USE OF SMALL ANGLE NEUTRON SCATTERING AND X-RAY POWDER DIFFRACTION WITH THE RIETVELD METHOD IN THE ASSESSMENT OF FIRING CONDITIONS IN HISTORICAL FIRED-CLAY BRICKS

It is well-known that the description of the complexity of fired-clay bricks can be tackled only by integrating information from different analytical techniques [1]. At the same time, a nondestructive or low impact approach, aimed at the conservation of cultural heritage objects, is prompting for the search of new analytical methods for their characterisation. As a widely accepted practice, in restoration works, the best compatible materials should be employed. To this aim, the characterization of the original bricks is essential in order to choose the most suitable replacement material and prevent further damages. Mechanical properties and performance of fired-clay bricks are directly related to their microstructure and mineralogical composition, which in turn depend upon the employed raw clay and firing temperature. In this work, the potential of small angle neutron scattering (SANS) and X-ray powder diffraction (XRPD) with the Rietveld method (including the quantification of the amorphous fraction), is evaluated in the investigation of historical fired-clay bricks. SANS is a powerful tool for the characterization of the microstructure of porous solids. In bricks, XRPD allows to identify and quantify the minerals and the amorphous fraction, in order to describe the phase transformations taking place during firing [1,2]. Bricks produced from 2 raw clays have been fired at 5 temperatures (from 800 to 1100 in laboratory. Results have been used to derive information on the firing process experienced by a number of historical fired-clay bricks produced in Czech Republic. SANS allowed to retrieve the pore size distribution, characterise the fractal surface of pores as well as suggest an empirical relationship between surface area per unit volume of pores and firing temperature. The fractal quality of the surface area of pores is also suggested as a method to distinguish between bricks produced from different raw clays. Unit-cell parameter of spinel and amount of hematite (phases detected in the fired bricks) from XRPD are proposed as indicators of the firing conditions.