Abstract The effect of chemical admixtures on ettringite formation and expansion of expansive cement containing a pre-hydrated high alumina cement — based additive (H-HAC) is discussed. Nine chemical admixtures including surfactants, accelerators, retarders and a bonding agent were investigated. Compacted samples were used to study the effect of chemical admixtures on relevant hydration reactions. Normally mixed paste samples were also used to study chemical and surface — chemical effects. The effect of admixtures on ettringite formation and reaction of gypsum at early hydration times was followed by XRD analysis of high w c wet samples containing large amounts of expansive additive. Suggestions are given for the control of ettringite formation and expansion in expansive cement — based systems.