The Influence of Temperature on the Hydration Rate of Cements Based on Calorimetric Measurements

Influence Mechanism of Initial Concreting Temperature and Water-Binder Ratio on Hydration Rate of Fly Ash Concrete

The hydration exothermic rate of fly ash concrete is significantly affected by the initial concreting temperature and water-binder ratio. Firstly, the adiabatic temperature rise and temperature rise rate of fly ash concrete at different initial concreting temperatures and water-binder ratios were obtained by a thermal test instrument; then, the effects of initial concreting temperature and water-binder ratio on the hydration kinetic parameters of the NG-I-D hydration process of fly ash concrete were analyzed by the theory of hydration kinetics; lastly, the effects of initial concreting temperature and water-binder ratio on chemically bound water and pore bulk of fly ash concrete during hydration were analyzed by applying a thermogravimetric analyzer and industrial CT scanning techniques. The results showed that the increase in initial concreting temperature and the decrease in water-binder ratio accelerated the rate of temperature rise, and the initial concreting temperature had a more significant effect than the water-binder ratio. During the hydration reaction, the I process was significantly influenced by the initial concreting temperature, and the D process was significantly influenced by the water-binder ratio; the content of bound water increased with the increase in water-binder ratio and age and the decrease in initial concreting temperature. The initial temperature had a significant effect on the growth rate of 1 to 3 days bound water, and the water-binder ratio had a more significant effect on the growth rate of 3 to 7 days bound water. The porosity was positively correlated with the initial concreting temperature and water-binder ratio and decreased with age, but 1 to 3 days was the key period of porosity change. Additionally, the pore size was also influenced by the initial concreting temperature and water-binder ratio.

Crack-Resistant Cements under Drying: Results from Ring Shrinkage Tests and Multi-Physical Modeling

Cementitious materials exhibit shrinkage strain on drying, leading easily to crack formation when internally or externally restrained. It is known that cements with a slow strength gain show higher crack resistance under external drying. The ring shrinkage test can be considered an accelerated method for cracking tendency due to existing historical correlations between ring cracking time and long-term surface concrete cracking. The experimental campaign used ring shrinkage tests on 25 mortars, covering 10 commercial cements and 15 cements produced on demand, covering Portland cements and blended cements up to a 30% slag substitution. The results show that the restrained ring cracking time generally increases with lower Blaine fineness and higher slag substitution in 6 to over 207 days’ span. Upper limits for crack-resistant cements were proposed for 2-day compressive strength and Blaine fineness, in the case of Portland cements: 27.7 MPa and 290 m²/kg, respectively. A hygro-mechanical model successfully replicated strain evolution with crack formation and brittle failure. Only two out of ten commercial cements were classified as crack-resistant, while the ratio increased to 10 out of 15 cements which were produced on demand.