<p>Polyacrylamide has been widely used as a viscosity-modifying admixture in cementitious materials. However, previous studies mainly focus on how polyacrylamide affects fresh properties of host materials, with scant attention on the hydrate of portlandite (CH) in hardened cement-based materials. In this study, the impact of polyacrylamide on the hydrate of portlandite in hardened cement paste was investigated. The isothermal calorimetry test was performed to reveal behaviors of the early cement hydration by adding polyacrylamide. X-ray diffraction analysis, thermogravimetric analysis, Fourier transform infrared spectroscopy, scanning electron microscopy and energy dispersive spectroscopy were performed to monitor the content and morphology of portlandite. The results indicate that CH content decreases with the increase in polyacrylamide dosage in hardened cement pastes. The orientation index and crystal size of CH crystal in hardened cement pastes decrease by adding polyacrylamide. The CH morphology changes from a large structure with typical stacked lamellar shape to a smaller structure with corroded edges. Due to the intermixing of CH microcrystals in C–S–H, the Ca/Si ratio of C–S–H increases by adding polyacrylamide. Additionally, isothermal calorimetry tests confirm the retardation effect of polyacrylamide on initial and induction periods because of the adsorption effect of polyacrylamide with cement particles and hydrates, as well as the complexation effect of –COOH/–COO⁻ and Ca²⁺, which would significantly change the morphology and content of CH phase.</p>