Andrea Hamilton

Sodium sulfate heptahydrate: direct observation of crystallization in a porous material

It is well known that sodium sulfate causes salt crystallization damage in building materials and rocks. However since the early 1900s the existence of the metastable heptahydrate has been largely forgotten and almost entirely overlooked in scientific publications on salt damage mechanics and on terrestrial and planetary geochemistry. We use hard synchrotron x-rays to detect the formation of this metastable heptahydrate on cooling a porous calcium silicate material saturated with sodium sulfate solution. The heptahydrate persists indefinitely and transforms to mirabilite only below 0 °C. At the transformation, which is rapid, the solution is highly supersaturated with respect to mirabilite. We estimate that crystallization of the heptahydrate and of mirabilite have associated Correns pressures of about 9 and 19 MPa, respectively, exceeding the tensile strength of building stones. We detect lattice strains in the salts from x-ray measurements consistent with these values.

Rehydroxylation (RHX) dating of archaeological pottery

We show that the rehydroxylation (RHX) method can be used to date archaeological pottery, and give the first RHX dates for three disparate items of excavated material. These are in agreement with independently assigned dates. We define precisely the mass components of the ceramic material before, during and after dehydroxylation. These include the masses of three types of water present in the sample: capillary water, weakly chemisorbed molecular water and chemically combined RHX water. We describe the main steps of the RHX dating process: sample preparation, drying, conditioning, reheating and measurement of RHX mass gain. We propose a statistical criterion for isolating the RHX component of the measured mass gain data after reheating and demonstrate how to calculate the RHX age. An effective lifetime temperature (ELT) is defined, and we show how this is related to the temperature history of a sample. The ELT is used to adjust the RHX rate constant obtained at the measurement temperature to the effective lifetime value used in the RHX age calculation. Our results suggest that RHX has the potential to be a reliable and technically straightforward method of dating archaeological pottery, thus filling a long-standing gap in dating methods.

Moisture dynamics in walls: response to micro-environment and climate change

A coupled sharp-front (SF) liquid transport and evaporation model is used to describe the capillary rise of moisture in monoliths and masonry structures. This provides a basis for the quantitative engineering analysis of moisture dynamics in such structures, with particular application to the conservation of historic buildings and monuments. We show how such a system responds to seasonal variations in the potential evaporation (PE) of the immediate environment, using meteorological data from southern England and Athens, Greece. Results from the SF analytical model are compared with those from finite-element unsaturated-flow simulations. We examine the magnitude and variation of the total flow through a structure as a primary factor in long-term damage caused by leaching, salt crystallization and chemical degradation. We find wide seasonal variation in the height of moisture rise, and this, together with the large estimated water flows, provides a new explanation of the observed position of salt-crystallization damage. The analysis also allows us to estimate the effects of future climate change on the capillary moisture dynamics of monoliths and masonry structures. For example, for southern England, predicted increases in PE for the period 2070–2100 suggest substantial increases in water flux, from which we expect increased damage rates.

In situ characterization of elusive salt hydrates. The crystal structures of the heptahydrate and octahydrate of sodium sulfate.

An important intermediate phase in the crystallization of aqueous solutions of sodium sulfate is the highly metastable sodium sulfate heptahydrate (Na(2)SO(4).7H(2)O). This has been structurally characterized for the first time by in situ single crystal X-ray diffraction. The crystal structure shows that each sodium cation is octahedrally coordinated to water molecules, with a slight distortion due to one of the water molecules being disordered. The hydrated sodium cations are hydrogen-bonded to form a three-dimensional bonded network, which is markedly different from the architecture of one-dimensional bonded chains observed in sodium sulfate decahydrate (mirabilite). This major structural difference explains the reconstructive nature of the transformation observed between the heptahydrate and mirabilite. High-pressure crystallization of a 3.41 mol/kg water aqueous solution of sodium sulfate at 1.54 GPa in a diamond-anvil cell resulted in the formation of a previously unknown sodium sulfate hydrate, which we have determined by single crystal X-ray diffraction methods to be an octahydrate, Na(2)SO(4).8H(2)O. In this structure the sulfate ions are coordinated directly to sodium ions. This resembles anhydrous sodium sulfate (thenardite) but contrasts with the heptahydrate and decahydrate in which the sodium ions are coordinated exclusively by water molecules. This observation demonstrates how the delicate balance of inter- and intramolecular bonds in the crystal structure can be significantly altered by the application of pressure.

Sodium sulfate heptahydrate: a synchrotron energy-dispersive diffraction study of an elusive metastable hydrated salt

We describe an unusual application of synchrotron energy-dispersive diffraction with hard X-rays to obtain structural information on metastable sodium sulfate heptahydrate. This hydrate was often mentioned in nineteenth and early twentieth century scientific literature but rarely in modern publications, and it had not been characterised structurally. Using a unique three-detector fixed-angle X-ray geometry, a good quality powder diffraction pattern was obtained directly from a stirred suspension of hydrate crystals in saturated aqueous sodium sulfate solution at about 14 °C. The suspension of crystals was contained in the 22 mm dia sealed cylindrical bottle in which crystallization occurred. Indexing showed that the heptahydrate has a tetragonal unit cell with a = 7.1668 A and c = 22.2120 A with a few weak unindexed reflections arising from the 2a supercell. New gravimetric data and the cell dimensions confirm the heptahydrate composition originally proposed by Loewel (Ann. Chim. Phys., 1850, 29, 62–127).

Physicochemical Characterization of a Hydrated Calcium Silicate Board Material

A hydrated calcium silicate insulation board was one of three construction materials used in the round-robin study of hygric properties carried out in the HAMSTAD project (Roels, S., Carmeliet, J., Hens, H., Adan, O., Brocken, H., Cerny, R., Pavlik, Z., Hall, C., Kumaran, K., Pel, L. and Plagge, R. (2004). Interlaboratory Comparison of Hygric Properties of Porous Building Materials, Journal of Thermal Envelope and Building Science, 27: 307-325). We report here the results of a physicochemical examination of this material. Analysis by synchrotron X-ray diffraction and nuclear magnetic resonance show that the mineralogical composition is essentially pure xonotlite. Bulk density and pore size distribution were obtained by mercury intrusion porosimetry and helium pycnometry; microstructural fabric was observed by high and low resolution electron microscopy; and organic content measured by Carbon-Hydrogen-Nitrogen (CHN) chemical analysis.

Direct measurement of salt–mineral repulsion using atomic force microscopy

The disjoining pressure between a mineral and soluble salt crystal in concentrated aqueous solution has been successfully measured with atomic force microscopy.

The mechanics of moisture-expansion cracking in fired-clay ceramics

Samian ware (or terra sigillata) is a type of fired-clay ceramic produced at a number of sites in France in the period 50 BC to 200 AD and widely traded in Western Europe. It has a characteristic high-gloss surface, formed by application of a non-calcareous clay slip to the green body before firing. New SEM observations show that the slip layer is frequently crazed, although the cracks are not usually visible to the unaided eye. We discuss the mechanics of the crazing, and show that the cracking is driven by rehydroxylation (RHX) moisture expansion. Observations and analysis aid in understanding the RHX dating of archaeological pottery by showing that craze networks permit efficient transport of moisture through the slip layer.

A Note on the Density of a Calcium Silicate Hydrate Board Material

A new analysis of the density of calcium silicate board material is reported. This takes account of revised estimates for the component densities.

Biogenic Hydroxyapatite: A New Material for the Preservation and Restoration of the Built Environment

Ordinary Portland cement (OPC) is by weight the worlds most produced man-made material and is used in a variety of applications in environments ranging from buildings, to nuclear wasteforms, and within the human body. In this paper, we present for the first time the direct deposition of biogenic hydroxyapatite onto the surface of OPC in a synergistic process which uses the composition of the cement substrate. This hydroxyapatite is very similar to that found in nature, having a similar crystallite size, iron and carbonate substitution, and a semi-crystalline structure. Hydroxyapatites with such a structure are known to be mechanically stronger and more biocompatible than synthetic or biomimetic hydroxyapatites. The formation of this biogenic hydroxyapatite coating therefore has significance in a range of contexts. In medicine, hydroxyapatite coatings are linked to improved biocompatibility of ceramic implant materials. In the built environment, hydroxyapatite coatings have been proposed for the consolidation and protection of sculptural materials such as marble and limestone, with biogenic hydroxyapatites having reduced solubility compared to synthetic apatites. Hydroxyapatites have also been established as effective for the adsorption and remediation of environmental contaminants such as radionuclides and heavy metals. We identify that in addition to providing a biofilm scaffold for nucleation, the metabolic activity of Pseudomonas fluorescens increases the pH of the growth medium to a suitable level for hydroxyapatite formation. The generated ammonia reacts with phosphate in the growth medium, producing ammonium phosphates which are a precursor to the formation of hydroxyapatite under conditions of ambient temperature and pressure. Subsequently, this biogenic deposition process takes place in a simple reaction system under mild chemical conditions and is cheap and easy to apply to fragile biological or architectural surfaces.