The time of setting, the rise of temperature in the setting, and the small piece compressive strength of the samples obtained by heating four mixtures, 3CaO⋅Al₂O₃, 5CaO⋅3Al₂O₃, CaO⋅Al₂O₃ and 3CaO⋅5Al₂O₃, at temperatures ranging from 1200° to 1500°C have been measured. The relation between the strength and the microstructure is also discussed.
All of the 3CaO⋅Al₂O₃ and 5CaO⋅3Al₂O₃ samples began their setting rapidly regardless of the content of free lime, although their final set became slow suddenly when the free lime exceeded certain amounts. The CaO⋅Al₂O₃ and 3CaO⋅5Al₂O₃ samples set rapidly as long as they contained free lime, but their final set was retarded by an excess of free lime.
The strength of the 5CaO⋅3Al₂O₃ samples increased almost in direct proportion to the degree of chemical combination, while that of the CaO⋅Al₂O₃ and 3CaO⋅5Al₂O₃ samples increased rapidly when the combination surpassed certain degrees. Generally speaking, curing in air was favorable than that in water.
It is probable that those samples which produce the crystals A described in the Report III at the early stage of hydration are apt to set in an instant. Weak test pieces contained much of the needle-shaped crystals A and small amounts of gel. Strong test pieces contained much spherulites consisting of radiating fibrus crystals and gel. The gel is produced by the hydration and also by the reaction between the hydrated aluminates and carbonic acid, and acts to increase the strength.

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