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Abstract: The technical quality of two compositionally different groups of solid bricks fired between 800 and 1000°C was evaluated. Five weight percentage of fly ash was added to both groups and they were compared with similar bricks with no added fly ash. The textures of the bricks with fly ash were very similar to the textures of those without it, except that the samples with the additive contained spherical fly ash particles with diameters ranging from 0.1 to 10μm. These particles led to a reduction in the density of the bricks and a substantial improvement in their durability, with less decay being caused by salt crystallization in the pores. This is because fly ash causes a reduction in the number of micropores, the pores that make porous materials most vulnerable to salt-induced decay. Use of this additive could have practical implications as a means of recycling and for achieving cost savings in brick production. [Copyright &y& Elsevier]

The changes in brick porosity upon firing (700 up to 1100 °C) and its relation to the mineralogical composition are examined. Two types of raw clay with a composition representative of that used in brick-making industry were selected to manufacture the bricks: one contains notable amounts of carbonates, with a grain size of under 1 mm, and the other is predominantly quartzitic and lacking in carbonates. We demonstrate that the presence or absence of carbonates strongly influences the porosity development and, therefore, the brick texture and physical-mechanical properties. The carbonates in the raw clay promote the formation of fissures and of pores under 1 μm in size when the bricks are fired between 800 and 1000 °C. The absence of carbonates results in a continuous reduction in porosity and a significant increase in the pore fraction with a radius (r)>1 μm as the firing temperature rises and smaller pores coalesce. Porosity and pore size distribution results obtained from the combined use of hydric tests (HT), mercury intrusion porosimetry (MIP) and digital image analysis (DIA) of scanning electron microscopy photomicrographs are compared. A clear correlation between the water absorption and drying behaviour of the bricks and the porosity plus pore size distribution is observed. DIA discloses the evolution of size, shape and connectivity of macropores (r> 1 μm) and evidences that MIP results underestimate the macropore content. Conversely, MIP gives a good estimate of the open porosity and of the distribution of pores with r<1 μm. It is concluded that the combined use of these complementary techniques helps to fully characterise the pore system of bricks. These results as well as the study of the evolution of the speed of ultrasound waves vs. time yield useful information to evaluate the bricks physical–mechanical behaviour and durability. The relevance of these findings in the conservation of historic buildings is discussed. [Copyright &y& Elsevier]

Abstract: Bricks proceeding from one of the bastions of the Riga medieval defence wall were studied. Two main type of bricks were identified: one rich in quartz and red coloured and the other with a calcium silicate phase and yellow coloured. The survey of the bastion reveals a partial deterioration of the bricks. It was observed that differences in mineralogy and texture of calcareous and non-calcareous bricks were correlated with their behaviour in hydric and weathering tests. Data from powder X-ray diffraction (presence of high-temperature mineral phase in yellow bricks), optical microscopy (vitrification in yellow bricks), Field Emission Scanning Electron Microscope (development of secondary bubbles due to extensive melting in yellow bricks) and ultrasounds measurements (lower velocity in red bricks) allow to deduce that yellow bricks were fired at a higher temperature than the red ones. The study of the pore system revealed a bad interconnection among pores in yellow bricks (hydric data) since melt isolated the pores and filled the smallest, whereas large pores increased in size (porosimetric data). As a consequence, considering the climatic conditions of the region, yellow bricks are the materials most subjected to environmental alteration. It is in agreement with the freeze–thaw test results. Finally, low contamination of the bricks with water-soluble salts in situ leads to exclude salt-caused decay process. The relevance of these results in the conservation of Cultural Heritage is discussed. [Copyright &y& Elsevier]