Moira Wilson

Dating fired-clay ceramics using long-term power law rehydroxylation kinetics

Fired-clay materials such as brick, tile and ceramic artefacts are found widely in archaeological deposits. The slow progressive chemical recombination of ceramics with environmental moisture (rehydroxylation) provides the basis for archaeological dating. Rehydroxylation rates are described by a (time)1/4 power law. A ceramic sample may be dated by first heating it to determine its lifetime water mass gain, and then exposing it to water vapour to measure its mass gain rate and hence its individual rehydroxylation kinetic constant. The kinetic constant depends on temperature. Mean lifetime temperatures are estimated from historical meteorological data. Calculated ages of samples of established provenance from Roman to modern dates agree excellently with assigned (known) ages. This agreement shows that the power law holds precisely on millennial time scales. The power law exponent is accurately 1 4, consistent with the theory of fractional (anomalous) ‘single-file’ diffusion.

Capillary water migration in rock: process and material properties examined by NMR imaging

We report the application of proton nuclear magnetic resonance (NMR) imaging to the measurement of water content distributions in Lépine limestone, a typical constructional stone. The method is used to observe the kinetics of the absorption of water into this material by capillarity. The water content distributions are consistent with the predictions of unsaturated flow theory. The hydraulic diffusivity of Lépine stone is found to be an approximately exponential function of the water content, in agreement with experimental data on other porous materials. The best estimate of the diffusivity function is D (m2s−1) = 6.3 × 10−9 exp (4.90θr), whereθr is the normalized volumetric water content. The sorptivity estimated from NMR data is in close agreement with the directly measured value (1.00 mm min−1/2). NMR imaging methods appear promising as a non-destructive and rapid laboratory means of determining moisture distributions, especially for the purpose of accurate measurement of the capillary transport properties of porous materials.ResumeOn décrit l’application de l’imagerie de résonance magnétique nucléaire (RMN) à la mesure de distributions de teneur en eau dans le calcaire Lépine, pierre de construction type. On présente les résultats d’une expérience où l’eau est absorbée librement par capillarité de l’extrémité d’une barre rectangulaire. On utilise l’imagerie RMN pour contrôler la vitesse d’absorption de l’eau. On obtient, par l’analyse des images, les distributions à l’intérieur de l’échantillon en fonction du temps.Les distributions de teneur en eau concordent avec l’application de la théorie de l’écoulement en milieu non saturé. La diffusivité hydraulique et le coefficient de sorption se calculent à partir des profiels d’absorption d’eau. Le coefficient de diffusion de la pierre de Lépine est une fonction approximativement exponentielle de la teneur en eau, conforme aux données expérimentales des autres matériaux poreux. La meilleure estimation de la fonction de diffusion est D (m2s−1) = 6.3 × 10−9 exp (4.90θr), oùθr est la teneur en eau volumétrique normalisée.Le coefficient de sorption évaluée à partir des données RMN concorde tout à fait avec la mesure directe (1,00 mm min−1/2). Les méthodes d’imagerie RMN semblent prometteuses en tant que technique de laboratoire non-destructive et rapide pour déterminer les distributions d’humidité, et en particulier, mesurer avec précision les propriétés de mouvement d’eau par capillarité de matériaux poreux.

Water anomaly in capillary liquid absorption by cement-based materials

The paper shows that the rate of capillary absorption of water in cement-based materials is anomalous when compared with that of other liquids.

The effects of lime and admixtures on the water-retaining properties of cement mortars

Abstract The water-retaining properties of wet mortar mixes are examined. The desorptivity is defined as a parameter characterising the water-retaining properties of such mixes. A test method based on the American Petroleum Institute filtration cell is described for measuring the desorptivity of wet mixes. Experimental results are reported for a range of wet cement mortars including mixes containing lime and air-entraining and water-retaining admixtures. These show that 1:3 cement:sand and equivalent mixes containing lime all have very similar water-retaining characteristics, but are all much less water-retaining than a 1:3 lime:sand mix. These results therefore demonstrate the strong water-retaining characteristics of lime.

Water movement in porous building materials—XIII. Absorption into a two-layer composite

Abstract The sharp wet front method of analysis is applied to the absorption of water into a composite bar consisting of two dissimilar materials in hydraulic contact. Equations describing both the rate of absorption of water and the cumulative absorbed volume of water per unit area of supply surface are derived. Experimental absorption data can be used to estimate the ratio of the hydraulic conductivities of the constituent materials. Experimental data on composite bars made up of various plaster I sand mixtures are included for the case of absorption through a high sorptivity into a lower sorptivity material. The results show that the absorption of water into the composite is controlled by the properties of the second material.

Moisture expansion and mass gain in fired clay ceramics : a two-stage (time)1/4 process

We report the first measurements of mass gain and expansive strain in clay ceramics immediately following firing. The results show that both mass gain and expansive strain follow a well defined two-stage process, each stage of which is linear with (time)1/4, with a transition from the first to the second stage commencing at t ~ 1.5 h. The first stage mass gain and strain rates are ~3.5 times greater than the second stage rates. The strain is found to show the same linear relationship with mass throughout both stages providing evidence that the underlying physical process is the same in each stage. The early time measurements are unique in providing experimental evidence for t1/4 kinetics for both strain and mass gain over times ranging from minutes to days and are consistent with earlier strain data showing t1/4 kinetics over times ranging from days to centuries.

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.

Anomalous water transport properties of Portland and blended cement-based materials

The paper presents new data giving further evidence of anomalies in the water transport properties of Portland cement-based solids, and show that the anomalous performance is much less evident when the Portland cement is partially replace by ground granulated blast-furnace slag (ggbs).

Water movement in porous building materials—X. Absorption from a small cylindrical cavity

Abstract The absorption of water from a cylindrical cavity is discussed in terms of the theory of water absorption by porous materials. The absorption process is analysed in terms of the sharp wet front model. Both theory and experiment show that the cumulative absorption from a cylindrical source varies as t 1 2 in the limit as t → 0 and that the time period for which t 1 2 beahviour is observed increases with source radius. A simple two-term approximation to the equation governing the absorption of water from a cylindrical cavity at longer times is developed. Three methods of data analysis are applied to experimental results on clay brick, aerated concrete and mortars to obtain estimates of the water sorptivity of these materials.

A falling head permeameter for the measurement of the hydraulic conductivity of granular solids

A falling head permeameter is described in which pairs of infrared emitters and detectors on a sight tube are used to measure the flow rate associated with the passage of water through a granular solid under the action of a diminishing pressure head. An equation relating pressure head to elapsed time is derived from which permeability may be calculated. In order to verify the accuracy and sensitivity of the instrument, permeability measurements carried out on a graded quartz sand are compared to those obtained by the more conventional constant head measurement. Excellent agreement is obtained between the permeability values obtained using both measurement methods. Experimental results are also reported for the measurement of the permeability of a range of sieved sand fractions. The falling head permeameter described here is particularly suitable for the measurement of the hydraulic conductivity of granular solids such as sands and soils through which a high flow rate may be expected.