DESALINATION OF HISTORIC MASONRY.
PRE-INVESTIGATION, TREATMENT AND FOLLOW-UP CARE
Rob P. J. van Hees, Barbara Lubelli, TNO Built Environment and Geosciences P.O. Box49, 2600AA, Delft, The Netherlands
Tel: +31-15-2763164 Fax: +31-15-276-3116, E-mail: rob.vanhees@tno.nl Delft University of Technology, Faculty of Architecture P.O. Box 5043, 2600AA, Delft, The Netherlands
Keywords: Desalination, poultices, pre-investigation, follow-up care
Abstract
Salt crystallization constitutes one of the most widespread decay mechanisms affecting historic buildings. Desalination is a conservation treatment of growing importance in the case of historic masonry.
Salts present in porous materials cause damage through their interaction with moisture. Consequently, in certain situations the prevention of further moisture ingress can provide a solution to the problem. However, damage can also occur due to the hygroscopic uptake of moisture from the air by the salt. Hygroscopic salts can dissolve and re-crystallize due to changes in air humidity (RH) that cross their equilibrium relative humidity value. In this case, climate control may provide stable RH values and thus avoid cycles of crystallization/dissolution. However, the desired climate for preservation is not always easy to be obtained in ancient buildings. In such situations, direct intervention in the form of desalination treatments may present a better solution to the problem.
This paper describes the approach from pre-investigation and treatment to follow-up care as the basis for a successful conservation of monuments suffering from salt decay. The necessity to tune the poultice properties to the substrate is underlined.
Introduction: Pre-investigation
Salt crystallization constitutes one of the most widespread decay mechanisms affecting historic buildings. Desalination is a conservation treatment of growing importance in the case of historic masonry. However, it is an intervention that needs a thorough analysis of the situation in order to guarantee success.
Pre-investigation
In practical cases the following steps should be taken:
- Historical assessment of the building, including past interventions;; historic events or interventions that may have contributed to the damage should be taken into consideration.
- Assessment of the technical state of conservation, including: - Damage assessment
- Conditions of exposure
- Description, identification and characterization of materials (total porosity, pore size distribution)
- Diagnosis, including risk assessment and description of the mechanism of decay;; which salts are present and in what concentration;; further the salt distribution, the presence of moisture present and the moisture source should be assessed
- Decision regarding the (technical) requirements for the treatment on the basis of substrate properties and salt distribution
- Decision on a suitable poultice mix [1]
- Decision on how to assure skilful execution and quality control on applied materials
- Decision on side measures (dealing with moisture sources, indoor climate,…) - Monitoring and follow-up care
is an example of a decision support system that assists this way of assessing historic buildings, as well as their damage types and damaging processes. Analysis and diagnosis
An assessment of the state of conservation is the first and necessary step in order to define the problem to be solved. This step includes also the question which investigations have to be performed. The investigations should lead to a clear diagnosis, leading to the cause of the damage, i.e. the damage process. It should become clear whether there still is an active source of moisture and/or salts. After the pre-investigation a decision should be made on the intervention, i.e. the treatment to be used and eventually on side measures.
Intervention: Desalination Treatment
Poultices for the extraction of salts are often used in conservation, both of valuable art objects (frescos, statues, etc.), and, recently, also of masonry surfaces. However, the obtained results appear often unpredictable.
Desalination with poultices can take place according to different principles, Figure 1: Location of salt damage and sampling in damaged wall section
which by the way are not always mutually exclusive:
-Diffusion: poultices stay wet. Salt ions are transported from the substrate into the poultice due to differences in ion concentration.
-Advection: poultices become dry. Dissolved salts are transported from the substrate into the poultice by capillary forces during the drying process. The direction of transport is dependent on the porous structures of the substrate and poultice, in such a way that the saline solution travels from larger pores into finer pores. Consequently, the poultice must have smaller pores than the substrate.
Desalination poultices used (both commercial products and self-made recipes) are generally chosen on the basis of experience and are not tuned to the properties of the substrates. Generally products are either based on cellulose fibers or on mixes of cellulose, clay and sand or light aggregates [3].
In the EU project desalination [4], a modular system of poultices was developed to optimize the desalination: tuning the pore size of the poultice to that of the substrate improves the efficiency of desalination (table 1 and Figure 2).
The modular system comprises 4 different categories of poultices, related to the pore size distribution of the substrate. Three of the poultice categories are intended for desalination according to the advection principle, and one of the categories is intended for diffusion based desalination. While this last poultice category can be used for all substrate pore size categories;; it is however most appropriate for the smallest pore sizes (< 0.1 µm), which can not be desalinated using advective poultices (table 1).
Table 1: Modular system of poultices: Each poultice category is suitable to desalinate certain categories of substrate pore sizes.
A = appropriate extraction poultice;; P = possible extraction poultice
Substrate Poultice principle
and type micro pores
< 0.1 µm small pores 0.1 – 1 µm meso pores 1-10 µm macro pores 10-100 µm 1. A P P 2. A P Advection 3. A Diffusion 4. A P P P
In order to have a sound basis for the choice of the poultice, the following information on the substrate should be obtained:
- Pore-size distribution and total open porosity - Moisture source, quantity and distribution - Salt type, quantity and distribution
Assessment of treatment efficiency
Before and after desalination the moisture and salt content and distribution in the wall (and not in the desalination poultice only) should be measured to evaluate the efficiency of salt extraction.
This assessment is usually done by means of minimally invasive techniques, based on drilling of powder from the substrate.
The powder drilled after each desalination treatment should contain less salt than before the treatment: the salt amount can either be gravimetrically assessed (after first dissolving the salts), or by means of Ion Chromatography (IC). Hygroscopic Moisture Content (HMC) measurements are also suitable [5].
The salt amount before and after the desalination treatment should be Figure 2: Modular system of poultices. The numbers indicate the category of desalination poultices. The arrows indicate the pore size the poultice should have for each category of pore
measured at least up to the depth reached by the water provided during desalination (depth of wetting).
This depth can be estimated on the basis of the (known) water amount in the poultice and the (to be measured) water absorption properties of the substrate.
Sampling for the determination of the salt amount should be performed at different depths (for example 0-1, 1-2, 2-3, 3-4, 4-5 cm etc.).
The efficiency (% of extracted salts with respect to the initial amount) can be calculated for the sampling depth:
Efficiency (0-x cm) = 100 * (salt before 0-X cm - saltafter 0-X cm) salt before 0-X cm
The salt amount can be expressed in different ways depending on the analysis method used to quantify the salt amount (e.g. if IC is used the total ion content can be used).
A model of the assumed possible salt distribution before and after desalination is given in Figure 3.
An efficient desalination is considered being achieved if the area under the curve f(depth)= total salt content , is lower after desalination than before desalination.
Effective treatment
Salts present in porous materials cause damage through their interaction with moisture. Consequently, in certain situations the prevention of further moisture ingress can provide a solution to the problem. Damage can also occur due to the hygroscopic uptake of moisture from the air by the salt. Hygroscopic salts can dissolve and re-crystallize due to changes in air humidity (RH) that cross the equilibrium relative humidity value of the salts. In this case, climate control may provide stable RH values and thus avoid cycles of crystallization/ dissolution. In many situations in monuments and historic buildings, climate control is difficult to implement and a desalination treatment could be suitable. As it has been explained before, a desalination poultice should be tuned to the specific substrate in order to be efficient. Apart from the poultice properties the application and execution are important (Figures 4 and 5) to reach a high efficiency.
Figure 4: The poultice should be applied with the right technique in order to obtain a good
The overall effectiveness of the treatment is generally not only dependent on the efficiency of the treatment itself, but is also influenced by conditions like the presence of rising damp, that may go together with transport of additional salts from the groundwater.
Therefore, apart from an efficient desalination treatment, the necessity of side measures (as for example treatment of capillary rising damp) should be judged for each individual situation.
Only is this way a good effectiveness in terms of user satisfaction and building preservation could be guaranteed.
Conclusions: Follow-up Care
The approach from pre-investigation and treatment to follow-up care as the basis for a successful conservation of monuments suffering from salt decay has been described.
A modular system of poultices was developed in the EU project Desalination in order to optimize the desalination efficiency.
Tuning the pore size of the poultice to that of the substrate clearly improves the quality of desalination.
Figure 5: After taking the poultice off, it is clearly visible in this case that a good contact even in the joint space was obtained in this case
Table 2: Steps ideally to be taken in the case of a desalination intervention
Phase Steps
Assessment and description Assessment of the history of the building
Assessment of the state of conservation
Identification and characterization of materials
Damage description (types / patterns)
Analysis and diagnosis Hypothesis on the damage
process
Measurement of moisture and salt load
Determination of the salt types, amount and distribution Monitoring of the climate Verification of the damage
process
Identification of the moisture and salt sources
Description of the damage process
Risk analysis Activity of moisture and salt sources
(Indoor) climate Intervention / execution Possibility and applicability of
desalination treatment Technical requirements for the desalination treatment
Choice of desalination principle Choice of poultice mix Pre-wetting
Pre-consolidation Quality control during
execution
Monitoring of the desalination process
Sampling and salt distribution measurements
Planning eventual side- measures
Climate control
Measures against moisture sources (for example rising damp)
After care Documentation of diagnosis
and intervention
Planning of future maintenance Monitoring of the state of conservation
The assessment of the overall effectiveness of a treatment should include both the efficiency of the performed treatment and the long term effect of desalination. This directly implies that long term monitoring is necessary after a desalination treatment.
It is considered of utmost importance that decisions made and measures taken, are properly described and archived, allowing that in future an evaluation of any measures taken, whether the effect is positive or negative, could be performed.
The value of monuments and historic buildings is such that all decisions taken in the conservation process should be documented and a sound follow-up care is considered necessary.
Table 2 gives a complete overview of the steps in the decision process. A maintenance plan should be made and a regular control of the effectiveness of the intervention should be carried out. In this way the intervention fits in the cost effective concept of preventive conservation [6].
References
1 Lubelli B., Hees van, R.P.J. (2010). “Desalination of masonry structures. Fine tuning of poultice pore size distribution to substrate properties”, in Journal of Cultural Heritage, 11, pp.10-18
2 van Hees R.P.J., Naldini S., Lubelli B., (2009) “The development of MDDS- COMPASS. Compatibility of plasters with salt loaded substrates”, in special issue Compass - Construction and Building Materials, 23 ( 5), pp. 1719-1730 3 Vergès-Belmin V. , Siedel, H. (2005). “Desalination of masonries and
monumental sculptures by poulticing: A review”, in Restoration of Buildings and Monuments (Bauinstandsetzen und Baudenkmalpflege), 11, pp. 1–18 4 EU project Desalination - Assessment of Desalination Mortars and Poultices
for Historic Masonry, Contract no.: 022714 (2006-2009)
5 Lubelli B., van Hees R.P.J., Brocken H.J.P., (2004) “Experimental research on hygroscopic behaviour of porous specimens contaminated with salts”, in Construction and Building Materials, 18 ( 5), pp. 339-348
