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Increasing Durability of Weak Lime Mortars Using
Introduction
Lime mortars have been broadly used in the
Medieval period in building as it has been considered as friendly mortar to the
building stones as well as to the environment. Chester City walls at Chester
City, UK as well as other Medieval structures had been built from natural
sandstone blocks cemented with such hydraulic lime mortar [1]. Not only that
but also the ancient Medieval Aachen City walls, Germany have also been built
from fine grained sandstone cemented with such type of mortars [2]. Recently, such
type of mortars become week and easily weathered by the severe and aggressive
weathering processes (particularly salts created from acid rains, de-icing salt
...etc.) acting on all building materials [2-5].
Increasing durability of such weak mortars
using some additives becomes a must. Unfortunately; a lack of experimentally
supported publications and detailed information about the impact of lime water
treatment on lime mortars particularly the weak ones have been found. Some
previous literatures regarding hardening weak mortars presented only marginal
data [6,7] or the results attained under certain limited conditions may be
broadly interpreted. It may be extrapolated into conditions where no enough
information is available, and so, no precise information or evidence can be
found regarding this point of view [8] Consequently, the number of relevant
articles is quite not enough to understand this topic i.e. it still requires
several investigations to cover it for the best of archaeological conservation
Angeli et al 2007 [9]. Only the most relevant experimental studies have been mentioned,
theoretical works analyzing very important questions of binding mechanisms [10]
analso literatures illustrating confusions in terminology (limewater against
lime wash) [11] or presenting discussions against the application of limewater
for consolidating mortar and/or building stone.
The efficacy of limewater applied in situ
for conservation of wall paintings on lime mortar rendering has been previously
studied [12]. Those researchers tested limewater treatments from the following
points: the application procedure, the number of applications that are ranging
from 20 – 60 cycles, dosage and maturation.
They concluded that continuous “wet”
applications ended with a consolidation effect, unlike applications with
“drying” breaks, which do not consolidate the wall paintings. However, the
observed consolidation tended to consider fixation of a released surface paint layer,
for which limewater was recommended to be applied in some publications [13].
Previous literatures had not considered measuring of mechanical characteristics
of the treated mortars before and after treating with limewater. On applying
limewater for 40 cycles on stone as well as on crushed limestone, a very small
increase in calcite content has been observed in the material, no observable change
in the mechanical characteristics (particularly compressive strength), and no
consolidation effect on the crushed material have been recorded [13]. Limewater
effects and the use of meta-kaolin as an additive in limewater were
investigated [14]. The limewater wasapplied for 40 cycles on stone and also on
crushed limestone sand, and it has been found a very limited increase in the
amount of calcite in the material, no observable change in the mechanical
characteristics, and no consolidation effect on the tested crushed material [14].
Applying the limewater and limewater with meta-kaolin for consolidating old
rendering with low cohesion at both of laboratory and in situ tests had been
conducted [13]. It was concluded that the tested consolidates increase the
mechanical resistance only of the superficial layers [14]. Based on the
previous investigations in this field of render consolidation. The current
study aimed to conduct an experimental program to reveal the fundamental
behavior of weak lime mortars when subjected to multiple saturations and evaporation
of distilled water, a saturated solution of calcium hydroxide in water
(“limewater”), limewater with added meta-kaolin, and saturated solution of
barium hydroxide.
Simply, the main aim of the current study
is to offer an objective evaluation of the impact of adding limewater to
friable mortar, and to ascertain the degree of consolidation. The consolidating
effects not only of limewater but also of multiple applications of some other
treatments e.g. distilled water, limewater blended with meta-kaolin, and barium
water (a barium hydroxide saturated solution in water). Barium hydroxide was
considered as an alternative consolidating agent to calcium hydroxide and is
expected to be more effective as it is much more soluble in water than calcium
hydroxide[13].
Methodology
To conduct and achieve the aims of the
current study, a well performed plan of research has been structured as shown
in Flow chart (1). The following sections are detailed explanation of the methods,
tests and results of the measurements conducted for the control and treated
mortar samples to find out which one of the suggested treating materials with
which regime of applying we get the ultimate enhancement in hardness and
durability of the weak lime mortar.
Lime
mortar test specimens
Surveying the previous literatures, it has
been indicated a very slight effects of multiple wetting of historic mortars or
stone with limewater, regarding its penetration depth and strengthening of the
material under consolidation. Test specimens have been prepared in the form of
short tubes for applying and testing compressive strength for these samples.
The specimens were made of lime mortar prepared in the laboratory from powdered
lime hydrate and sand. It is the same composition as that of the mortar
composing a binding material in the walls of ancient Aachen City (Figure 1).
For this purpose, the historic lime render
with quartz aggregate was sampled from the medieval Aachen City walls and the sample
of about 200g was dissolved by the acid dissolution. The aggregate separated
from a disintegrated mortar by filtration was dried at 60 °C till reaching
constant weight. In accordance with the historic render, the sand was mixed and
used for preparation of the model laboratory mortar’s samples in a ratio of 1:8
by volume were accurately mixed in the laboratory to prepare a weak lime
mortar.
The steps of mixing lime-mortar components
have been listed as follows: First, water was poured into the mixing bowl then
the lime hydrate was added, and the lime mixture was mixed for about 25 minutes
in a laboratory mortar mixer to ensure perfect mixing of these components.
After that, the sand was poured into the lime and finally, the mortar was
properly mixed for about 40 minutes to ensure complete mixing and preparation
of this created lime mortar. The fresh mortar was stored in a closed plastic
bag to keep it away from carbonation process. The specimens were prepared by casting
of the fresh mortar in a stainless-steel cast, and they were well compacted.
This enabled the specimens to be safely pushed out from the cast immediately
after molding and prevented the development of shrinkage defects in the
prepared mortar samples.
The tubular cast shape of specimens for the
compressive test increased the ratio of surface/cross section area and
intensified the measurable compressive strength. As the study was focused
mainly on compressive strength of the consolidated mortar, Eight tubes for
compressive strength testing were prepared for each mode of consolidation
treatment (consolidation agent, application regime, and number of applied
cycles). The specimens were cured by slight spraying of distilled water, for
about one month, to support carbonation process for these samples. All specimens
were kept in a conditioned room before testing at a controlled environment
(i.e. temp.20 °C, and Rh 70%). After that, the consolidation agents were applied,
and on completion of the consolidation treatment, the specimens were left to
mature for another 60 days, which was sufficientto allow carbonation of the calcium
or barium hydroxides applied into lime mortar specimens. The other tests (Pore
size distribution “PSD”, microscopic examination and durability investigation)
were carried out on samples prepared as a copy of that used for conducting
compressive strength testing. Therefore, the tests follow a sequence from
destructive to non-destructive ones.
The micro-texture of the created weak
mortar can be noted, with its components, under the magnification power of the
transmitting polarizing microscope (Figure 2) and for more clarification on the
texture of this mortar, it has been examined under the high magnification power
of scanning electron microscope (Figure 3).
Consolidation
of mortar specimens
In the current study, four consolidating
substances were applied, for the created mortars, they are namely: distilled
water, calcium hydroxide saturated solution in water (known as “lime water”),
limewater mixed with meta-kaolin and barium hydroxide saturated solution in
water (known as “barium water”). Water solutions of calcium hydroxide and
barium hydroxide were prepared, i.e., for the limewater solution 3g of Ca(OH)2
were dissolved in one liter of distilled water; while for the barium water
solution, 6g of Ba(OH)2·8H2O were dissolved in one liter of distilled water.
The solubility of barium hydroxide in water made it possible to prepare “barium
water” with a higher concentration of barium hydroxide (6% weight) than for the
limewater (0.18% weight) [15-18]. The limewater with meta-kaolin was prepared
by mixing 3g of calcium hydroxide and 3g of meta-kaolin in one liter of
distilled water. Meta-kaolin used in the current study for modification of the
limewater was a finely ground burnt clay-stone with relatively high amount of alumina
(52.5% SiO2, 43.3% Al2O3). The meta-kaolin has the particle diameter at 40% of
particles equal 4μm and 60% of particles size was less than 12μm. Limewater,
barium water and limewater with added meta-kaolin were prepared and stored in
closed glass barrels at controlled laboratory conditions (temp. 25 °C, and 40% RH)
throughout the experiment. For consolidation treatments, a solution above the
solid sediment was poured off and used. Each agent was applied, on the created weak
mortar, by continually dripping it from a syringe on to tubes fixed in a
horizontal position on a rotating shaft (Figure 4).
The mortar specimens were fully saturated
during each application of each consolidating agent. The treatment by limewater
has been conducted by means of many applied cycles of sprayed limewater into
the created lime render samples. The same schedule was used for the distilled
water treatment to find out the difference between effects of limewater and
distilled water. In respect to other two studied agents (limewater with
meta-kaolin and barium water), the applications were realized with a purpose to
compare the obtained effects with limewater applied at the same condition.
Two different regimes of the drying time
interval between two following saturations were tested for the limewater: first
regime, two applications per day were conducted, and the mortar tubes were allowed
to dry completely before the following saturation (wet to dry alternative); and
second regime, three applications per day were performed. The new dose of the
lime or distilled water was applied to the created mortar once the mortar was
capable to absorb it, but before it dried out completely (wet to wet
alternative). However, the intended number of application has been precisely
managed in the experimental work and actually the applied cycles of console dating
agents have slightly varied from the original schedule (60 cycles were realized
for the wet to wet alternatives and 200 cycles for the wet to dry alternative).
The main aim of this study is to examine the influence of lower and higher
repeated applications number for the consolidating materials on the created
render that imitate that of Aachen City walls.
Mechanical
characteristics
The mechanical measurements for the created
mortar samples have been conducted, in the current study at laboratory
conditions (Rh 60%, Temp. 20 °C), the crosshead velocity movement of 0.45 mm/min.
The short mortar tubes were loaded along the tube axis as shown in Figure 5.
The attained compressive strengths had been
checked for the untreated reference (control) specimens of tube shape (of dimensions
40mm diameter and 60mm length). The average compressive strength was calculated
as an average value, from tests conducted, for eight samples of the created mortar
at each regime of the considered consolidating material (fresh water, lime
water, limewater with meta-kaolin; and barium water). The results of the
mechanical tests conducted in the current study are presented in Figure 6 and listed
in Table 1. Mostly, five specimens had been tested, in few cases, the fragile mortar
samples had not sustained treatment and was damaged before testing, then, only
four specimens were tested. The amount of new calcite after limewater treatment
is sufficient to make a slight improvement in the shear cohesion
characteristics, not only that but also in the surface cohesion characteristics.
Regarding data listed in Table 1 and graphically presented in Figure 6, it can
be noted that barium water treatment is about three times (particularly for wet/dry
regime applying it 200 cycles) as that of the control samples. Not only that
but also, it is two times more efficient than lime water treatment that
corresponds to a higher concentration and the high solubility of the barium hydroxide
in the solution. Increasing number of applied cycles (for each of limewater and
barium water) from 60 cycles (at wet/wet regime as three applications/day) to
200 cycles (at wet/dry regime as two applications/day), an increase in the
compressive strength of the created mortar sampleshas been noted (Figure 6).
The results show that there is no apparent difference in compressive strength
between distilled water application and limewater application mixed with
meta-kaolin (wet/wet application for 60 cycles) (Figure 6). There is a
considerable increase in the compressive strength of a poor lime mortar after 200
cycles (in the case of two saturations regime, wet/dry regime either for
limewater or barium water, Figure 6). The combination of limewater with
meta-kaolin did not provide any noticeable input or advance in the mortar’s
strength (Figure 6). This indicates that the products of the pozzolanic reaction
of meta-kaolin and calcium hydroxide in limewater were not water-soluble, i.e.,
did not penetrate throughout the mortar under investigation, and therefore did
not improve its compressive strength (Figure 6). The lime presents in the lime
meta-kaolin suspension was partially consumed due to a pozzolanic reaction with
meta-kaolin, and the following consolidation treatment of the mortar with lime–meta-kaolin
water was less effective than treatment with simple limewater. Samples undergoing
the same number of treatments show a higher compressive strength when barium
water was employed regarding to limewater.
The observed improvement in mechanical
properties is not high as it is expected considering the higher concentration
of Ba(OH)2 in the saturated solution. In fact, the strength after
consolidation, using Ba(OH)2, was three times higher than that of the untreated
(control) mortar. Distilled water did not show any consolidating effecton the
tested mortar with a low lime content. In this case, only compressive strength
was measured, and the difference from the control specimens was insignificant
(Figure 6). Probably the repeated dissolution and precipitation of the calcium
carbonate presented in the treated mortar with a low content of lime, was not
associated with a significant re-distribution within the volume of the specimen
and no relevant micro-structural changes was occurred.
Change
in structure
Solutions with higher content of the active
agents are considered as enough salt sources to a given building material e.g.,
brick, concrete, mortar. Such agents act as binding material for the unit components
at the beginning till the whole pores and fractures are completely filled with
such salt then, such agents (particularly the salts) start exerting stresses on
the unit structure leading to its deformation [19-21]. Figure 7 (a-b)
illustrates the basic differences between untreated (control) mortar and
mortars treated with limewater and barium water, respectively, at the high
power of magnification using the scanning electron microscope.
It has been noted that calcium carbonate
has grown in the columnar form within the texture of the untreated mortar,
together with tabular crystals. The mortar’s matrix is quite thin, with weak bridges.
After 200 cycles of limewater treatment in the regime of full drying between
subsequent applications, the matrix is filled with layers of newly formed
discontinuous clusters of calcium carbonate. The difference between the
consolidating matrices of lime and barium water can be noted (Figure 7). Barium
water obviously resulted in a denser and better-connected microstructure, i.e.,
barium hydroxide presents higher efficiency as consolidating material compared
with limewater.
A microscopic study of the cross-sections
conducted in the created and treated mortar samples, focused on the
distribution of consolidates into the texture of the created mortar specimen through
a depth profile starting from sample’s surface to its deep inside. It has been
noted that calcium carbonate on the surface layer of the created mortar, in the
first case for the reference mortar, and in the second case for the mortar
treated with 200 cycles of lime water, where a much thicker layer of calcium
carbonate is visible. Barium carbonate presents dense structure on the treated
mortar surface. Only the mortar samples treated with barium water presented a
significantly higher deposition of barium (in the form of barium carbonate)
mainly on the surface of created and treated mortar samples.
Change
in porosity
The pore size distribution had examined for
the reference and consolidated mortars using Mercury intrusion porosimetry (MIP
AutoPore IV 9500) with pressure range of 13,000 to 30,000 MPa to achieve all
pores within the examined samples. This test aims to find out the impact of
different consolidating materials on the pore size distribution (PSD) of the
created mortars and such PSD has been previously reported to be the main
culprit behind material durability to weathering particularly by salts [22,23].
Five mortar samples copying those used for measuring each consolidation treatment
(followed by calculating average values) for more data accuracy were also used
for PSD measurements. The MIP results of the examined mortar samples indicated
a slightly reduction for the porosity of the mortars particularly treated with
limewater and barium water (Table 2a). For the lower number of consolidates (limewater
or barium water) applications (60 cycles; wet/wet application; 3 apps./day),
the porosity has been slightly shifted from mega-pores to meso-pores and
micro-pores and the total porosity has been slightly decreased regarding its
original porosity (Table 2a). While for the higher number of applications (200
cycles; wet/dry deposition; 2 apps./day), the porosity had been noticeably shifted
from mega-pores to almost meso and micro-pores and the total porosity had been
noticeably reduced regarding its original value (Table 2a). No significant
difference, in reduction of samples’ porosity, was noted for limewater with
meta-kaolin compared to simple limewater consolidating materials (Table 2a).
The shifting of mortars’ pore size distribution at each consolidating material
at each regime of application, regardless water and limewater mixed with
meta-kaolin that haven’t result in any progress in mortars durability, has been
noted on the MIP curve (Figures 8a - 8e). The MIP enabled defining the salt
susceptibility index (SSI) based on the PSD of the given mortar samples, and
the interpretation of the SSI values has been based on [23] classification
(Table 2b). It indicated a progress in mortars durability (by shifting SSI to
salt resistance trend, Table 2a) particularly for those treated with barium
water (wet/dry regime).
Durability
test
It is a measure of materials resistance to
weathering/damage on its exposure to artificial weathering at conditions as
those dominate at the study area but with condensed limits to highly reduce time
from years scale to days scale [24]. The durable material is that expresses low
weight loss percentage at the end of the test and vice versa. The weight loss
percentage has been computed, at the end as well as every two cycles of the
sixteen cycles of artificial salt weathering, for each mortar before and after
treatment with each consolidating material following the equation of [25] given
below:
Weight loss (%) = ((W1 – Wn) / W1)* 100
where W1 is the initial weight of a given sample, Wn is the sample’s weight at
the end of the test, the results are listed in Table 3, and graphically represented
(Figure 9).
From the graphical representation of the
sample’s weight through the sixteen cycles of artificial weathering (for the
control and the treated mortar samples, Figure 9), it is clear that the first three
cases of mortar behave nearly the same. In another words, they present slight
increase in their weight as salt ingresses and fills mortars’ pores in the
first four cycles of durability test, then, anoticeable weight loss (decrease in
samples’ weight) has been reported (Figure 9 and Table 3). On the other hand,
the mortar samples treated with either lime water or barium water at each of
the two (wet/wet or wet/dry) regimes of application, present a noticeable
withstand against weathering (particularly for barium water applied for 200
cycles, wet/dry regime) till the 12th cycle of attack.
Then, slight to very slight weight loss can
still be noted till the end of this test (Figure 9). This regime of weight loss
is corresponding with the microscopic investigation of the treated samples that
indicated noticeable ingress of barium water (more than limewater) within
mortar’s pore reducing pore size distribution (as indicated from MIP
measurements conducted for these samples, Table 2a). This indicated the
efficacy of barium water and limewater particu larly limewater as well as
barium water at wet/dry regime of application for that weak mortar. By plotting
the weight loss percentage of the control and treated mortar samples (Table 3)
on [26] (Figure 10), the difference in durability can be noted among these
cases. The three cases (control, treated with water, treated with limewater
mixed with meta-kaolin) of the mortars have nearly the same durability without
any impact of treating with water or limewater mixed with meta-kaolin, they are
in durability class E i.e. very low durability class (Figure 10). While the
mortar treated with limewater at its two regimes are falling within Class D
i.e. low durability class which is better than class E [27]. Lastly, the mortar
samples treated with barium water at its two regimes are within class C i.e.
moderate durability class (Figure 10). This progress in cohesion/durability
class of these treated mortars to the limewater and barium water trend (Table
3) is highly matching with SSI determined by MIP (Table 2a) (Figure 11).
Conclusion
Limewater treatment of a specific lime
mortar had proved to be effective after a considerably large number of
immersions (200 cycles of immersion) into a weak lime mortar. Some poor mechanical
characteristics (compressive strength) were improved substantially after a
large number of saturations. No consolidating effect of distilled water on the
compressive strength of tested mortars with a low lime content (1:8) was
observed. The higher concentration of barium hydroxide in its saturated solution
resulted in higher limits of compressive strength than in specimens treated in
the same mode with limewater, but the increase was not as large as would have
been expected according to the concentration of the barium water. The
improvement in compressive strength after consolidation was more than three
times higher than the strength of the untreated (control) mortar samples. A
microscopic study verified differences between the consolidating matrices of
lime and barium water, where barium water clearly built a denser and better coherent
substance. There was no detectable benefit of modifying limewater with
meta-kaolin in terms of the mechanical characteristics of the treated mortar.
The evaluated consolidating materials, in particular the barium water, reduced
the porosity and pore size distribution of the investigated mortar. Concerning
the distribution of the consolidates into the mortar specimens, a higher
deposition of barium carbonate on the surface layer of the mortar was detected
by scanning electron microscope. This finding corresponds with the results of
the durability test conducted for these mortars over sixteen cycles of
artificial weathering. The research reported here did not aim to optimize the
application of various agents, only to make a comparison under specific
conditions. This should be considered in order to avoid misinterpretation of
the results.
Acknowledgement
The authors are greatly thankful for the
intensive reading and discussion of the results with Prof Martin Derek and his
remarkable editing of English language of this work. They are also thankful for
the financial support of DAAD during the fellowship at Aachen University
(RWTH).
Conflict
of Interest.
No conflict of interest.
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