High alumina cement is black in color and develops very fast in strength. About 80% of the ultimate strength is developed after 24 hours, and even 8 hours is strong enough to remove the formwork. The high growth rate of strength is due to its rapid hydration. It also has a high calorific value.
It has a heat development rate of 9 calories/hour per gram of cement (9 cal/g/h) and for fast hardening cement (3.5 cal/g/h), which is 2.5 times the fast hardening Portland cement. However, the total heat of hydration for both cements is of the same order of magnitude. Ordinary Portland cement has a thermal development rate of 1.6 cal/hour/gram.
The compressive strength of 1:2:4 concrete prepared with high alumina cement was 420 kg/㎠ after 24 hours and 490 kg/cm after 3 days, and its ultimate strength was also higher than that of ordinary cement. As mentioned above, its heat development is very rapid, occurring in about the first 10 hours. The high heat development rate makes this cement unsuitable for mass concrete construction, but when the concrete is placed in freezing weather, the fast heat development rate has great advantages.
High alumina cement is slow setting cement. The initial setting time is 4 to 5 hours, and the final setting is about 30 minutes after the initial setting. The setting time of high alumina cement is greatly affected by the addition of plaster of paris, lime, Portland cement and organic matter. Therefore no additives should be used. To reduce the setting time of this cement, 1% to 2% slaked lime can be added. When rapid setting is critical, i.e. preventing water ingress or temporary construction between tides, etc., a mixture of ordinary Portland cement and high alumina cement in the proper proportions is used, but the ultimate strength of this slurry is rather low.
In normal concrete construction, the two cements should not be allowed to come into contact with each other, as flash setting will occur in this case. The flash setting or accelerated setting time is due to the formation of C4A hydrates by the addition of lime in ordinary Portland cement to calcium aluminate in high alumina cement. The gypsum contained in Portland cement may also react with the calcium aluminate hydrate and cause the meat to set.
If two layers of cement are to be laid, they should be laid at different times. If the first layer of high alumina cement is laid, the Portland cement layer should be laid at least 24 hours later. If the first layer is made of Portland cement, the concrete made of high alumina cement should be laid after 3 to 7 days. Contamination through equipment or tools should also be avoided. In the case of the same water-cement ratio and equal mixing ratio, high-alumina cement is used as the raw material of high-alumina cement, because the surface of high-alumina cement particles is smoother than that of Portland cement particles, so compared with Portland cement concrete, it has better workability, fully integrated in the hearth.
