Mechanism of calcium sulfate variety on strength of portland cement-calcium aluminate cement blends

Varieties of calcium sulfate on the strength of Portland cement-calcium aluminate cement blends were investigated by means of calorimetry, X-ray diffraction, environmental scanning electron microscopy. Results show that the variety of calcium sulfate determines its solubility and dissolution rate, which directly lead to the differences in the formation content, time and size of ettringite (AFt), and affects the volume stability, which finally determines the strength of pastes. When the calcium sulfate dosage is 7.5%, the formation content of AFt firstly increases and then decreases. However, the time when AFt content reaches the maximum and the size of AFt have close relationships with the variety of calcium sulfate. When anhydrite is added, the transit time from AFt to monosulfate is the latest, and the size of AFt is much longer and thinner than those with other calcium sulfate, additionally the AFt content before 8 hours is prominently lower than others, which leads to lower strength. When the calcium sulfate dosage is 15%, secondary gypsum also forms besides AFt, is ten times bigger than AFt. This can lead to the expansion and even complete lost of strength for the systems with hemi-hydrite