Chemical sulfate attack performance of partially exposed cement and cement + fly ash paste

When concrete elements are partially exposed to sulfate rich environment, the upper part of concrete in contact with air will be deteriorated more severely than the underground part. Fly ash additions seem to accelerate the collapse of concrete in such an environment. Although concrete technologists attribute concrete damage mainly to salt crystallization or physical sulfate attack, the influence of chemical sulfate attack cannot be neglected and should also be studied.The objective of this paper is twofold. First, pore solution expression test was conducted to squeeze pore solution of different parts of cement paste partially exposed to Na2SO4 solution. The sulfate concentration and pH value of pore solution were measured. Results showed that the sulfate concentration of the pore solution in the upper part of paste in contact with air was much higher than in the lower submerged part. Fly ash additions could draw more sulfates into the paste in a shorter time, forming a higher concentration sulfate pore solution than in normal concrete.The second test was designed to simulate the effect of severe exposure condition on reactive products of cement paste. Pure cement and cement + fly ash (25% dosage) pastes were immersed in 5%, 15% and 30% at 30 °C and 15% at 40 °C Na2SO4 solutions. Thermogravimetric analysis was used to analyze the reaction products of the paste. The results indicate that more ettringite and gypsum were formed in cement + fly ash paste than pure cement paste.

► Pore solution expression test is used to study pore solution zone in partially exposed cement paste.
► An almost saturated sulfate pore solution is generated in the part of paste in contact with air.
► Fly ash additions can draw more sulfates into the paste in a shorter time, forming a higher concentration sulfate pore solution than in normal paste.
► More ettringite and gypsum are formed in cement + fly ash paste than pure cement paste.