Scroll over image for captionAir hardened, mineral-bound foam concretes that can be produced cost effectively for load-bearing structures, are now available, say researchers at the Technische Universitaet Dortmund, Germany.
Ultraporcrete is said to combine the strength, low density and good heat insulation of cellular and foam concrete in one material, without the need for energy intensive autoclave manufacturing.
‘Residential buildings made using standard concrete need additional heat insulation,’ explains Dr Bernhard Middendorf from the university. ‘Chemically altered foam concretes provide these properties but cannot be used as a construction building material for load-bearing applications due to their unstable, undefined void structure.’
To increase their strength, foam concretes must be hardened using costly and energy-intensive autoclaving processes under high pressure and high temperature.
Middendorf and his team have developed a way of controlling the porous structure of these foams by adjusting the optimal amount of the conventional foaming agent, aluminium powder, making the materials strong enough for load-bearing applications.
‘By using different grain sizes of the aluminium powder, the air void distribution is controllable and the air void volume can be adjusted by the total quantity,’ explains Middendorf. ‘The structure of the hardened cement paste matrix can then be optimised using reactive additive microsilica in the form of a suspension.’
The water/cement ratio also influences the volume of cement paste during expansion. ‘Reducing the water content leads to a stiffer fresh mortar consistency, the pore size becomes smaller and the mass density and compressive strength rise. In order to ensure an expansion in cement pastes with low water volume, the introduction of a superplasticiser based on polycarboxylatether was necessary,’ he adds.
The results show compression strengths of 14-20N/mm2 with densities of 400- 1,200kg/m3, comparable with autoclaved aerated concrete (which is between eight and 10N/mm2 at densities from 800-1,000 kg/m3), but lower than traditional concrete, which has values of 20-80N/mm2 and 2,300kg/m3.
Middendorf says that, because an autoclave is not needed, Ultraporcrete is cheaper and more sustainable to produce than strengthened mineral-bound foam concrete, but is slightly more expensive to produce than traditional concrete.
Dr Phil Purnell, Director of the Institute for Resilient Infrastructure at Leeds University, UK, agrees, but still questions the overall cost benefits.
‘Reducing material costs is always welcome, however the operating costs may increase due to the large amounts of microsilica needed to strengthen it.
‘We also need to see what happens when Ultraporcrete carbonates,’ he adds. ‘Autoclaved concrete reacts with carbon dioxide in the air, but very slowly. We need to check that the carbonation process is not going to make the material too brittle for use in construction.’
Middendorf says Ultraporcrete can be produced using existing manufacturing methods and the university is working with industrial partners to get the material to market.Materials World Magazine, 01 Mar 2010