Hydration degree of cement and slag in slag blended cement pastes

As hydration of pure cement pastes proceeds, unhydrated cement particles hydrolyse, hydration products are formed and the degree of hydration continuously increases. Moreover, the ultimate hydration degree mainly depends on the water/cement ratio and can be estimated by the formula of Mills. However, the situation becomes more complicated when the cement is partly replaced by blast-furnace slag since the hydration reactions of both components are influenced by each other. Furthermore, the hydration degree of slag in paste is difficult to predict and depends on the curing conditions, water/cement ratio, test age, cement/slag ratio and the slag reactivity.
In this research, the extent to which slag particles are hydrolysed and take part in the hydration process was visualized by backscattered electron (BSE) microscopy at 2, 7, 14, 74 days and 28 months. Besides, thermogravimetric analyses were performed to determine the bound water content, Ca(OH)2 and CaCO3 content: the differences between pastes with and without slag gave an indication about the extent to which slag hydration products are formed.
The test results indicated that the cement hydration is enhanced in the presence of blast-furnace slag. The ultimate cement hydration degree of 74%, which has been reached after about 2 years in pure cement pastes with a water/cement ratio of 0.5, increased above 90% for slag blended pastes. The hydration of the slag itself strongly depended on the cement/slag ratio. For replacement levels of 50% and 85%, the hydration degree after about 2 years amounted to 70%, respectively 39%.

Author: E. Gruyaert, N. De Belie

Section: Science and production

Keywords: blast-furnace slag, Portland cement, hydration, electron microscopy, thermogravimetry