Formation of calcium aluminate hydrates in Portland cement modified by organic admixtures

The formation of calcium aluminate hydrates in Portland cement pastes modified with two organic admixtures, hydroxyethyl methyl cellulose (HEMC) and ethylene-vinyl acetate redispersible polymer powder (EVA), was evaluated by using X-ray diffraction analysis. The results show that both the HEMC and the EVA retard the formation of ettringite and postpone the time to reach maximum ettringite content. However, the stability of ettringite is enhanced and the conversion of ettringite to monosulfate (AFm) is delayed by these two organic admixtures to retain a higher ettringite content in cement pastes at later hydration time. With calcite powders contained in EVA as the anti-blocking agents, the formation of two AFm phases, hemicarboaluminate and monocarboaluminate, is promoted by EVA after 3 d of hydration. Accordingly, more types of calcium aluminate hydrates coexist in the cement paste modified with EVA than in the control paste or in cement paste modified with HEMC. Also, having been synchronously mixed in the cement paste, these two organic admixtures have obvious superposition effects on the formation and evolution of calcium aluminate hydrates.