The hydration of cement containing quick setting agents, where the setting agents were based on calcium aluminate, was investigated to discuss the relationship between quick setting properties and hydration characteristics of calcium aluminates. The calcium aluminate with different crystallinity and different CaO/Al₂O₃ molar ratio of 1.74 and 2.53 were synthesized by means of air quenching method. The following results were obtained.
(1) A mixture of cement and calcium aluminate without additives did not have quick setting properties.
(2) when a quick setter composed as a mixture of calcium aluminate with suitable amount of NaAlO₂, Al(OH)₃, CaO, Na₂CO₃ and CaSO₄, the addition of 7% of the quick setter for cement mixture with W/C ratio of 0.6 resulted to set the paste within 30s to 2min. A mortar with W/C ratio of 0.6 and SO₃/CaO ratio of 0.33 set within 10min.
(3) Non-crystallinity and high CaO/Al₂O₃ molar ratio were very important for quick setter based on calcium aluminate.
(4) A cement paste with 7% of quick setter evolved a large amount of heat of initial stage of hydration, which corresponded to the amount of ettringite formed. The larger the heat of hydration, the faster the setting time. In a paste having a good quick setting property, ettringite with 5 to 10μm in size were formed and these hydration products were well connected each other.