Criteria and methodology for the evaluation of conservation interventions on treated porous stone susceptible to salt decay

Susceptibility of stone to salt decay is directly related to its microstructural and mechanical characteristics. In the present work, a porous stone from a quarry in Cyprus was examined. Samples of this stone were consolidated and protected with certain conservation materials. The modification of microstructural characteristics of the stone was evaluated by mercury intrusion porosimetry, while its mechanical characteristics were measured by compressive and bending strength tests. From these data, the susceptibility of Cyprus stone to salt (NaCl) decay was estimated. The values of crystallization pressure of NaCl were calculated and compared to the experimentally measured compressive and tensile strength of the stone, in order to examine the probability of stone disruption due to salt crystallization. The development of the phenomenon after coarse pores’ filling with crystals, i.e. crystallization of salts in capillaries or mechanical failure of the stone, was determined by thermodynamic analysis. Additionally, artificial weathering tests of marine salt spray were performed for the evaluation of stone’s durability after the conservation. The treated stone proved to be resistant to salt decay, as it presented ameliorated microstructural and mechanical characteristics, concerning decay due to soluble salt crystallization. The most possible thermodynamic scenario was small pores repletion with crystals, avoiding mechanical failure of the stone.     

Utilization of jarosite/alunite residue for mortars restoration production

The present study was carried out to produce artificial hydraulic lime mortars for repair and conservation of historic masonry using a jarosite/alunite precipitate, a waste product of a novel Greek hydrometallurgical process developed to treat economically low grade nickel oxides ores. Alternative mortars were prepared by mixing lime powder, quartz sand and the above residue, substituting lime up to 50%. The mortars were prepared and tested according to European Norm EN 1015. They were cured for periods of 28 and 90 days and the compressive and flexural strengths were determined. The best mechanical behavior was observed for the mortar with 50% lime replacement, which also presented a low ratio of compressive to flexural strength (f _c/f _f). X-Ray diffraction, TG-DTA and mercury porosimetry were used to characterize the hydration products at 28 and 90 days. The results showed that the jarosite/alunite residue was dissolved in the high alkaline environment of the mortar, producing CaSO₄ · 2H₂O and AlOOH. During hydration, gypsum and some of the Ca(OH)₂ were consumed, together with aluminum hydroxide in order to produce ettringite, a fact that improved the mechanical behavior of the produced mortars.     

HygroScope: A Moisture Transfer Simulator for Buildings

Moisture transfer is one of the most important factors related to the energy performance, indoor quality, and maintenance of buildings. HygroScope is a simulator that takes into account the moisture transfer mechanisms, the wall configuration, the construction materials properties, and seasonal meteorological data for the region and calculates the seasonal wall moisture content along with the corresponded equilibrium moisture height. HygroScope is an application developed under Visual Studio 2005 using the C# programming language and consists of three units: the database, the mathematical model library, and the graphical user interface.     

Consolidation effectiveness of modified Si-based nanocomposites applied to limestones

One of the main issues in the field of Monument Protection is the degradation of limestones as a result of the action of various weathering mechanisms. The modification of widely used silicon-based materials for stone consolidation is intended to overcome the well-known drawbacks of these materials, such as shrinkage and cracking tendency during drying. The addition of nano-dispersions into a silica matrix aims to enhance their effectiveness in several ways, by improving their properties and their viscoelastic behavior. The objective of the current research was the application and evaluation of Si-based modified nanocomposites of optimized composition. The materials were applied to two types of porous stone and the assessment of their compatibility and performance was carried out by using both laboratory techniques and methods (SEM, MIP, TMA, Water Absorption by Capillarity, determination of Water Vapor Permeability) and non-destructive techniques (Ultrasound Velocity determination, Colorimetry). To study the resistance of the treated samples to soluble salt crystallization, accelerated aging tests were performed in sodium sulfate cycles. The modified consolidants consist of an ethyl silicate matrix reinforced with colloidal silica (SiO₂) nano-particles and titania (TiO₂) particles. Based on the results, the consolidating material does not significantly alter the characteristics of the microstructure and the appearance of stones, allowing the passage of water vapor, while increasing their mechanical properties. Furthermore, the accelerated ageing tests revealed that the treated samples have a higher resistance to the action and crystallization of soluble salts in comparison to untreated.     

Salt Crystal Growth as Weathering Mechanism of Porous Stone on Historic Masonry

It is well known that in porous stones, NaCl precipitation on and beneath a surface depends on the solution supply and the evaporation rate according to the microclimate and the effective pore structure.In the present work, the study of salt crystal growth mechanisms in large heterogeneous systems like porous stone masonries is attempted. The historic masonries of the Medieval City of Rhodes act as a pilot, due to the intense marine environment and the climatic conditions in favour of NaCl crystallization into the highly porous stones.Weathered monument samples from various depths on masonries exposed to the sea are examined systematically under SEM and EPMA. The obtained results permit the differentiation of crystal growth patterns occurring during distinguished phases of the evaporation process within the porous stone masonry in depth.In the first phase, salt crystals grow favourably in the larger pores, connecting with the empty evaporation channels, and being supplied by solution from the next smaller pores. The isometric crystal habits attaining an equilibrium form correspond to those growing immersed in the solution, when a granular crust is formed.In a second phase, the crystals already exceed the pore size and overlap other smaller pores. As the rate of evaporation exceeds the solution supply, the solution retires and the substrate dries out, the area where the crystal contacts the solution is reduced, and consequently, columnar crystals grow.The pressure exerted by crystallization against the pore walls, when the crystals filling entirely the coarse pores continue growing, leads to disruption.     

Provenance and Technology Investigation of Agia Sophia Bricks, Istanbul, Turkey

Bricks from the Agia Sophia in Istanbul, Turkey, were investigated to better formulate a plan for restoration. According to an ancient text, these bricks are extremely light and were manufactured on the island of Rhodes. Results of neutron activation analysis, grouped by multivariate statistics, show that the brick samples do not correlate with clays thought to be local and used in contemporary churches in Istanbul. The bricks are similar to those used in the Great Basilica of Rhodes. However, the Agia Sophia bricks are lighter (45% porosity) than the bricks from the other churches (35% porosity). The Agia Sophia bricks are made of a noncalcareous, fine paste with quartz temper, fired at low temperature (∼750°C). The exceptional characteristic is their homogeneity and small pore-size distribution (0.3–0.8 μm). The tensile strength of the dome bricks is unexpectedly high (up to 1.3 MPa), even though the bricks have high porosity.     

Physical and mechanical characteristics of TEOS-modified nanocomposites

Journal of Polymer Research - Sodium 2,2′-methylene-bis(4,6-di-tertbutylphenyl)-phosphate (NA11) is an α-type nucleating agent for improving the mechanical and crystallization...     

Study of the substitution of limestone filler with pozzolanic additives in mortars

In this work several specimens of mortars were prepared with the addition of 5% fly ash and 5% perlite and their mechanical properties and porosity were tested and compared to those of mortars with no additives (reference sample).Specifically, it was studied the influence that these additives have on the elastic modulus and porosity of the mortars.After conducting chemical, mineralogical and granulometric analysis of additives, a series of measurements were made in fresh and hardened mortars. In fresh mortars, measurements were conducted according to the EN1015-2/3/8 Standards concerning flow value, wet bulk density, air content, water demand and water retention.In hardened mortars, the measurements were made in the age of 28 days. The rate of carbonation and the formation of hydraulic phases were investigated using XRD, scanning electron microscopy (SEM) and thermal analysis (DTA–TGA). Porosity and pore size distribution of mortars were investigated by mercury intrusion porosimetry. Compressive and bending strength and adhesion were measured according to the EN1015-11/12 Standards. Finally, the Young’s elastic modulus was measured in cylindrical specimens sized 50/100 mm (diameter/height) according to ASTM C469-02 Standard.The results of the present study indicated a differentiation in the microstructure of the mortars that can be contributed to the use of different additives, such as fly ash and perlite. The three materials proved to be compatible in their mechanical behavior and appropriate for construction use.The modulus of elasticity value is related to the ratio of compressive to flexural strength and in particular, with the increase of the value of the ratio, the value of the Young’s modulus of elasticity increases. The development of the microstructure represents a major parameter to improve existing mortars and to formulate new mortars.     

Digital processing of SEM images for the assessment of evaluation indexes of cleaning interventions on Pentelic marble surfaces

In this work, digital processing of scanning-electron-microscopy images utilized to assess cleaning interventions applied on the Pentelic marble surfaces of the National Archaeological Museum and National Library in Athens, Greece. Beside mineralogical and chemical characterization that took place by scanning-electron-microscopy with Energy Dispersive X-ray Spectroscopy, the image-analysis program EDGE was applied for estimating three evaluation indexes of the marble micro-structure. The EDGE program was developed by the U.S. Geological Survey for the evaluation of cleaning interventions applied on Philadelphia City Hall. This computer program analyzes scanning-electron-microscopy images of stone specimens cut in cross-section for measuring the fractal dimension of the exposed surfaces, the stone near-surface fracture density, the shape factor (a surface roughness factor) and the friability index which represents the physico-chemical and physico-mechanical stability of the stone surface. The results indicated that the evaluation of the marble surface micro-structure before and after cleaning is achieved by the suggested indexes, while the performance of cleaning interventions on the marble surfaces can be assessed.