Margaret Carter

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.

Sources of error in using silver/silver chloride electrodes to monitor chloride activity in concrete

The silver/silver chloride electrode is discussed as a chloride-monitoring device in concrete rather than as a reference electrode. The effects of temperature, bromide ions, and applied potential fields on the reliability and accuracy of these electrodes are evaluated. The effect of temperature is assessed using a novel mathematical approach and the other two effects are examined experimentally.

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.

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.

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.

A high pressure permeameter for the measurement of liquid conductivity of porous construction materials

Equipment is described which provides measurements of the pressure and volume of unidirectional liquid flow through sample cores of porous solids. The real-time measurement of flow pressures enables any changes in conductivity resulting either from interactions with the flow liquid or from changes in effective stress levels to be monitored directly. The equipment can use a range of flow liquids and be adapted for miscible and immiscible displacement studies. It is particularly suitable for cement-based and stone construction materials.

Theoretical development and validation of a Sharp Front model of the dewatering of a slurry by an absorbent substrate

The absorption of water from a slurry into an absorbent substrate is analysed using Sharp Front theory. The analysis describes the relationship between the sorptivity S of the substrate, the desorptivity R of the slurry and the transfer sorptivity A between slurry and substrate, and leads to the relationship 1/A2 = 1/R2 + 1/S2. Experimental data are presented which validate this equation for the practically important case of the absorption of water from soft mortar mixes by fired clay bricks. A unique feature of the experimental work is the measurement of the desorptivity of the mortars at a pressure equal to the wetting front capillary pressure of the clay brick substrate. Analysis of the experimental data also enables, for the first time, the calculation of the capillary potential at the slurry/substrate interface. The analysis has relevance to many aspects of ceramic and mineral processing, industrial filtration and construction engineering.

Direct measurement of the wetting front capillary pressure in a clay brick ceramic

The absorption of a liquid into a rectangular bar of an initially dry porous material that is sealed on all surfaces except the inflow face is analysed in terms of Sharp Front theory. Sharp Front models are developed for both complete and incomplete displacement of air ahead of the advancing wetting front. Experiments are described from which a characteristic capillary potential of the material is obtained by measuring the equilibrium pressure of the air displaced and compressed ahead of the advancing wetting front. Results for the absorption of water and n-heptane by a fired clay brick ceramic suggest that this wetting front capillary pressure (or capillary potential) scales approximately with the surface tension and also that the permeability scales inversely with the liquid viscosity. The pressure of the air trapped in the wetted region is found to be the same as the pressure of the displaced air. For this material the wetting front capillary pressure for water at 20°C is 0.113 MPa, equivalent to a hydraulic tension head of 11.5 m and to a Young–Laplace pore diameter of 2.6 µm. The capillary pressure so measured is apparently a fundamental percolation property of the material that can be interpreted as the air pressure at which liquid phase continuity and unsaturated conductivity both vanish. The method described can be applied generally to porous materials.

The Role of the Kaolinite-Mullite Reaction Sequence in Moisture Mass Gain in Fired Kaolinite

Kaolinite (2SiO2.Al2O3.2H¬¬¬¬2O), an aluminosilicate mineral, is the most common constituent mineral in clay used in manufacturing traditional ceramics such as whitewares, some refractories and structural clay products. On firing, kaolin (mainly kaolinite) undergoes several phase transformations. Immediately following firing, kaolin starts to chemically combine with atmospheric moisture. This reaction causes a mass increase that is proportional to the fourth root of time. The consequence of this mass gain is an accompanying expansive strain. Kaolinite transformations following firing at a range of temperatures between 700 and 1200 oC were examined by XRD. Following firing, high accuracy mass gain measurements were carried out using a microbalance under precisely controlled conditions of temperature and relative humidity. It was found that the formation and development of crystalline phases increases with increasing firing temperature. This causes a reduction in the amorphous phase which, in turn, leads to reduced mass gain. Kaolin fired at 1200 oC exhibited the least reactivity with moisture and mullite was found to be the dominant crystalline phase after firing at that temperature. SEM was used to examine microstructural changes in the fired specimens. The reactivity of the ceramic with moisture is shown to be directly related to the crystallinity of the fired clay.