Built Heritage 2013 Monitoring Conservation Management
Ammonium oxalate treatment application in the presence of soluble salts: laboratory results on soft limestone
Tabitha Dreyfuss; Joann Cassar
Department of the Built Heritage, Faculty for the Built Environment, University
of Malta, Msida, Malta
1. Aims of study
The aims of this research were multiple: to investigate the different possible
outcomes when ammonium oxalate treatment is applied to Globigerina Limestone that contains different types of soluble salts; to understand the role
played by the stone’s pathology vis-à-vis the treatment; and to assess the effect of treatment contact time so as to consider the impact that this may have
on the resulting formation of calcium oxalate. This paper focuses on that part
of the research concerning the resistance to salt crystallization of weathered
limestone samples treated with ammonium oxalate.
1.1. Introduction
The Maltese Islands, which are located in the centre of the Mediterranean
Sea, consist of a small archipelago measuring 316 sq. km. The Islands have
numerous historic limestone buildings and monuments that span the millennia. These are mostly built in Maltese Globigerina Limestone - a highly porous
(total porosity up to 40%) calcareous stone which deteriorates, often catastrophically, in an environment that is exposed to both high moisture levels and
elevated amounts of soluble salts. Globigerina Limestone exists as one of
two types - the more durable franka and the less durable soll [Cassar, 2002].
Many of the historic stone edifices in Malta and Gozo were built before the insertion of a damp proof course became mandatory, thus allowing water entry
in the form of rising damp together with any soluble salts present . Additionally, traditional wall construction generally utilised soil infill, usually salt laden,
between two masonry wall leaves. The Island environment further enhances
salt contamination through wind driven and aerosol borne salts. The context
is therefore a porous limestone which has a continual supply of moisture and
soluble salts. Treatment of exposed Globigerina Limestone which has lost cohesion, manifested as powdering/ granular disintegration must therefore take
this context into account.
1.2. Ammonium oxalate treatment
Ammonium oxalate treatment of calcareous stone has, over the past 30 years,
emerged as a conservation treatment with both consolidating and protective
properties [Matteini and Moles, 1986; Matteini et al., 1994; Cezar, 1998; Miliani et al., 2007; Matteini, 2007; Bracci et al., 2008; Charola et al., 2010; Conti
et al., 2011; Booth et al., 2012]. The surface conversion of calcium carbonate
to calcium oxalate is ideal for calcareous stone. Results from studies on Globigerina Limestone have confirmed that the resulting calcium oxalate is harder
and more resistant to acid attack, with a lower solubility than calcium carbonate, and with an improvement in surface cohesion occurring after treatment
[Mifsud, 2006; Mifsud and Cassar, 2006; Dreyfuss and Cassar, 2012]. Althou1271
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gh the treated stone undergoes some reduction in surface porosity (detailed
studies in this respect are ongoing), its wetting and hydrophilic properties are
retained [Croveri, 2003; Croveri et al., 2004]. Furthermore, treatment in the
presence of high levels of sodium chloride does not impede the conversion
to calcium oxalate [Mifsud and Cassar, 2006; Pinna et al., 2011]. This is an
important factor, especially in a marine environment like the Maltese Islands
where marine aerosols are ever present. Other salts that are commonly present in historical masonry include both sulphates and nitrates. This study has
centred on Globigerina Limestone of the franka type and includes both these
salt types - as sodium sulphate and sodium nitrate - as well as sodium chloride, in a comparative study where ammonium oxalate treatment was applied
to samples of this soft limestone under laboratory conditions.
2. Sample selection and preparation
In an attempt to recreate site conditions in a controlled laboratory environment, the range of franka samples tested included artificially weathered types
as desalinated samples as well as salt contaminated samples. In addition,
stone that was treated with different contact times was also included. The selection of samples included in this part of the research programme is summarised in Table 1 below. The sample types outlined in this table were prepared
as 50mm x 50mm x 50mm cubes as described in Sections 2.1 to 2.4 below.
2.1. Sample preparation
Quarry franka samples in the form of stone blocks (approximately 410mm W
x 230mm D x 267mm H) were obtained from the main quarry area of Qrendi
from the area known as Ta’ l-Iklin (quarry coordinates: 51500, 66500) at a
depth of 12m below ground level. Cubes measuring 50mm x 50mm x 50mm
were dry cut from the stone blocks. The samples were then dusted/ brushed
with a dry nylon brush to remove superficial surface dust resulting from the
Table 1 Range of samples included in the study. Treatment is with a saturated (5%) ammonium
oxalate (monohydrate) solution applied as a poultice
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cutting procedure. The weathered pathology of the franka samples was then
induced through artificial weathering through 2 cycles of salt weathering as per
EN12370:2000 using anhydrous sodium sulphate. This resulted in a powdering surface texture with an increased surface area, visible to the naked eye.
2.2. Desalination
The desalinating procedure was first carried out on all the samples. These
were immersed in distilled water, repeatedly changing the water until its conductivity revealed that soluble salts were no longer present. The conductivity
of the distilled water used (≤3μS) was measured by means of a conductivity
meter (HI98308 PWT – Hanna instruments). The conductivity of the distilled
water containing the immersed samples was measured after each immersion
until its conductivity was ≤3μS. All of the samples- including those that were
to be salt contaminated - were thus desalinated and then oven dried for 24
hours at a temperature of 105ºC, then cooled in the laboratory to constant
mass at 20 ºC room temperature. One fourth of the samples were then retained as is, to represent the desalinated type samples, while the remaining
samples were selectively salinated as described in Section 2.3 below.
2.3. Selective salt contamination (chlorides, sulphates, nitrates)
Following the desalination procedure, those samples that were designated for
salt contamination as per Table 1 were selectively contaminated. The chloride
group were immersed in a saturated solution of sodium chloride, the sulphate
group in a saturated solution of sodium sulphate and the nitrate group in a
saturated solution of sodium nitrate – all for a duration of 2 hours. Following
immersion, the samples were air dried to constant weight.
2.4. Ammonium oxalate treatment
A saturated (at 5%) ammonium oxalate (monohydrate) treatment was applied
in a cellulose pulp poultice for 2 different contact times – 5 hours and 24 hours.
Calcium oxalate had already been found to form on franka limestone after a 5
hour contact time of an ammonium oxalate paper pulp poultice (Mifsud 2006).
The 24 hour contact time is however considered to be more practical in the
field since this allows treatment to be applied up until the end of the working
day rather than limiting treatment to take place 5 hours before the end of the
working day. For this reason samples with a 24 contact time of treatment were
included in this study. Following treatment, the poultice was manually removed and the samples left to air dry at room temperature. The excess pulp was
brushed off with a soft and dry nylon brush. Only the top surface of the cube
samples was treated. For every treated sample type, an untreated control
sample was included.
3. Testing
The determination of resistance to salt crystallisation was carried out in accordance with EN 12370:2000 on the treated and untreated samples, to determine whether or not the ammonium oxalate treatment improved the stones’
ability to resist this type of deterioration. The number of cycles carried out was
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4 and the resistance to salt crystallisation was calculated as the percentage
mass of material lost against the initial dry mass. The results obtained are
given in Figure 1.
4. Results and Discussion
Fig.1 - % mass lost after 4 cycles of the sodium sulphate weathering test. (WF = weathered
franka; DES = desalinated; CL = contaminated with sodium chloride; SUL = contaminated with
sodium sulphate; NIT = contaminated with sodium nitrate; UN = untreated; T1 = treated for 5
hours; T2 = treated for 24 hours)
For all sample types – desalinated, contaminated with sodium chloride, contaminated with sodium sulphate and contaminated with sodium nitrate – all of
the untreated samples were found to have lost a greater percentage of their
initial mass when compared to treated samples of the same type. Although
the ammonium oxalate treatment was only applied to the top face of the cube
samples, the difference between treated and untreated samples was significant for all cases. Untreated samples in the desalinated group, chloride group,
sulphate group and nitrate group lost 47%, 62%, 56% and 59% respectively
while treated (5hours) samples lost 22%, 39%, 23% and 35% respectively
and treated (24 hours) samples lost 22%, 37%, 25% and 33% respectively.
This suggests that ammonium oxalate treatment does in fact provide protection from salt crystallization even when this is formed in the presence of
sodium chloride, sodium sulphate or sodium nitrate. This is also evidenced
visually - Figure 2 - where the top untreated row is significantly more deteriorated, less sound and with less pronounced corners and edges than the
bottom two rows (which were treated). The differences in resistance to salt
crystallization between those samples treated for a 5 hour contact time and
those treated for a 24 hour contact time were less significant, indicating that
the increased contact time is probably neither crucial for nor detrimental to salt
crystallization protection.
Given the increased salt crystallization resistance in the weathered franka
samples following ammonium oxalate treatment, the treated and untreated
sample types were studied through water absorption tests, to verify whether
the treatment had affected the water transport properties. Samples shown in
Table 1 were prepared as per Sections 2.1 to 2.4 above and then tested. The
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Fig.2 - photographs of the samples after the 4th cycle of the salt crystallization test
samples were evaluated by means of the water absorption test through capillarity in accordance with EN15801:2009. The samples were placed over a
10cm bed of paper pulp, the bottom 8cm of which was saturated with distilled
water. The treated samples were placed face down with the treated face in
contact with the paper pulp. The reason for using paper pulp as opposed to
filter paper as specified in the EN15801:2009 was due to the uneven surface
of the naturally weathered stone samples which were seen not to have had
full contact when placed on a wad of filter paper. The quantity of water absorbed per unit surface area during a given amount of time was measured. The
results are expressed graphically in Figure 3, where the mass of water absorbed per unit surface area (y-axis) is plotted against the square root of time
taken (x-axis). The graphs obtained show that, for all cases, they are similar
in shape. This confirms that although ammonium oxalate treatment improves
the salt crystallization resistance of weathered franka, it does not inhibit the
mode or amount of water uptake through capillarity through the treated face.
Fig.3 - water absorption graphs for weathered franka samples (WF = weathered franka; DES =
desalinated; CL = contaminated with sodium chloride; SU = contaminated with sodium sulphate;
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NIT = contaminated with sodium nitrate; UN = untreated; T1 = treated for 5 hours; T2 = treated
for 24 hours)
Built Heritage 2013 Monitoring Conservation Management
5. Conclusion
This stage in the understanding of ammonium oxalate treatment of Globigerina Limestone under different soluble salt content conditions is an important
step towards carrying this treatment forwards to real site conditions, where
soluble salts are naturally present in the stone and where desalination may
prove to be difficult or impossible. The results obtained have shown that even
in the presence of sodium chloride, sodium sulphate or sodium nitrate, a degree of protection from salt crystallization is achieved with ammonium oxalate
treatment. This protection is achieved both for treatment applied for a 5 hour
contact time as well as for treatment applied for a 24 hour contact time. Additionally, the pathology of powdering weathered franka is conducive to the successful consolidation of the stone as seen through the increased resistance to
salt crystallization of weathered types. Moreover, this increased salt crystallization resistance does not alter the water transport properties of the stone.
This is an important aspect to retain when considering historical limestone
buildings and monuments that are exposed to moisture and soluble salts. The
progression from laboratory to site is an important step to be considered in the
development of this treatment for the conservation of Globigerina Limestone
and it is hoped that through this research programme, a reliable conservation
procedure may emerge that will assist within the overall conservation of Globigerina Limestone in real restoration projects.
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