Construction and Building Materials 181 (2018) 394–407Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier .com/locate /conbui ldmatNanolime for the consolidation of lime mortars: A comparison of three
available productshttps://doi.org/10.1016/j.conbuildmat.2018.06.055
0950-0618/ 2018 Elsevier Ltd. All rights reserved.
⇑ Corresponding author.
E-mail address: b5039083@my.shu.ac.uk (J. Otero).J. Otero a,⇑, V. Starinieri a, A.E. Charola b
aMaterials and Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, UK
bMuseum Conservation Institute, Smithsonian Institution, Washington, DC, USA
h i g h l i g h t s

 The consolidation effectiveness of Calosil
 and Nanorestore Plus
 is reduced with time.

 L’Aquila nanoparticles are more reactive which tend to grow better developed calcites.

 L’Aquila nanolime provides higher durability to dissolution.

 All treatments reduce the porosity in the surface that reduces evaporation rate.

 The whitening effect caused by nanolime treatment is reduced after accelerated weathering.a r t i c l e i n f o
Article history:
Received 21 January 2018
Received in revised form 3 May 2018
Accepted 6 June 2018
Keywords:
Nanolime
Nanoparticles
Calcium hydroxide
Consolidation
Limestone
Lime mortar
Calosil
Nanorestorea b s t r a c t
Nanolime products are one of the most promising consolidation methods for historic calcareous sub-
strates. Whilst the popularity of nanolime has been growing, its consolidation mechanism still needs
to be fully understood when applied to highly porous substrates. The aim of this paper is to compare
the three available nanolime products in terms of consolidation efficacy on lime mortar specimens. It
is shown that repeated applications of a low concentrated nanolime can increase the superficial cohesion
and the mechanical strength of the mortar within 1 cm from the surface, while also reducing porosity,
number of micro-pores and capillary water absorption coefficient. Nanorestore Plus
 yielded the highest
short-term consolidation effect. However, L’Aquila nanolime showed a higher durability which was
attributed to a better developed crystalline structure.
 2018 Elsevier Ltd. All rights reserved.1. Introduction
The consolidation of degraded calcareous materials is one of the
main challenges in the conservation of historic structures. Calcare-
ous substrates degrade by weathering processes such as crystalli-
sation of salts, biological activity, freeze-thaw action and
chemical attack by acid atmospheric pollutants [1–3]. Consolidants
can help in recovering the strength of degradedmaterials as well as
decreasing the deterioration rate of the substrate. In general, suit-
able consolidants must meet the following criteria: i) be physically,
mechanically and chemically compatible with the substrate; ii)
have a good adhesion to the substrate; iii) increase the substrate’s
mechanical properties; iv) not induce colour or aesthetic changesto the substrate; v) reduce porosity but not hamper moisture
transport through the substrate. The effectiveness of the consolida-
tion depends on the interaction of a number of factors such as the
characteristics of the substrate, the properties of the consolidant
and its compatibility with the substrate, and the application meth-
ods and conditions [4].
Limewater, which is a saturated solution of calcium hydroxide
with a maximum of concentration of 1.5 g/L with lime particles
in a colloidal suspension [5], has been used over centuries to con-
solidate deteriorated limestone or plaster. It has the advantage of
being durable and compatible with the substrate as it is based on
the precipitation of calcium carbonate into the pores of the treated
material by reaction of the calcium hydroxide with the atmo-
spheric carbon dioxide (CO2). The main constraint of the limewater
technique is a low consolidation depth due to limited penetration
of carbon dioxide into the substrate [6]. Furthermore, the
Fig. 1. Sand grading curve.
J. Otero et al. / Construction and Building Materials 181 (2018) 394–407 395application of limewater can cause whitening of the treated sur-
face. The effectiveness of limewater has been greatly discussed in
literature. According to Price et al. [7] when using limewater most
particles are deposited within 2 mm from the surface yielding an
ineffective consolidation. Price’s research has led some stone con-
servators to be skeptical of this treatment. Nonetheless, Brajer [8]
demonstrated that a prolonged uninterrupted application pro-
duces a noticeable consolidation effect and some authors brought
up new perspectives to its practical work such as the use of lime
poultices and of an increased number of limewater applications
[3,9,10].
During the last century, organic consolidants (i.e. synthetic
polymers, such as Paraloid B-72, Mowilith 30 and Primal AC33)
were extensively used in restoration treatments for calcareous
materials due to their immediate strength enhancement, ease of
application and the limitations shown by limewater [11]. These
consolidants proved to be effective in the short and medium term
for some calcareous substrates. However, the low compatibility
with the mineral substrate and their short durability caused more
substrate degradation in the long term, particularly in environ-
ments where temperatures increase above 40 C [3,9,11]. Specifi-
cally, physical and mechanical incompatibility between organic
consolidants and calcareous substrates can cause crack develop-
ment, aesthetic changes and interference with future treatments
[12,13], sometimes with severe consequences [14,15].
Nanolimes were developed to overcome the limitations of the
traditional limewater treatment. Their consolidating effect takes
place by the same mechanism as for the limewater technique but
the smaller size of the lime particles (nanoscale) improves their
performance. The advantages of nanolime compared to limewater
are: i) nanolimes contain higher amounts of calcium hydroxide
particles; ii) lime nanoparticles are more reactive due to their
higher specific surface thus increasing the carbonation rate; iii)
nanolimes penetrate deeper into the substrate because of their
smaller particle size; iv) nanolimes have better colloidal stability
due to their smaller particle size and the electrostatic repulsion
forces between them; and, v) reduced whitening of the treated sur-
face when nanolimes are used. An overview of nanolime syntheti-
sation methods and use as a consolidant for calcareous substrates
can be found elsewhere [16].
The first nanolime to become available on the market was Calo-
sil
 (IBZ-Salzchemie GmbH & Co.KR, Germany) in 2006, followed
by Nanorestore
 (CSGI – University of Florence, Italy) in 2008.
Recently, the University of L’Aquila has patented a new method
to synthesise nanolime where nanoparticles are produced by an
anion exchange process at ambient room conditions in aqueous
suspensions [17–19]. Nanolime produced through this method is
currently in the process of being commercialised.
Both Calosil and Nanorestore have been extensively used for the
conservation of wall paintings and stuccoes, achieving good re-
adhesion of detached particles or pigment flakes [5,16,20] and con-
solidating powdering surfaces [15,20]. However, the effectiveness
of the nanolimes decreases when mass consolidation of a porous
substrate such as deteriorated stone or mortar is required
[21–23]. The effectiveness of nanolime as a consolidant appears
to be influenced by several factors: i) multiple applications of
low nanolime concentration suspensions (i.e. 5 g/L) reduce the
accumulation of consolidating product near the surface and
improves the yield of carbonation within the pores [24,25]; ii)
the type of alcohol used can influence the nanolime deposition in
the pores [26,27] as well as the carbonation process [28], iii) exter-
nal factors such as relative humidity (RH) and exposure time
appear to influence the carbonation rate and precipitation of poly-
morphs [29,30]; iv) age and storing temperature of the nanolime
affects the conversion of Ca(OH)2 particles into Ca-alkoxides which
can decrease its effectiveness [31].The aim of this work is to compare the consolidation effective-
ness of the three available nanolime products on lime mortars and
investigate their long term performance. The influence of the nano-
lime treatments on mortar superficial cohesion, water absorption
by capillarity, drying rate, drilling resistance, pore structure and
aesthetic properties have been investigated.
2. Materials and methods
2.1. Lime and sand
Singleton Birch Ultralime CL90 (98% Ca(OH)2, measured by
XRD and XRF) and silica sand from Pentney (UK) were used
throughout this work for the mortar mixes. The sand grading is
shown in Fig. 1. The mineralogical composition of the sand, which
was determined by XRD (PANalytical XPert PRO) using Rietveld
refinements, is 96.3% Quartz (SiO2, ICSD #00-046-1045) and 3.7%
Potassium Feldspar (KAlSi3O8, ICSD #01-076-0831).
2.2. Nanolime
Three nanolime dispersions were used throughout this work:
- Nanorestore Plus Propanol 5
 (CSGI Consortium – University
of Florence, Italy): 5 g/L calcium hydroxide in 2-propanol. Parti-
cle size 100–300 nm. This dispersion is referred to as NAN.
- Calosil IP5
 (by IBZ Salzchemie GmbH & Co.KG, Germany): 5 g/
L calcium hydroxide in 2-propanol. Particle size 50–150 nm.
This dispersion is referred to as CAL.
- Nanolime synthesised through the method developed by
Taglieri et al [17] at the University of L’Aquila: 5 g/L calcium
hydroxide in 50–50% water – 2-propanol. Particle size 20–80
nm. This dispersion is referred to as LAQ.
LAQ was synthesized through an anionic exchange process car-
ried out at room temperature and ambient pressure by mixing
under moderate stirring an anion exchange resin (Dowex Mono-
sphere 550A OH from Dow Chemical) with an aqueous calcium
chloride solution following a methodology described by Taglieri
et al. [17,18] and Volpe et al. [19]. When these two components
are mixed together, the substitution of OH groups in the resin with
chloride ions (Cl) in solution leads, in conditions of supersatura-
tion, to the formation of pure Ca(OH)2 nanoparticles, following
the reaction:
CaCl2 þ 2 R - OHð Þ ! Ca OHð Þ2 # þ 2 R - Clð Þ
The concentration of chloride was monitored during the process
(Vernier Chloride Ion-Selective Electrode CL-BTA) and when this
396 J. Otero et al. / Construction and Building Materials 181 (2018) 394–407reached a constant value below 30 mg/L, the stirring was stopped
and the aqueous suspension was separated from the resin by
means of a sieve (80 mm). The supernatant water was then
extracted through a pipette and replaced with 2-propanol in order
to maintain a concentration of 5 g/L in a 50–50% vol. water-2-
propanol solvent. This is considered the optimal formulation for
this nanolime in terms of carbonation and kinetic stability [18,32].
The decrease rate of chloride content during the synthesis was
very rapid, with about 97% of the reduction occurring within the
first minute of the process. The synthesis was stopped after 15
min when the ion exchange process was completed (zero kinetic
exchange), with a total reduction of chloride content of 99.82%
and a residual chloride content of 0.18% (29.4 mg/L). Similar results
have been reported by Taglieri et al. [17–19,32].
2.3. Mortar preparation and testing
Mortars were produced with a volumetric binder:sand ratio of
1:2.5 to a constant flow of 16 cm measured according to BS EN
1015-3:1999 [33]. Although mix proportions are expressed by vol-
ume, mortars were batched by weight after accounting for compo-
nent densities, which were measured in accordance with BS EN
459-2:2010 [34]. The water:binder ratio was 1.56 to obtain the
desired flow. Mortars were produced by mixing the dry ingredients
with water in a Hobart mixer. The mix protocol was as follows: 1)
dry mix sand and lime for 2 min at 62 rpm; 2) add water while
continuing mixing at 62 rpm for 30 s; 3) stop the mixer for 30 s
and scrape the mixer bowl; 4) mix for 5 min at 125 rpm. Samples
were cast in 40  40  160 mm steel moulds in two layers and
vibration-compacted. Immediately upon floating off the fresh mor-
tar, the moulds were transferred to a temperature and humidity
controlled room maintained at 20 C and 65% RH. Mortar beams
were de-moulded after 5 days and stored in the same room for
the following 23 days. On the 28th day of curing, each prism was
cut into 4 cubes measuring approximately 40  40  40 mm prior
to curing for further 28 days in the same conditions.
The mineralogical composition of the mortar was determined
by X-ray Diffraction. Samples obtained from the core of the cube
were ground and sieved through an 80 mm sieve mesh and placed
over an XRD zero-background sample holder; the XRD patterns
were recorded with a step size of 0.0262h, in the angular range
5-702h. Quantitative analyses were carried out by means of
Rietveld refinement [35,36]. X-ray data were fitted using the
pseudo-Voigt profile function. Specimen displacement, polynomial
coefficients for the background function, lattice parameters,
profile parameters, and Gaussian and Lorentzian profile
coefficients were refined. Quantitative analysis by Rietveld refine-
ment shows the mineralogical composition of the mortar is 82.3%
Quartz (SiO2, ICSD #00-046-1045) and 17.7% Calcite (CaCO3, ICSD
#00-005-0586).
2.4. Nanolime characterisation
The size, shape and degree of agglomeration of the calcium
hydroxide particles in the three nanolime products were deter-
mined by TEM (Philips CM200), while the crystalline phase was
analysed by XRD. TEM samples were prepared by dispersing 0.2
ml of nanolime suspension in 20 ml of ethanol and placing a few
drops of the resulting liquid on a carbon-coated copper grid. XRD
samples were prepared by using the zero-background silica sample
holder. Both TEM and XRD samples were prepared in nitrogen
atmosphere, to prevent carbonation of the lime.
The kinetic stability (KS) of the three products was determined
by turbidity measurements, analysing their absorbance at k = 600
nm by means of a UV/VIS Spectrophotometer (UV–vis Spectropho-
tometer Varian 50SCAN). Before the test, nanolimes were agitated,instead of sonicated. Sonicating has been the normal practice in
previous studies [18,20,26,28], however, agitation was preferred
in this work to simulate in-situ conditions where conservators nor-
mally have no access to an Ultrasonic Bath. Measurements were
taken over a period of up to 2 h. The relative kinetic stability (KS
%) was calculated using the following formula:
KS% ¼ 1 ½ðA0  AtÞ=A0  100
where A0 is the starting absorbance and At the absorbance at time t,
both at a wavelength of 600 nm [5,28]. KS % decreases as a result of
the nanoparticles settling; values range from 0 (unstable disper-
sion) to 100 (no deposition of nanoparticles).
2.5. Nanolime carbonation process characterisation
The carbonation process was investigated by means of TEM,
SEM (SEM, NOVA 200 NanoSEM 450) and XRD. For TEM observa-
tions, samples were prepared as described in Section 2.4 and
exposed to air for 1 min prior to carrying out the observations.
For SEM investigations, 0.10 ml of each suspension were placed
on a copper SEM sample holder and exposed to outdoor conditions
in a sheltered area (T  5–15 C, RH  60–80%) for 7 days prior to
carrying out the observations. SEM micrographs were taken with
an ETD detector, a working distance of approximately 3 mm, an
optimum accelerating voltage of 15 kV and a spot size of about
30 nm. Specimens were coated with a 20 nm thick layer of gold
using a Quorum Q150T coater unit to prevent surface charging.
Samples for XRD were prepared by exposing 0.12 ml of each
suspension (NAN, CAL and LAQ) to outdoor conditions in the same
sheltered area mentioned previously for 1 h and 7 days prior to
analysing by the diffractometer. XRD patterns were recorded in
step size of 0.0262h. Each experimental diffraction pattern was
elaborated by means of a Profile Fit Software (HighScorePlus,
PANalytical), and each crystalline phase was identified using the
ICSD and ICDD reference databases. Quantitative analyses were
carried out by means of Rietveld refinements.
2.6. Nanolime treatment application
Each of the three nanolimes was agitated and applied by brush
on the top face (as cast) of three 40  40  40 mm mortar cubes,
allowing the nanolime to be fully absorbed by the mortar between
two consecutive brushstrokes. Nanolimes were also agitated
between each brushstroke. The treatment was continued until no
further absorption was observed for a period of at least one minute
after a brushstroke. At this time, the application was interrupted
and was repeated 24 h later when the specimen was dry. Samples
were weighed before and after each application (after full evapora-
tion of the solvent) to determine the total amount of nanolime
absorbed by the mortar. The treatment was considered complete
when each mortar cube absorbed 500 mg of calcium hydroxide
(approximately 100 ml of nanolime). This required about 20 con-
secutive days of application for each nanolime. Upon treatment
completion, the samples were stored outdoor in the sheltered area
for a period of 28 days (T  5–15 C, RH  60–80%). A set of
untreated control samples was also stored in the same conditions.
2.7. Evaluation of treatment effectiveness
Following 28-day outdoor exposure, the mortar cubes were
dried to constant mass at 60 C in a fan assisted oven and subse-
quently stored in a desiccator until testing. The effectiveness of
the nanolime was evaluated by means of the tests described below.
The influence of the treatments on pore size distribution and
porosity of the mortars was investigated by Mercury Intrusion
Porosimetry (MIP) using a PASCAL 140/240 instrument. Each test
J. Otero et al. / Construction and Building Materials 181 (2018) 394–407 397was carried out on one fragment measuring approximately 5  5
 10 mm collected within 5 mm from the top surface (as cast) of
both treated and control samples. The samples were dried in a
fan-assisted oven at 60 C until constant weight prior to testing.
The mercury contact angle was taken to be 140.
The water absorption coefficient by capillarity (WAC) of both
treated and control samples was measured according to EN
13,755 [37]; three samples being tested for each mortar. Upon
completion of this test, once asymptotic water absorption was
reached, samples were immersed in water for 24 h and their appar-
ent porosity was calculated following ASTM C 97–96 (American
Society for Testing and Materials, 1996) [38], although the actual
immersion time was half of that recommended in this standard.
Their drying behaviour was sequentially calculated according to
CEN EN 16,322 [39]. This sequence simulates a real situation in
an outdoor condition (cycles of dry-wet-dry).
The influence of the nanolime treatments on the surface cohe-
sion of the mortar was evaluated by means of the ‘Scotch Tape Test’
(STT) carried out in accordance with ASTM D3359 [40]. The test
was performed on both treated and control samples and results
taken as the average of 9 measurements per sample.
The consolidation effectiveness of the three nanolimes was also
assessed by means of a Drilling Resistance Measurement System
(DRMS) from SINT-Technology. The DRMS measures the force
required to drill a hole at constant rotation (rpm) and lateral feed
rate (mm/min). It is generally considered the most suitable
methodology for quantifying consolidation effectiveness and depth
of penetration of consolidants, particularly in soft stones [41,42].
Tests were performed on both treated and control mortar cubes
using drill bits of 5 mm diameter, a rotation speed of 200 rpm, a
rate of penetration of 15 mm/min and a drilling depth of 20 mm.
Drilling resistance values per each treatment were calculated as
the mean of 6 tests carried out on 2 cubes per each of the
nanolimes.
The surfaces of both treated and control samples were observed
under a Scanning Electron Microscope in order to evaluate the
morphology and distribution within the pores of the calcite crys-
tals originated from the carbonation of nanolime.
Any surface colour variations induced by the nanolime treat-
ments were evaluated with a spectrophotometer (Minolta
CM508D Colorimeter) with the CIELab system [43]. 30 measure-
ments were taken in different areas of the surface of each of the
treated and control mortar cubes. Total colour variation (DE) was
calculated by the formula:
DE ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
DL2 þ Da2 þ Db2
qFig. 2. TEM images of: a) NAN; b) CAL; c) LAQ. Please note that the magwhere DL*, Da* and Db* are the change in luminosity for white-
black, red-green and blue-yellow parameters, respectively.
The durability of the treatments was evaluated using a QUV/SE
Accelerated Weathering Tester from Q-Lab Europe ltd. Both treated
and control mortar cubes were exposed to alternating cycles of UV
light and moisture condensation at controlled temperatures in
accordance to ASTM G154 CYCLE 4 [44] for a period of 340 h. Each
UV cycle has duration of 8 h and uses an irradiance of 1.55 W/m2 at
a temperature of 70 C. The moisture condensation cycles are car-
ried out at 50 C with no UV irradiation. The influence of the accel-
erated weathering on the mortars surface colour and drilling
resistance were assessed by colorimetry and DRMS as described
above.3. Results and discussion
3.1. Characterisation of the nanolime suspensions
Fig. 2 shows TEM photomicrographs of the three nanolime
products. Hexagonal Ca(OH)2 nanoparticles with identical mor-
phology can be observed in all three of the nanolimes. The crystals
tend to agglomerate due to the nanoparticle phenomenon caused
by their high surface energy [26,28,31] (Fig. 2a and b). The size
of the observed nanoparticles is 250–100 nm for NAN (Fig. 2a),
150–80 nm for CAL (Fig. 2b) and 80–20 nm for LAQ (Fig. 2c). These
results are in line with those reported by the developers of the
three products [5,17–20,45,46].
The XRD analysis in nitrogen atmosphere (Fig. 3) shows that the
only crystalline phase of the three nanolimes is Portlandite (Ca
(OH)2, ICSD #01-087-0674), confirming the results of the observa-
tions by TEM. For the three samples the strongest peak corre-
sponds to the {0 0 1} basal plane. However, XRD Rietveld
refinements showed that LAQ Ca(OH)2 particles have a preferred
orientation to the side plane {0 1 0}. NAN and CAL did not show
any significant preferred orientation. The Rietveld refinement fac-
tors are included in Table 1.
Fig. 4 shows the absorbance (at 600 nm) of NAN, CAL and LAQ
dispersions over a period of 2 h following a moderate agitation. A
high colloidal stability was observed for NAN and CAL Ca(OH)2
nanoparticles. Previous studies have shown that NAN and CAL
nanolimes can keep the colloidal state for more than one week
[47]. In contrast, LAQ Ca(OH)2 nanoparticles start to settle about
2 min after agitation (sedimentation rate  6% per h) and that
12% of the particles settle after 2 h of the testing period. This sed-
imentation process is considerably slower than that occurring in
limewater, where more than 90% of the particles settle in 2 hnification bar is 100 nm for NAN and CAL, and only 20 nm for LAQ.
Fig. 3. XRD patterns for NAN (a), CAL (b) and LAQ (c) samples dried in nitrogen atmosphere. Red line corresponds to Intensity observed, blue line to Intensity calculated and
green line to the Intensity observed – Intensity calculated difference curve. The crystal phases are shown in brackets and the ‘‘+” symbol corresponds to the main peaks.
Table 1
Rietveld refinement factors of samples dried in Nitrogen atmosphere and exposed to air in outdoor conditions (60–80%RH) for 1 h and 7 days.
Nitrogen 1 h in air 7 days in air
NAN CAL LAQ NAN CAL LAQ NAN CAL LAQ
R-expected 10.1 9.35 13.89 20.24 20.24 20.03 19.53 15.67 20.03
R-profile 11.69 7.98 11.77 16.13 16.13 15.64 18.52 16.31 15.64
Weighed R profile 14.66 10.38 16.11 20.67 20.67 20.17 24.1 20.81 20.17
Goodness of fit 3.49 1.23 4.34 2.04 2.04 3.01 1.52 1.76 4.31
Phase proportions 100% P 100% P 100% P 90.3% C 100% P 100% C 100% C 75.8%C 100% C
9.7% P 24.2% V
Direction of preferred orientations NPO NPO 10 NPO NPO NPO NPO NPO 104
NPO (No preferred orientation), P (Portlandite); C (Calcite); V (Vaterite).
0 20 40 60 80 100 120
0
20
40
60
80
100
K
S
 (%
)
Time (min)
LAQ
NAN CAL
Fig. 4. Kinetic stability KS (%) of NAN, CAL and LAQ dispersions.
398 J. Otero et al. / Construction and Building Materials 181 (2018) 394–407[48]. The colloidal stability of LAQ dispersion during the first 2 min
after agitation can be considered acceptable for practical purposes
as most of the particles remain in colloidal state for the whole
length of each application [28]. High colloidal stability is critical
to prevent the formation of undesired surface whitening as col-
loidal particles penetrate better into the pores reducing accumula-
tion on the surface [48].3.2. Nanolime carbonation process
The XRD patterns of nanolimes exposed to air for 1 h (Fig. 5)
show the LAQ nanoparticles to be the most reactive, with calcite
(ICSD # 01-085-1108) being the only detected crystalline phase
(Fig. 5c). NAN nanoparticles (Fig. 5a) also showed good reactivity
within the 1 h of air exposure time, with 90.3% of the sample being
composed of calcite (ICSD # 01-072-1652) and 9.7% of portlandite
(ICSD # 01-087-0674). The only crystalline phase detected for CAL
was portlandite (ICSD # 01-072-0156), indicating a slower carbon-
ation process (Fig. 5b). The relatively high background combined
Fig. 5. XRD patterns of NAN (a), CAL (b) and LAQ (c) samples exposed to air in outdoor conditions for 1 h (60–80%RH). Red line corresponds to Intensity observed, blue line to
Intensity calculated and green line to the Intensity observed – Intensity calculated difference curve. The crystal phases are shown in brackets and the ‘‘+” symbol corresponds
to the main peaks.
J. Otero et al. / Construction and Building Materials 181 (2018) 394–407 399with broad and weak Bragg peaks recorded for these samples sug-
gest the presence of poorly crystalline phase(s) (Fig. 5).
XRD analysis after 7 days of air exposure shows a better defined
crystalline structure for all three samples (Fig. 6) resulting from the
completion of the carbonation process. Both NAN and LAQ samples
consist entirely of calcite (CaCO3, ICSD# 01-086-2334 and ICSD
#01-083-0578 respectively) while CAL is composed of 75.8% of cal-
cite (CaCO3, ICSD# 01-081-2027) and 24.2% of vaterite (ICSD# 01-
072-0506). Vaterite is the least stable polymorph of calcium car-
bonate, calcite being the most stable one [29,30]. In previous stud-
ies [24], Calosil was found to precipitate as calcite and aragonite.
XRD-Rietveld shows the calcite crystals in LAQ (Fig. 6c) are fully
oriented to {1 0 4} while the crystals in NAN and CAL have no pre-
ferred orientation. The Rietveld refinement factors are included in
Table 1.
TEM images taken after 1 min of air exposure show that during
the carbonation process NAN and CAL tend to grow mainly as
hexagonal plates (Fig. 7a, b) while LAQ nanoparticles can be seen
to grow preferentially through the {0 1 0} side plane (Fig. 7c).
SEM images taken after 7 days exposure to outdoor conditions
show NAN and CAL being characterised by calcium carbonate crys-
tals of irregular shape (Fig. 7d–e), while LAQ shows calcite in well-
formed hexagonal prisms and parallelepiped shapes (Fig. 7f), pre-
senting larger size (10 mm) than NAN and CAL ones, despite the
smaller original nanoparticle size. Prismatic calcite crystals with
similar shape to that observed for LAQ have been described by
Ukrainczyk et al [49] when examining the influence of additives
on the growth of calcite crystals.3.3. Effectiveness of the nanolime treatments
The pore structure properties of treated and control mortars are
summarised in Table 2. It is evident that all three treatments affect
the pore structure of the mortar by reducing the modal pore diam-
eter and the porosity while increasing the total pore surface area.
The highest porosity decrease was observed for the mortar treated
with NAN (26% decrease), followed by those treated with LAQ
(21%) and CAL (16%). The pore size distributions of control and
treated mortars are shown in Fig. 8. It can be seen that the control
sample has a tri-modal pore size distribution, coarser pores with
diameters between 25 lm and 100 lm, intermediate pores with
diameters between 3 lm and 25 lm and finer pores with diame-
ters between 0.06 lm and 0.7 lm. Following the treatments, a
complete removal of the coarsest mode and enhancement of the
intermediate mode can be observed (Fig. 8). Such enhancement
is more prominent for the mortar treated with CAL (Fig. 8), which
is in line with the lower decrease in porosity observed for this
mortar.
The water absorption and drying curves are reported in Fig. 9.
Apparent porosity by immersion, water absorption and drying
characteristics are reported in Table 3.
Capillary rise in a porous material is strongly influenced by pore
size distribution. The pore diameter range affecting water capillary
rise and transport could theoretically be considered to range
between 1 mm and 1 mm diameter [50]. It is shown that all three
treatments yield a decrease of the capillary water absorption coef-
ficient of the mortar. This decrease can be attributed to the reduc-
Fig. 6. XRD patterns of NAN (a), CAL (b) and LAQ (c) samples exposed to air in outdoor conditions for 7 days (60–80%RH). Red line corresponds to Intensity observed, blue line
to Intensity calculated and green line to the Intensity observed – Intensity calculated difference curve. The crystal phases are shown in brackets and the ‘‘+” symbol
corresponds to the main peaks.
400 J. Otero et al. / Construction and Building Materials 181 (2018) 394–407tion in the number of pores with diameters between 17 mm and
100 mm, which results from the applied treatments and was also
recorded by the MIP measurements. Fig. 9a and Table 3 show that
the NAN treatment yielded the biggest decrease in WAC (42%), fol-
lowed by CAL (14%) and LAQ (4%). Control samples required about
7 h of contact with water to reach asymptotic values while treated
samples took nearly 9 h (Fig. 9a). The latter absorbed less water by
both capillarity and 24-hour immersion, with the NAN treated
sample showing the greatest decreases.
The treatments also reduced the apparent porosity of the mor-
tar, with the NAN treated samples showing the highest reduction
(11.4%) followed by LAQ (5.7%) and CAL (2.1%). These results are
fully compatible with the results of the porosity measurements
by MIP reported above. The average apparent porosity of the trea-
ted samples show a relatively high standard deviation reflecting
the inhomogeneity of the applied treatment, with the CAL (±0.40)
being only twice the value of the control, while LAQ (±0.65) is
thrice higher than the control and NAN (±1.63) being significantly
higher (Table 3).
The drying curves (Fig. 9b) and the measured initial and final
drying rates (Table 3) show that treated mortar samples take far
more time to dry than the control ones. This is an undesirable
behaviour as it increases the risk of spalling when the mortar is
exposed to freeze-thaw cycles (limited to the external 1 cm plus
where the nanolime precipitates) and/or biological attack [51].
The drying rate reduction is attributed to the finer pore structure
of the mortar near the surface of the sample which results from
the nanolime treatment. The smaller pores in the denser mortarlayer reduce the liquid water transport towards the surface hence
slowing down the drying kinetics [51]. The lowest initial drying
rate was observed for the mortar treated with CAL while NAN
and LAQ yielded similar rates. Whilst control samples were com-
pletely dry after 50 h, treated samples still contained some residual
moisture after 72 h (Table 3).
Table 4 shows the results of the Scotch Tape Test (STT). A signif-
icant decrease of removed material (DV  90%) was observed for
all treated mortars with no significant differences in the perfor-
mance of the three nanolimes. The standard deviation of the DV
(%) (SD) shown in the table confirms a good reliability for this test
in the measuring of surface cohesion.
Drilling resistance results are shown in Fig. 10. It can be seen
that whilst the control sample shows a steady drilling resistance
throughout the drilling depth (average force  0.07 N), the treated
samples show increased resistance within the first 10 mm from the
surface. The highest average drilling resistance was observed for
the mortar treated with NAN (average force  0.57), followed by
those treated with CAL (average force  0.32) and LAQ (average
force  0.27). This is fully compatible with the porosity results
which showed a more marked decrease of porosity for NAN treated
samples than for CAL and LAQ treated samples. The average drilling
resistance in the outer 5 mm is 0.09 N for Control, 1.12 N for NAN,
0.68 N for CAL and 0.64 N for LAQ, which is also in line with the
results of the MIP measurements. This trend was not observed in
the results of the Scotch Tape Test, possibly due to a lower sensitiv-
ity of this technique compared to that of the DRMS. The DRMS pro-
files in Fig. 10a-c show that the drilling resistance of the treated
Table 2
Pore structure properties of treated and control mortar samples measured by MIP.
Sample Porosity
(%)
Modal pore diameter
(mm)
Total pore surface area
(m2/g)
Control 22.63 18.09 0.45
NAN 16.75 15.23 0.575
CAL 18.92 13.55 0.585
LAQ 17.87 14.15 0.781
Fig. 7. a) TEM image of NAN after 1 min of air exposure at 124,000; b) TEM image of CAL after 1 min of air exposure at 213,000; c) TEM image of LAQ after 1 min of air
exposure at 28,800; d) SEM image of NAN after 7 days at 8,000; e) SEM image of CAL after 7 days at 10,000; f) SEM image of LAQ after 7 days at 3,000.
J. Otero et al. / Construction and Building Materials 181 (2018) 394–407 401samples decreases with depth until it reaches the resistance of the
control sample at about 10 mm for NAN and CAL and about 6 mm
for LAQ. This data seems to indicate for the NAN and CAL treat-
ments a deeper penetration into the mortar compared to the LAQ
treatment.
The drilling resistance tests were repeated after exposing the
samples to alternating cycles of UV light and moisture (Fig. 10d-
f) and the results show for the NAN and CAL treated samples a
reduction of the depth at which the drilling resistance reaches that
of the control samples. This could indicate that a certain amount of
material got partially dissolved from the surface of these samples
as a result of the exposure to the alternating cycles of UV light
and moisture condensation at elevated temperatures in the tester.
The presence of deposits of white powder material in the water
pan just below the sample holder panel seems to support this
interpretation. The DRMS profile of the LAQ treated sample is sim-
ilar to that recorded before the accelerated weathering test(Fig. 10f), which seems to indicate that less or no material loss
occurred on the surface of this sample. The apparent higher resis-
tance to weathering of the LAQ treated sample compared to that of
the NAN and CAL treated ones could be attributed to a better devel-
oped crystalline structure and larger crystals. As described in Sec-
tion 3.2, LAQ produces perfectly shaped prismatic and
parallelepiped calcite crystals of a larger size compared to NAN
and CAL, which present poorly crystallized ones. The ‘‘well-
shaped” calcite crystals are more resistant to dissolution than
irregular shaped crystals as is well known [52]. In the case of the
CAL treated sample, a reduction of the drilling resistance was
observed within the first millimeter of depth (Fig. 10e), which
could be due to the solubility of metastable amorphous carbonate
phases such as vaterite (see Section 3.2).
Both treated and untreated mortar surfaces were observed by
SEM in order to study the distribution of the nanolime in the pores
and the morphology of the calcite crystals. SEM micrographs
showed for the control samples a great number of large pores (pore
diameter > 100 mm), which are outside of the measurement range
of the used MIP technique (Fig. 11a). Following the treatments,
the reduction of porosity and pore size due to the deposition of cal-
cite inside the pores is clearly visible (Fig. 11b-d).
In Fig. 11b-d it can be seen that the nanolime is distributed
homogeneously in the pores of the mortar and seems to adhere
well to the substrate to the point that it is not easy to distinguish
between the mortar and the newly precipitated calcite which fills
the pores reducing their size. Fig. 12a shows that the calcitic
Fig. 9. Capillary absorption (a) and drying curves (b) for control and NAN, CAL and LAQ treated mortars.
Table 3
Apparent porosity by immersion, capillary water absorption and drying characteristics.
Parameter CO NAN CAL LAQ
Apparent porosity (%) 11.84 (±0.20) 10.49 (±1.63) 11.60 (±0.40) 11.17 (±0.65)
Water absorption coefficient (103 g/cm2 s0.5) 9.5 (±0.78) 5.5 (±1.7) 8.2 (±2.5) 9.1 (±0.5)
Water absorbed at asymptotic value (g) 11.00 (±0.06) 8.95 (±1.05) 10.60 (±0.52) 10.32 (±0.68)
Water absorbed after 24-hour immersion (g) 11.57 (±0.16) 10.34 (±1.61) 11.49 (±0.38) 11.07 (±0.50)
Initial drying rate (103 g/cm3 h) 4.9 (±0.1) 3.9 (±0.3) 4.4 (±0.5) 4.4 (±0.3)
Final drying rate (103 g/cm3 h) 1.0 (±0.3) 1.0 (±1.0) 1.2 (±0.6) 1.2 (±0.5)
Moisture content after 72 h (g/cm3) 0.01 (±0.01) 1.81 (±0.01) 1.94 (±0.02) 2.02 (±0.02)
Time for total drying (h) ±50 >72 >72 >72
Values in parentheses are standard deviation of the three cubic samples.
Fig. 8. Differential volume of intruded mercury versus pore diameter of treated and control mortar samples.
402 J. Otero et al. / Construction and Building Materials 181 (2018) 394–407material resulting from the carbonation of NAN within the mortar
presents a different variety of shapes (mostly rhombohedral with
scalenohedral terminations), which were observed also in previous
studies for the same nanolime [24]. CAL precipitated in the form of
calcite rhombohedra and some vaterite (Fig. 12b). Fig. 12c showstypical calcite prisms on the LAQ sample. These are larger than
those formed by NAN and CAL hence resulting in a smaller specific
surface which is associated with higher resistance to weathering
[52]. This morphology is similar to that visible on the SEM images
(Section 3.2).
Fig. 10. Drilling resistance of: a) NAN; b) CAL; c) LAQ; d) NAN after AWT; e) CAL after AWT and f) LAQ after AWT.
Table 4
Scotch tape test results. Values determined on 9 measurements.
Treatment Removed material (mg/cm2) DV (%) SD
Before treatment After treatment
NAN 26.85 (±1.24) 2.54 (±3.96) 90.54 5.1
CAL 26.71 (±1.56) 1.71 (±0.73) 93.60 2.3
LAQ 26.76 (±1.04) 3.16 (±2.57) 88.19 3.6
Scotch area: 3  1.5 cm; Values in parentheses are standard deviation for the nine measurements taken on each of the three cub samples.
J. Otero et al. / Construction and Building Materials 181 (2018) 394–407 403Ideally consolidation treatments should improve the physical-
mechanical properties of the treated material without affecting
its aesthetic appearance. However, a common side effect of the
use of nanolime is a whitening of the surface following treatment.
In order to investigate the occurrence of this phenomenon in asso-
ciation with the use of the tested nanolimes, spectrophotometric
analyses were carried out to measure changes in L* (white-black
parameter) and DE* (total colour variations) following treatment.
DE* values <5 are considered suitable for practical conservation
interventions and visually imperceptible [28]. The results (Table 5)
show that all of the three treatments cause whitening of the
surface with both DE* and DL* values above 5. NAN yielded the
lowest whitening effect (DL*  6 and total colour alteration
DE*  6). CAL and LAQ caused a more significant whitening effect
with DL*  7.5 and DL*  11.94, respectively, and total colour
change with DE*  9 and DE*  14, respectively. In previous
studies, NAN and CAL yielded chromatic alterations of aboutDE*  6 and DE*  4, respectively [24]. The lower colloidal stability
of LAQ is considered responsible for the more significant formation
of undesired surface whitening. Nonetheless, the observed whiten-
ing and total chromatic alterations are still lower than those
caused by the lime-water technique, which normally vary between
DL*  50 to 70 [9,25].
The whitening effect and total colour changes associated with
the application of nanolime decreased after the samples were
exposed to UV light and moisture cycles in the accelerated weath-
ering test, with both DE* and DL* decreasing below 5. This could
indicate that the induced weathering dissolved a certain amount
of material from the surface of the samples, as also detected by
the DRMS for the NAN and CAL samples. It is apparent that in
the case of the LAQ samples the amount of material that got dis-
solved from their surface during the accelerated weathering test
was sufficient to reduce the whitening but too small to be detected
by the DRMS.
Fig. 11. SEM micrographs of mortar samples (100 X): a) control sample; b) surface treated with NAN; c) surface treated with CAL; d) surface treated with LAQ.
404 J. Otero et al. / Construction and Building Materials 181 (2018) 394–4074. Conclusions
This study has shown that NAN, CAL and LAQ can be used effec-
tively for the consolidation of highly porous calcareous materials
such as lime mortars and represent a viable alternative to organic
consolidants for their consolidation. It has been proven that
repeated applications of low concentration dispersions of these
nanolimes can restore the substrate superficial cohesion, improve
its mechanical properties, reduce its porosity, number of microp-
ores (10–100 mm) and water absorption coefficient.
It has been shown that:

 The Ca(OH)2 nanoparticles of the three nanolimes consist of
crystalline hexagonal plates with similar shapes. However,
there is a difference in size: LAQ nanoparticles are smaller than
both NAN and CAL. XRD-Rietveld refinements showed that LAQ
Ca(OH)2 particles tend to align in a preferential direction along
the side plane {0 1 0} whereas NAN and CAL did not show any
significant preferred orientation. Nanorestore Plus
 and Calo-
sil
 are stable colloidal dispersions while LAQ nanoparticles
start to settle about 2 min after agitation (sedimentation rate 6% per h) and the 12% of the particles has settled after 2 h.
This sedimentation rate is considered acceptable for practical
purposes as most of the particles remain in colloidal state dur-
ing the time of the application. However, a moderate agitation
is highly recommended before the application of this nanolime.
Moreover, the application should ideally be completed within
the first 2 first minutes after the agitation, before the particles
start to settle.

 Nanolime particles created by anion exchange synthesis (LAQ)
are the most reactive with calcite being the only detected crys-
talline phase after 1 h of air exposure. NAN particles also
showed a good reactivity with 90% of the sample being com-
posed of calcite following the same exposure time. CAL particles
showed slower carbonation and no calcite had formed after 1 h
exposure. After 7 days both LAQ and NAN consisted entirely of
calcite, while CAL was composed approximately of 75% of cal-
cite and 25% of vaterite, which is a metastable phase of calcium
carbonate, less stable than calcite.

 During the carbonation process, NAN and CAL tend to grow
mainly as hexagonal plates while LAQ nanoparticles seem to
grow preferentially through the {0 1 0} side plane. After 7 days
Fig. 12. SEM detail micrographs of carbonated nanolime: a) NAN 50,000; b) CAL 50,000; c) LAQ 50,000.
Table 5
Chromatic alterations of treated samples before and after the Accelerated Weathering Test.
DL* Da* Db* DE*
Treated AWT Treated AWT Treated AWT Treated AWT
NAN 6.33 (±0.20) 3.18 (±0.43) 1.48 (±0.16) 0.48 (±0.20) 0.43 (±0.91) 1.96 (±0.55) 6.51 3.76
CAL 7.50 (±0.51) 2.53 (±0.90) 0.75 (±0.13) 0.37 (±0.16) 4.64 (±0.69) 0.88 (±1.15) 8.85 2.7
LAQ 11.94 (±0.96) 4.04 (±0.63) 1.25 (±0.15) 0.55 (±0.0.26) 6.96 (±1.07) 1.17 (±1.45) 13.87 4.24
Values determined on 30 measurements; AWT (Accelerated Weathering Test).
J. Otero et al. / Construction and Building Materials 181 (2018) 394–407 405of carbonation, LAQ shows well-formed hexagonal calcite pris-
matic crystals with larger size than NAN and CAL, which consist
of calcite crystals of irregular shape. XRD-Rietveld shows the
calcite crystals in LAQ are fully oriented to {1 0 4} while the
crystals in NAN and CAL have no preferred orientation.

 All treatments affect the pore structure by reducing the porosity
of the superficial portion of the mortar. The reduction in poros-
ity is attributed mainly to a reduction of the number of pores
with diameter between 17 mm and 100 mm, which also
decreases the capillary water absorption coefficient. Nanore-
store Plus
 presented the highest reduction of the porosity
and water absorption by capillarity rate.
 All three nanolimes increase the drilling resistance of the mor-
tar surface. The penetration depth of nanolime appears to be
approximately 1 cm for NAN and CAL and 0.6 cm for LAQ.
NAN treated mortars showed the highest increase in drilling
resistance, which is in line with their highest reduction in
porosity; followed by those treated with CAL and LAQ. Follow-
ing an accelerated weathering test, the DRMS detected the loss
of a certain amount of material from the surface of mortars trea-
ted with NAN and CAL. This was not observed for LAQ treated
samples, possibly due to LAQ better developed crystalline struc-
ture providing higher resistance to dissolution processes during
the accelerated weathering test. The detected loss is slightly
406 J. Otero et al. / Construction and Building Materials 181 (2018) 394–407more pronounced in Calosil
 samples, possibly as a conse-
quence of the dissolution of metastable vaterite. A study of
the calcite morphology and structural features of LAQ nanolime
must be addressed in future studies to better understand the
performance of this nanolime.

 Following treatment, the samples take far more time to dry due
to a finer pore structure of the mortar near the surface. This is
an undesirable behaviour which may lead to deterioration pro-
cesses. Specifically, spalling can occur where moisture is pre-
sent and the mortar is exposed to freeze-thaw cycles.
Furthermore, longer drying times can favour biological growth
and the dissolution of poorly crystallised calcite and vaterite.
A full study of the durability of mortars treated with the three
nanolimes will be carried out in future research.

 All treatments produced an undesired whitening of the surface
after application. LAQ treatment induced the highest whitening
effect which can be attributed to the lower colloidal stability of
this nanolime. The whitening effect associated with all treat-
ments decreases to values which are imperceptible to the naked
eye after exposing the samples to the Accelerated Weathering
Test. This could indicate that a superficial layer of the consoli-
dated mortar was partially dissolved during the accelerated
weathering test, as also observed by DRMS. In the case of LAQ
treated mortars the dissolution decreased the whitening with-
out causing significant thickness reduction.
One final conclusion of this study is that substrates treated with
the studied nanolimes will require regular maintenance (i.e. peri-
odic application of nanolime) in order to achieve long term perfor-
mance in terms of mechanical and physical properties. Further
work carried out by the authors has focused on the influence of
the use of different types of solvents on the performance of LAQ
nanolime and a report will be submitted for publication in due
course.5. Declaration of conflicting interests
The authors declare that there is no conflict of interest and take
a neutral position to offer an objective evaluation of the consolida-
tion process.Acknowledgements
This research has been funded by a Vice Chancellor’s Scholar-
ship within the Doctorate Program at Sheffield Hallam University
(UK). The authors want to thank Dr. Anthony Bell for his support
with Rietveld refinements.References
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