Abstract In this paper, water seepage of cement asphalt emulsified mortar (CA mortar) was studied in a small-scale track model, including the seepage rate, the chemical composition of the seeping water and its effects upon CA mortar. It is shown that the seepage time is negatively correlated to the environment temperature. A higher temperature would lead to a shorter seepage time, while the cumulative amount of seepage is similar. Seepage may be induced by the volume expansion of CA mortar in the early age, which is evidenced by a positive correlation between the seepage rate of the small-scale mortar sample and the expansion rate measured on a cylindrical sample. The solid components in the seeping water is low, initially less than 12% and ultimately at 3–4%, thus has negligible effect on the mix proportion. In addition, a “size effect” is noted, in which the water seepage is more significant in the field, compared with the small-scale model.