This paper studies the setting behaviour of alkali-activated slag/fly-ash blends using ultrasonic P-wave velocity and standard Vicat Needle test method. Four binders were formulated with different slag/fly-ash ratios (100:0, 75:25, 50:50, and 25:75) and were activated with sodium silicate alkaline solutions. The results show that the ultrasonic velocity developed slowly with increase in fly-ash replacement. Initial setting time of alkali-activated slag/fly-ash blends occurs around the time of appearance of first inflection point in ultrasonic curves. Final setting time corresponds with the time of starting of stage III of UPV curves. Moreover, final setting time can also be estimated accurately from the time of ultrasonic velocity reaching a value of around 1700 m/s for slag/fly-ash blends activated with sodium silicate.
Alkali activated materials (AAM) are generally cured at high temperatures to compensate for the low reaction rate. Higher temperature accelerates the reaction of AAM as in cement-based materials and this effect is generally predicted using Arrhenius equation based on the activation energy. While apparent activation energy is calculated from parallel isothermal calorimetry measurements at different temperatures, instantaneous activation energy is typically measured using a differential scanning calorimeter. Compared to the apparent activation energy, instantaneous activation energy has minimal effects on the microstructural changes due to the variation in temperature. In this work, the evolution of activation energy was determined by traditional methods and was compared with the instantaneous activation energy. It was found that while the activation energy changed with the progress of reaction over traditional methods, the instantaneous activation energy did not show any changes / or remained the same. The instantaneous activation energy was also found to be higher compared to the apparent activation energy determined with traditional methods.
The autogenous shrinkage increased with increasing the concentration of sodium sulfate solutions. Increasing curing temperature decreased the autogenous shrinkage of AASF systems. The S ions provided by the activating solutions have limited influence on the reaction process.
The purpose of this study is to investigate the autogenous shrinkage of alkali-activated slag/fly ash (AASF) mortar blends. A series of tests was performed to determine the effect of type and dosage of activators on autogenous shrinkage deformation. Heat progression in AASF systems was characterized by means of isothermal calorimetry. The reaction products of alkali-activated slag/fly ash (Class F) blends was characterized using X-ray diffraction (XRD). From those results, two main phases (C-A-S-H and N-A-S-H gels) are detected in slag/fly ash blended systems and with increase in fly ash content, the amount of the C-A-S-H gel decreases and the amount of N-A-S-H gel increases. Test results show that the slag/fly ash mass ratio, type and dosage of activator are the significant factors influencing the autogenous shrinkage and rate of reaction in AASF system. With increasing fly ash content and decrease in activator-to-cementitious materials ratio, the autogenous shrinkage (up to 7 days) of the AASF system decreases.
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