Evolution of Damage Due to Sulphate Attack in Cement Mortar with and Without Ground Coal Bottom Ash

Abstract

Siliceous coal bottom ash is a residue originated in thermo-electrical power stations as a result of the hard coal combustion. It is expected that some characteristics of the coal bottom ash would be similar to those of the coal fly ash formed together in the same boiler. Coal bottom ash has a larger size than coal fly ash. Then, the first one was ground to achieve a particle size similar to the cement size. Therefore, to assess the sulphate resistance of cement-based materials made of ground coal bottom ash, sixteen Portland cement mixes were prepared by combining a cement CEM I 42.5 N according to the European standard EN 197-1:2011, a ground coal bottom ash and a coal fly ash. Both ashes were formed in the same boiler. The expansion measurements are considered to be an adequate parameter to assess damage due to sulphate attack of continuously submerged specimens. This procedure is the basis of the American standard ASTM C-1012/C1012 to evaluate the resistance of Portland cement and other cementitious materials to sulphate attack wherein the expansion measurements are taken with a standardized length comparator along the time. In this research program, the extent of sulphate attack was quantified by the percentage expansion of slender bars submerged in 5% sodium sulphate solution according to the ASTM C-1012 standard. This standard specifies an expansion limit of 0.01% for ordinary Portland cements CEM I and 0.035% for blended cements after a period of one year of exposure. The Portland cement CEM I 42.5 N made without ashes exhibited the largest expansion at 330 days (0.09%); whereas a cement with 10% of coal ash, CEM II/A-V, the expansion was much lower (0.03%) for both types of ashes. The expansion decreases when the ash content increases. In this property, no difference was found between ground coal bottom ash and coal fly ash provided by the same thermo-electrical power station.