Title: Calcined Paper Sludge-Pozzolanic Admixture to Improve Sulfate Resistance of Concrete

Author(s): S. Rols, J. Ambroise, J. Pera, and M. Chabannet

Publication: Symposium Paper

Volume: 192

Issue:

Appears on pages(s): 353-368

Keywords: ammonium compounds; expansion; gypsum; microstructure

DOI: 10.14359/5759

Date: 4/1/2000

Abstract:
Ammonium sulfate-(NH4)2SO4-is harmful to concrete as it reacts with calcium hydroxide to form expansive gypsum which, in the presence of aluminates, may in turn give expansive ettringite. The improvement in the ammonium sulfate resistance induced by pozzolanic additions does not always seem to be effective, and it has been reported that some low C3A portland cements are able to resist ammonium sulfate attack better than blended cements containing silica fume. A new admixture, calcined paper sludge, which is a mixture of metakaolin and calcium carbonate, was used to improve the resistance o both portland cement and ground-granulated blast furnace cement to 20% ammonium sulfate solution. Five concrete samples containing a cementitious material content of 350 kg/m3 were subjected to sulfate attack in alternate wetting-drying cycles. After 4 or 5 cycles, only concrete made with blast-furnace slag cement and containing the calcined sludge presented some residual strength, a small mass loss, and lower expansion. The microstructural studies by SEM showed that after the sulfate attack, gypsum was present at the aggregate-paste interfaces, and the size of gypsum crystals depended upon the binder used.