Decalcification of cement paste in NH4NO3 solution: microstructural alterations and its influence on the transport properties

Leaching of cement-based materials changes its properties such as a reduction in pH, an increase in porosity and transport properties and a detrimental effect on properties related to long-term durability. Therefore, a better understanding of leaching process is important including the relevant long-term effects for concretes used in waste disposal systems. However, the decalcification process is not easy to capture because it is extremely slow. In this study, an ammonium nitrate (NH4NO3) solution of 6 mol/l was used to accelerate the leaching kinetics. The experiments were performed on cement paste samples with different water/powder and limestone filler replacement ratios. The change of sample mass over time was monitored, and the amount of calcium ion leached out during the test was determined. Different post-analysis techniques like SEM, MIP and N2-adsorption were used to characterize the microstructural changes, while the degraded front was determined by phenolphthalein spraying. The effect of accelerated leaching on transport properties was studied by measuring the change in water permeability. Results show that (i) NH4NO3 solution is an aggressive but suitable agent to be used to accelerate the Ca leaching in cementitious materials while still keeping the “nature” of leaching; (ii) the square-root-time law of degradation is applicable under accelerated conditions; (iii) the porosity of the leached samples increases significantly and the critical pore size is shifted to larger radius; and (iv) the BET specific surface area of the leached sample is also significantly increased. These changes result in a significant increase in water permeability.