An Approach to the Use of Glycol Alkoxysilane–Polysaccharide Hybrids in the Conservation of Historical Building Stones

Stone consolidants have been widely used to protect historical monuments. Consolidants and hydrophobic formulations based on the use of tetraethoxysilane (TEOS) and alkylalkoxysilanes as precursors have been widely applied, despite their lack of solubility in water and requirement to be applied in organic media. In the search for a “greener” alternative based on silicon that has potential use in this field, the use of tetrakis(2-hydroxyethyl)silane (THEOS) and tris(2-hydroxyethyl)methyl silane (MeTHEOS) as precursors, due their high water solubility and stability, is proposed in this paper. It is already known that THEOS and MeTHEOS possess remarkable compatibility with different natural polysaccharides. The investigated approach uses the water-soluble silanes THEOS–chitosan and MeTHEOS–chitosan as a basis for obtaining hybrid consolidants and hydrophobic formulations for the conservation of siliceous and calcareous stones. In the case of calcareous systems, their incompatibility with alkoxysilanes is known and is expected to be solved by the developed hybrid consolidant. Their application in the conservation of building stones from historical and archeological sites from Guanajuato, México was studied. The evaluation of the consolidant and hydrophobic formulation treatment was mainly conducted by determining the mechanical properties and contact angle measurements with satisfactory results in terms of the performance and compatibility with the studied stones.

Stone consolidation: a critical discussion of theoretical insights and field practice

Stone consolidants have been extensively used for the preservation of historical structures since the 19th century. However, their true effectiveness in practice is often a source of debate, largely because of known cases where badly chosen treatments were unsuccessful, or even caused an accelerated degradation of the substrate. Researchers have therefore strived to better understand, and possibly predict, the behavior of consolidants on-site, in order to assist practitioners in their decision making. Despite the large number of publications available on the subject, however, the contribution of scientific research for practical applications remains scarce. Reasons for this include the limited accessibility of scientific publications and the lack of documentation or preparatory studies from the field. This unfortunately compromises the knowledge exchange between researchers and practitioners, which we consider to be a main challenge that this field must overcome. The target of this letter is to reconnect the critical problems identified on-site through practical experience with the conceptual research outcomes that could help solve them. For this, we present an informed evaluation of the most needed research, along with a synthetic overview of the insights that scientific research can offer in terms of consolidant selection, application and monitoring.

Effectiveness of Nanolime as a Stone Consolidant: A 4-Year Study of Six Common UK Limestones

Protecting stone buildings from weathering and decay is a major challenge in the conservation of built heritage. Most of the stone consolidants currently available are well suited to silicate stones, but are less compatible with limestone. In this paper we present for the first time the results over a 4-year period of various consolidation treatments carried out using nanolime on 6 of the most representative and significant stones used in historic buildings in the United Kingdom. Tests investigated the influence of stone type, environmental conditions and pre-treatments on the effectiveness of the consolidation treatment. A comprehensive and rigorous testing programme was carried out to evaluate the short (12 weeks) and longer-term (4 years) effects. Stone samples were characterised before and after treatment using light and electron microscopy, sorptivity tests and a novel methodology employing drilling resistance to interrogate the near surface effects. Results show that for some of the stones, such as Clunch and Bath Stone, the positive effect of the treatment with nanolime is noticeable after 4 years since application. However, results for other stones such as Portland and magnesian limestone showed that the initial beneficial effect of the treatment is reduced after 4 years. Nanolime treatment of Ham Stone produced an unnoticeable effect on the continuous natural reduction of the drilling resistance of the specimen over time. The results presented are of immense value to conservators as they provide essential guidance on the most appropriate repair approach. Impact to the conservation industry will be to avoid the use of nanolime on stones where there is no perceivable benefit, reducing the risk of adverse effects, including potential damage to buildings. In additional costs will be saved which might otherwise have been spent on ineffective treatments.

Neutron Radiography Study of Laboratory Ageing and Treatment Applications with Stone Consolidants

A nano-silica consolidant and nano-titania modified tetraethyl-orthosilicate were applied on two building stones, a carbonate and a silicate, by brush, poultice or capillary absorption. Neutron radiography was used to monitor capillary water absorption, and to analyse changes in physical properties caused by heat treatment of specimens for the purposes of artificially ageing and different treatment applications with stone consolidants. Moreover, ultrasonic pulse velocity and gravimetrically determined water absorption were analysed to cross-validate neutron radiography. The results reveal that reactive systems like tetraethyl-orthosilicates need an unknown period for polymerisation, which makes nano-silica consolidants more favourable for construction follow-up work. While polymerisation is incomplete, hydrophobic behaviour, water trapping and pore clogging are evident. Within the tetraethyl-orthosilicate treatment, poultice and brushing are strongly influenced by the applicant, which results in wide ranging amounts of water absorbed and anomalous water distributions and kinetics. The carbonate lithotype displays polymerisation initiated in the core of the specimen, while the lateral surfaces are still mostly hydrophobic. Reaction time differences can be attributed to the different amounts of consolidants applied, which is a result of the chosen application settings. Artificial ageing of stone specimens is a prerequisite when mechanical strength gain is studied, as demonstrated by sound speed propagation.

Polyethylene glycol oligomers as siloxane modificators in consolidation of carbonate stones

The overall performance of alkoxysilanes as stone consolidants is constrained by stone mineralogy (particularly in the carbonate varieties) and by their tendency to crack during drying. In an attempt to overcome these problems, polyethylene glycol “chains” with two carboxylic acid end-groups (PEG-CA) were introduced in siloxane sols obtained by sol-gel chemistry using tetraethoxysilane (TEOS) as precursor. Different pre-condensation degrees (by varying the stirring times of sol-gel reaction: 10 min, 2, and 24 h) and PEG-CA chains with different molecular weights were studied as variables affecting the initial efficacy of the consolidants when applied into a limestone. The sol containing siloxanes with the lowest pre-condensation degree (10 min stirring) was quite susceptible to the carbonate media and thus a poor consolidation was achieved. The sol with the highest pre-condensation degree (24 h stirring) together with the PEG-CA chains with intermediate molecular weight produced significant and uniform strength gains along the stone depth. The consolidation also showed to be highly dependent on the molecular weight of the PEG-CA chains, the PEG-CA with highest molecular weight produced a non-uniform strength increase with potential harmful side effects. The results confirmed the role of carboxylic acid end-groups as efficient sol-gel catalysts and their ability to be incorporated into the silica matrix in the presence of carbonate stone.

Impact of Relative Humidity on Modified Alkoxysilane Gels

Silicic acid esters have been used as stone consolidants for 50 years now. During this time they became the most popular consolidants for building materials, mainly thanks to their good properties during application. Despise all these positives, the use of alkoxysilanes brings also some drawbacks. The gel of the silicic acid esters, which is responsible for the consolidation, cracks and decays during several years after the treatment so that the consolidation has to be repeated. To suppress the cracking, the alkoxysilanes are often modified. In recent studies, we focused on modification of silicic acid ester gels by SiO2 and Al2O3 nanoparticles. Some of the prepared mixtures cracked less than the unmodified mixture under ambient condition. This work builds on these experiments and works with the selected modified mixtures. As the consolidants are widely used outdoors, it was necessary to compare the degree of cracking of modified gels under various conditions. In this experiment we focused on their cracking under different levels of relative humidity. As the objective evaluation of cracking was needed, we believe to come up with partial solution to this problem – to assort the extent of cracking to some defined category. The categories were defined according to our former experiences with observation of cracking states of the gels. We found out, that the change of relative humidity has huge impact on the way and extent to which the gels cracked. Knowing this information we may be able to prepare the modified consolidant, that better withstands outdoor conditions and thus to preserve our cultural heritage for longer time.