Cement-asphalt mortar (CA mortar) is composed of Portland cement, asphalt emulsion,water and other admixtures,which combines both advantages of the mechanical property of cement and flexibility of asphalt.Therefore, it has already been applied in high-speed railway and used as semi-flexible pavement material.As a basic and key composition of asphalt emulsion, emulsifier is the reaction medium between cement and asphalt, and the effect of asphalt emulsion on cement hydration is mainly through the interaction between emulsifier and cement particles.Hence, The main purpose of the investigation was to clarify the retardation effect of emulsifier LA on cement hydration and reveal its mechanism.