Stabilization/solidification has been found to be a relatively sustainable and economical method for managing risks associated with contaminated land. The technology has been employed extensively over the last three decades, particularly for the containment of heavy metals. However, because contaminants are not removed, there is a need to validate the long-term effectiveness of the technique. This requires knowledge of the containment mechanisms, the kinetics of contaminant release (equilibrium and disequilibrium conditions), and identification of the reactive surfaces that induce containment. In the present study, cement-stabilized zinc-contaminated kaolin clay containing 1% humic acid was tested to evaluate chemical performance in the long term. Time-dependent (chemical kinetics) and pH-dependent (equilibrium conditions) and equilibrium porewater leaching were evaluated, using LeachXS and geochemical speciation modelling using ORCHESTRA (embedded in LeachXS). Results showed that zinc was effectively contained within the waste form matrix under the prevailing chemical conditions, with immobilization increasing with hydration. Presence of humic acid increased the availability of zinc at an early stage, but this decreased at later stages and was comparable with results for organic-free matrices.