Characterisation of the pore-forming process in lightweight aggregate based on silica sludge by means of X-ray micro-tomography (micro-CT) and mercury intrusion porosimetry (MIP)

In the case of lightweight aggregate (LWA), porosity can be achieved by means of a high temperature foaming process. During this process a glassy matrix is created, and almost simultaneously the added foaming agent degasses and the resulting gasses remain trapped inside the glassy structure. The present paper deals with the foaming process which occurs in the case of silica sludge to which fly ash, which creates liquid phase, has been added, as well as SiC, which acts as a foaming agent. The development of porosity within the structure of this material was investigated at different temperatures and dwelling times by means of X-ray micro-tomography (micro-CT), and mercury intrusion porosimetry (MIP). The results were compared by means of both X-ray micro-tomography and mercury intrusion porosimetry, the first technique being applied within the pore size distribution range of between 50 μm to more than 1 mm, and the second within the range, between 0.0055 and 360 μm. Both techniques have certain limitations as well as certain advantages, but in the case of the investigated system micro-CT gives much more reliable results about porosity development over a prolonged firing time, at the selected temperature. The results showed that, at the selected temperature, which in this case amounted to 1220 °C, porosity as well as median pore size increased with increased dwelling times. In the case of prolonged dwelling times, the number of pores decreased, but, on the other hand, the volume of these pores increased.