Conversion of high alumina cement results in a loss of strength because the density of the converted cubic calcium aluminate hexahydrate is higher than that of the unconverted calcium aluminate decahydrate. Therefore, if the total volume of the body remains the same, the transformation leads to an increase in the porosity of the slurry, which has a huge impact on the strength of the cement paste or cement concrete. Strength loss due to conversion is a function of temperature and water-cement ratio.
At moderate and high w/c ratios, the residual strength is so low that it is unacceptable for most structural purposes. Even at low w/c ratios, the conversion rate increases the porosity so much that chemical attack can easily occur. Therefore, HAC is no longer used for structural concrete above or below ground level due to conversion effects, but it is a useful material for limited-life repair work and temporary works. The hydration of high aluminium cement is about 50% of the dry cement weight, which is about 100 times the amount of water required for the hydration of ordinary Portland cement, but the ratio to high-alumina cement and low-water cement is about 0.35, which is indeed desirable.
Long-term field studies and experiments conducted in the laboratory show that:
1. Calcium aluminate concrete made of high alumina cement with a free water/cement ratio less than 0.4 and stored in water at 18°C during its initial curing period and subsequent life will reach its minimum strength after approximately 5 years. However, this minimum intensity will not be significantly lower than a day's intensity.
2. If concrete made of high alumina cement is stored in water at 18°C for 24 hours and then stored in water at 38°C, it will rapidly transform to the highest strength and reach the lowest strength in about 90 days. However, this intensity is significantly lower than the 1-day intensity.
3. If concrete made of high alumina cement is stored in water at 18°C for a long period of time, such as up to about 8.5 years, and then immersed in water at 38°C, it will rapidly transform into strength and reduce to 38°C.
4. The 38°C temperature is the upper limit of temperature that may occur during curing or normally heated buildings. Therefore, it is recommended that the design of high-alumina concrete should be based on the minimum strength at 38°C.
5. Highly transformed high alumina cement concrete is susceptible to chemical attack in the presence of long-term moisture and chemical attack agents. This risk may be more severe for concrete with high water-cement ratios.
