure 5.

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The two unpainted mortars, NR, present quite similar coefficients of water absorption, 0.14 and 0.13,

for mortar C and CH, respectively. However, for mortar C, it was obtained a greater disperse of values,

as can be verified for the error bars in Figure 5, tendency which is transversal in this mortar, since

this situation is verified for all the tested paint solutions. It can still be added that the effect of the

application of the paints on the hindering of the water absorption is more effective in mortar C. In both

mortars, the greater reduction of the CWA happens for silicate paints, SA and SB, which present the

same value between them, 0.06 in mortar C and 0.09 in mortar CH. On the other hand, the paint

solution that less influence the water absorption by capillarity is the simple limewash, Ca.

Figure 1 – Water absorption kinetics of the unpainted mortars (NR)

Figure 2 - Water absorption kinetics – Mortar C, painted and unpainted (NR)

Figure 3 - Water absorption kinetics – Mortar CH, painted and unpainted (NR)

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For mortar CH, the CWA values for the unpainted and the simple limewash is equal. In an

intermediate situation, there are the modified limewash, CaA, and the water paint, TA. This paint

coatings present the same CWA value for mortar C, 0.08, and a quite close value for mortar CH, 0.11

and 0.10 kg.m-2.h-0.5, for the modified limewash and the water paint, respectively.

Regarding this results, some considerations could be made. First of all it was expected that the water

paint presented a greater hindering effect on the water absorption by capillarity of the substrate, due

to its composition based on styrene-acrylic resins. What was verified was an approximation of this

paint behavior with the modified limewash, CaA.

Concerning the two limewashes, the results are clarifying. The modified limewash introduces a

greater resistance to the water penetration by capillarity in the substrate than the simple limewash,

without being too different from the behavior of the unpainted specimens, difference that is more

expressive in mortar C. This results are consistence with the conclusions reached by Gonçalves and

Aguiar [2002] in a similar study.

As for the silicate paints, both presented a similar behavior, significantly reducing the water

absorption by capillarity of the substrate. They are the paint coatings with the smallest CWA. Similar

studies conducted by Tavares et al. (2002) and Brito (2009) also demonstrate the great tendency that

this paints have to reduce the water absorption by capillarity.

Figure 5 - Coefficients of water absorption of Mortar CH Figure 4 - Coefficients of water absorption of Mortar C

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3.2. INFLUENCE OF PAINT COATINGS ON WATER VAPOR PERMEABILITY

Figure 6 e Figure 7 present the obtained results of water vapor diffusion coefficients, not as averages,

but as absolute values, so it becomes easier to identify tendencies of variation. The horizontal lines

represent the maximum and minimum values obtained for the unpainted specimens, NR.

As can be seen, the measured water vapor diffusion coefficients do not show pronounced differences

regarding the water vapor permeability of the two mortars. This result is consistent with the study

carried by Silva et al (2014), which indicated that blended mortars of lime and hydraulic lime with

percentages of substitution, of the former binder for the latter, inferior to 50%, present similar water

vapor diffusion coefficients to the lime-based mortars. It can also be said that the influence in the

water vapor permeability of the applied paints is similar in both mortars.

Regarding the simples limewash, Ca, it is not possible to draw conclusions since the results are highly

dispersed and do not show any tendency of variation. This sharp dispersion of results has a probable

origin in the fact that the limewash does not create a homogeneous film on the surface where it is

applied, but rather an inorganic porous structure which, in different points of the substrate, interacts

differently with it.

The incorporation of the addition in the limewash homogenizes the results for the measured water

vapor diffusion coefficients, resulting in an approximation to the maximum values of water vapor

permeability for both mortars. Contrary to the simple limewash, the modified limewash, due to its

organic compound, originates a more homogeneous film, on the surface of the substrate and,

consequently, a fewer dispersion of the results.

Figure 6 - Water vapor diffusion coefficients of Mortar C Figure 7 - Water vapor diffusion coefficients of Mortar CH

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Both silicate paints presented a tendency to decrease the water vapor permeability. However it is not

a reduction that could jeopardize the compatibility between these paints and the renders of old

buildings, since the spot of results is close to the inferior limit of the measured water vapor diffusion

coefficients of both unpainted mortars. This findings are consistency with the studies carried by

Tavares et al. (2002) and Birto (2009).

Has it was expected, the water paint presented the biggest reduction of water vapor diffusion

coefficients in both mortars.

3.3. INFLUENCE OF PAINT COATINGS ON DRYING KINETICS

For a better characterization of the mortars, it were determined the curves of water flow output in

function of time. These curves allow to identify the drying stage I, which corresponds to a constant

drying rate period. Hence, it is possible to determinate the evaporation flow, qc [g.cm-2.h-1], the

duration of the constant drying rate stage, tc [h], and critical saturation, Sc [%],which is the saturation

of the specimens at the end of the constant drying rate stage. This information is presented in Table

2 and the average of the drying curves of both unpainted mortars in Figure 8.

As seen in Table 2 and Figure 8, the two mortars have resembling drying kinetics, but also some

significant differences. The mortar CH has a slightly superior DI but an inferior evaporation flow in the

constant drying rate stage. Furthermore, the biggest difference in the drying kinetics of the two

mortars is the duration of the constant drying rate stage, being verified that the mortar CH as a

duration of the constant drying rate stage that is 24% inferior to the mortars C. Nevertheless these

differences, both mortars reach critical saturation values of 36%. In Figure 9 are represented the

drying curves of each painting solution and in Figure 10 the drying indexes, for both mortars. The

quantities that characterize the constant drying rate period were also determined for the painted

specimens and are presented in Table 3.

The previous data shows that, in general, the several paint solutions studied present drying kinetics

typical of porous materials with 3 drying stages: a first linear section where the drying rate period is

constant, a second concave section, where the drying rate decreases with time, and a third and last

section, where the moisture content stabilizes for the moisture content of hygroscopic equilibrium.

Regarding the limewashes, it is verified a widespread acceleration of the drying process, which

confirms the conclusions of Brito and Gonçalves (2013), that the limewashes not only do not hinder

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the drying of the of the tested specimens, but strikingly, they even accelerate the process. The

mentioned authors suggest that one likely explanation for this phenomenon is that the application of

the limewash coating increases the effective surface of evaporation, i.e., the dimension of the

evaporative surface during the constant drying rate period in which the surface of evaporation is

coincident with the surface of the specimens. The effective surface of evaporation depends on the

porosity of the material and the pore size distribution. The complex pore network created by the

mineral interaction between the substrate and the limewash gives rise to evaporation surfaces with

irregular morphology, whose area may exceed that of the projected surface (Brito and Gonçalves,

2013). Another important result attained in the carried drying tests is that the previous conclusion is

also valid for modified limewashes. Meaning that the incorporation of an organic addition do not

hamper the effect of drying acceleration of limewashes.

Table 2 - Drying indexes and characterization of the drying stage I of mortars C and CH

Mortar C Mortar CH

IS 0.20 0.21

Tc [h] 92 70

qc [g.cm-2.h-1] -0.007 -0.008

Sc [%] 36.5 36.4

Final moisture content [%] 9.1 8.6

Inicial moisture content [%] 0.80 0.95 Figure 8 - Drying kinetics of the unpainted substrates

Figure 9 - Drying kinetics of the painting solutions

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Table 3 - Characterization of the drying stage I of the painting solutions

tc [h] qc [g.cm-2.h-1] Sc [%]

C CH C CH C CH

NR 92 70 -0,00700 -0,00800 36 36

Ca 46 46 -0,01013 -0,01010 38 45

CaA 46 46 -0,01020 -0,00609 40 48

SA 92 92 -0,00553 -0,00567 37 43

SB 21 21 -0,00820 -0,00977 78 74

TA 46 46 -0,00820 -0,00857 54 54

The silicate paint SA has a different behavior for each mortar. For the mortar C, its drying kinetics is

very close to the unpainted substrate. For the mortar CH, there is a clear reduction of the drying rate.

Despite being the only paint coating that presented a higher duration of the constant drying rate stage

than the unpainted specimens, its evaporation flows are substantially inferior to the substrates. For

the mortar C, the application of the silicate paint SA only originates an increase of 5% in the drying

index, whereas this increase was 14% for the mortar CH, with a clear damage on this mortars drying

kinetics.

Of all the tested paint coatings, it was the silicate paint that introduced the biggest resistance to the

dying process. The drying index suffered an increase of 55% for the mortar C and 24% for the mortar

CH. For both mortars, this paint presented the lowest duration of the constant drying rate stage, 21h,

and the highest critical saturation, 74%. This data proves this paint to be unsuitable to use in old

buildings. In fact, Brito (2009) had also pointed the heterogeneous behaviors of silicate paints. It is

Figure 10 - Drying Indexes of the painting solutions

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necessary to actually understand the behaviors of this paints before assuming their compatibility with

the old buildings.

Finally, the water paint also introduced some resistance to the drying of the substrate, but not as

pronounced as the silicate paint SB.

3.4. INFLUENCE OF PAINT COATINGS ON SALT DECAY PHENOMENA

The decay patterns obtained for the uncoated mortars were very similar, Figure 11, with the

occurrence of efflorescence on a high percentage of the surface area. Throughout the test it was

verified an increase of the efflorescence density and height development. The main difference

between the decay patterns of the two mortars is the extension and intensity of the damages. The

mortar CH presented slower damage evolution than mortar C.

The simple limewash presented the same decay pattern of the uncoated specimens. It was also noted

in this case the slower tendency of decay evolution of the mortar CH.

As for the modified limewash, Figure 13, it presented the same decay patterns for both mortars, but

very distinctive from the previous situations. The formed efflorescence only occupied 50 to 70% of

Figure 11 - Salt decay pattern of the unpainted specimens at the end of the test. a) Mortar C. b) Mortar CH.

Figure 13 - Salt decay pattern of the specimens painted with the modified limewash at the end of the test. Mortar C

Figure 12 - Salt decay pattern of the specimens painted with the silicate paint SA, at the end of the test. Mortar C

17

the surface area, had low density and little height development. In this painting solution also appeared

dark stains surrounding the efflorescence, formed since the early stages.

The silicate paint SA revealed different behaviors for each substrate. At the end of stage 1, in mortar

C, Figure 12, it was possible to observe some punctual efflorescence. Throughout the next stages,

there was an evolution of the efflorescence accompanied with the blistering and cracking of the

finishing in the areas where the efflorescence appeared. At the end of the test, the coating was

completely damage in the areas of the surface affected by efflorescence and well-preserved in the

areas without salt crystallization.

In mortar CH it was observed 3 different decay patterns. One of the specimens, Figure 14 – a),

presented bome blistering at the end of stage 1 that developed and resulted in the cracking of the

finishing at the end of the test. Another specimen had the generality of its surface covered with

efflorescence, Figure 14 – b), and the last specimen only presented some minor cracking at the end

of the test.

Both the silicate paint SB and the water paint TA bahaved in a similar way of the substrates with the

formation of efflorescence in a high percentage of the surface area, with the agravation of the

crystallized volume throughout the stages. As had happened for the unpainted specimens, the

development of the efflorescence was more progressive in mortar CH.

It is now presented the obtained capillarity curves of the capillarity tests conducted before and after

the crystallization tests.

Analyzing Figure 15, it is concluded that both mortars present reductions in the coefficients of water

absorption, reduction that is higher in the mortar CH. As for the final quantity of absorbed water, both

mortars have reductions of 17%.

Regarding the limewashes, the two tested solutions presented very distinctive behaviors. The simple

limewash had similar variations of the capillarity curves to the unpainted specimens in mortar C,

Figure 14 - Salt decay pattern of the specimens painted with the silicate paint SA, at the end of the test. a) Blistering and cracking. b) Efflorescence

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Figure 16. In mortar CH, it was observed an increase of the coefficient of water absorption and final

quantity of absorbed water, situation that is, in the authors view, unexpected and not normal. As for

the modified limewash, Figure 17, it is verified a pronounced reduction of the coefficient of water

absorption and reductions of the final quantity of absorbed water close to 50%.

The silicate paint SA, Figure 18, had an aggravate effect of the water absorption by capillarity similar

to the modified limewash and the silicate paint SB, Figure 19, behaved close to the unpainted

Figure 15 - Water absorption kinetics of the unpainted mortars, before and after the salt crystallization test

Figure 16 – Limewash - Water absorption kinetics, before and after the salt crystallization test

Figure 17 – Modified Limewash - Water absorption kinetics, before and after the salt crystallization test

Figure 18 - Silicate paint SA - Water absorption kinetics, before and after the salt crystallization test

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specimens. The water paint TA presented a distinct effect in each mortar, Figure 20. For mortar CH

there was a clear hindering of the water absorption by capillarity and for the mortar C the results are

practically the same, before and after the salt crystallization test. This disparity was not expected.

The overall results obtained allow to state that both mortars behaved as “salt transport renders”,

affirmation that is more explicit in mortar C. Due to the minor duration of the stage I of drying of the

mortar CH, the front of evaporation recedes from the substrates surface to the interior sooner than in

the mortar C, which implies that a higher quantity of salt crystallizes in the mortars interior. When a

paint coating that hampers the evaporation is applied, causing the reduction of the duration of the

stage I of drying, therefore resulting in a bigger quantity of salt crystallizing in the interior of the mortar,

its effect was more severe in the mortar CH.

Through the water absorption by capillarity tests that were executed, it is possible to deduce that the

modified limewash, the silicate paint SA and the water paint TA (only in mortar CH) led to greater

quantity of salt to crystallize in the interior of the mortars, since these painting solutions originated

reductions superior to 50% in final quantity of absorbed water.

It is important to mention that old buildings renders

possess low mechanic resistances, so a good behavior regarding the salt crystallization phenomenon

in this materials, is one that guarantees their transport to the surface, preventing them to crystallize

in the interior of the mortars, where they can originate tensions greater than the mortars mechanic

resistances. Thus, regarding the compatibility between decorative painting solutions and renders of

old buildings, those solutions with salt decay patterns mainly as efflorescence are, in principle, more

indicate to use in this kind of renders. In this terms, the limewash presented a better behavior than

Figure 19 - Silicate paint SB - Water absorption kinetics, before and after the salt crystallization test

Figure 20 - Water paint (TA) - Water absorption kinetics, before and after the salt crystallization test

20

the modified limewash, behaving closely to the unpainted specimens. The silicate paint SB and the

water paint TA also presented a satisfactory behavior, contrarily to what happen for the silicate paint

SA. This last paint behaved as a barrier to the salt crystallization on the surface, which only happened

when the salts disrupted the coating.

CONCLUSIONS

This experimental work indicates that the limewashes decisively are the paint coatings more

compatible with the construction typology of the old buildings. The use of an organic addition

introduces very interesting improvements in the performance of the limewash. The obtained painting

solution becomes easier to apply, introduces a higher protection of the substrate in terms of water

absorption, homogenizes the water vapor permeability of the specimens, which turn out to be very

close to the unpainted substrates, and does not hinder the acceleration effect that the limewashes

have on the drying process. The only feature of the limewash that was damaged by the mixing of the

organic addition was the behavior regarding the salt decay phenomenon. However, due to the

essential qualitative character of the carried test procedure, it is not possible to quantify the extension

or the impact of this difference in the overall behavior of the renders.

Knowing that the limewashes present reduced durability, and their use in urban or polluted

environments is not recommended, it is necessary a compatible alternative with the old buildings

renders. The silicate paints clearly assume this role. Nevertheless, there is a widespread idea that

the suitability of this paints for the use in old buildings is absolute, which this experimental work proved

to be a misconception. Even though there were only tested two silicate paints, chosen purposely for

their differences, it was observed a not negligible difference in their behavior. Both silicate paints

tested presented high protection of the substrate in terms of water absorption and a satisfactory water

vapor permeability. However, silicate paint SB had a clearly unsatisfactory influence in the drying

kinetics of the tested specimens and the silicate paint SA reveled the worst response to the salt

damage features. Both the carried experimental work and the consulted bibliography indicate that

there is a high diversity of these paints, and to state that they are compatible with old buildings may

be true, but only until certain point. There is a clear lack of unambiguous criteria expressing

acceptability limits as exists, for instances, in factory-made replacement mortars used in rehabilitation

works.

On the other hand, the tested water paint presented a performance higher than expected in the

generality of carried tests, indicating that its utilization in old buildings could not be as damaging as it

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is usually anticipated and it could have some interesting applications in old buildings with little

historical value and good environmental conditions.

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