Strength Characteristics of Hardened Alumina Cement

Abstract

The strength changes due to the “conversion” in alumina cement mortar have been investigated under various conditions, mainly on the stand point of the mineralogical changes.The characteristics of the strength change have been found in the hardened alumina cement (Fig. 6.). The strength of alumina cement mortar or concrete decreased with the proceeding of the conversion reaction from CAH10 to C3AH6. The strength reached the minimum value (point A in Fig. 6.), when the reaction had been completed, C3AH6 and AH3 being the only observed hydrates. Thereafter, the strength increased again with aging time up to the maximum value, point B. Through the stage from A to B the species of hydrates did not change, but the crystals of hydrates grew up and formed a tight structure, resulting the increase in the strength above mentioned. During the further aging the strength diminished again gradually and reached a constant value, the hydrates changing into an unhydrated compound, C12A7.These characteristic points, A, B and C, were plotted as functions of the temperature and the aging time in Fig. 7. From the Figure the following experimental equations were derived for each point, A log t=101.87×T-0.758 (1)B log t=102.25×T-0.806 (2)C log t=102.51×T-0.833 (3)where, t=aging time (minute), T=temperature (°C).From the equation (1) the term of strength-fall at 20°C is calculated to be about 76 years. Therefore, if the concrete made with alumina cement is kept at low temperatures, it is considerd that the decrease in long-term strength is very little at the practical point of view.On the basis of these results, the test method for predicting the long-term strength was discussed.