Abstract
In the preliminary study, the property of gypsum cement composite is investigated to use as a binder. The mixture of a type gypsum hemihydrate and normal portland cement, containing normal portland cement proportioning 20 % by weight, makes a good composite system in which hardened crystal gypsum is fully covered by cement hydration products. The composite system improves its water resistant property remarkably, and makes the most of preferable properties of gypsum for building materials. Some experiment are carried out to find out a retarder for the composite system, on which setting time is exceedingly short without retarder and is susceptibly influenced by the temperature of mix condition. In several chemicals, citric acid and starch have efficacy to retard the setting time but their actions are quite different. As the temperature of mixture increases, the setting time becomes shorter by citric acid, and becomes longer by starch. Then a combination method, adding citric acid and starch together in mixture, is examined to control the setting time independent of the change of temperature. A certain amount of citric acid and starch, which can retard the setting time about 1 hour constantly under different temperature, is considered as the most preferable combination method. By the combination method, the mixture with water binder ratio of 30 % has good consistency, and its hardened specimen in 3 hours after mixing with water indicates high strength near 300 kgf/cm^2.
