Investigation of the conditions for a thaumasite form of sulfate attack in SCC with limestone filler: Fid-Bau Portal

Investigation of the conditions for a thaumasite form of sulfate attack in SCC with limestone filler

Trägardh, J. / Kalinowski, M.

The parameters investigated include sulfate load (concentration), cement type, amount of limestone filler and fineness of limestone filler. The self-compacting concrete (SCC) test specimens have been stored at 5 deg C for approximately 2 years in three different MgSO4-solutions with concentrations of 0.01 % (100 mg/l), 0.05% (500 mg/l) and 0.14 % (1400 mg/l) as SO4(2-). An ordinary sulfate resistant Portland cement (CEM I) with low sulfate and C3A contents and two blended Portland limestone cements (CEM II) with different C3A contents were tested. The mix proportions used correspond to normal Swedish SCC and included 0, 50, 100 and 180 kg/m3 added limestone filler. The test program included compressive strength, wear rating, weight and pH measurements. The microstructure was studied by SEM-EDS techniques and optical light microscopy. The results so far indicate that SCC with blended Portland limestone cements (CEM II/A-LL), with and without added limestone filler, are vulnerable to TSA when exposed to the moderate magnesium sulfate solution, 0.14 % SO4(2-). At weak sulfate concentrations (0.01 and 0.05 %), the SCC with blended Portland cements has not yet shown any sign of TSA. SCC containing cement with sulfate resistant Portland cement with a low C3A content (SRPC, CEM I) remains intact in all solutions after 1 year and 10 months of exposure. Evidence from the microstructural study indicates that the reaction mechanism included formations of ettringite and ettringite solid solution compounds prior to thaumasite formation. Thus, the results indicate that the Woodfordite- rather than the direct C-S-H reaction route was predominant. The C3A content played an important role.