The sensitivity of transmitted ultrasound signals to accelerator type (alkaline aluminate or alkali-free) and dosage were evaluated. A stepwise increase of the accelerator dosage resulted in increasing values for the ultrasound pulse velocity at early ages. In the accelerated mortar no dormant period could be noticed before the pulse velocity started to increase rapidly, indicating a quick change in solid phase connectivity. The alkaline accelerator had a larger effect than the alkali-free accelerator, especially at ages below 90 minutes. The increase of ultrasound energy could be related to the setting phenomenon and the maximum energy was reached when the end of workability was approached. Only the alkaline accelerator caused a significant reduction in compressive strength and this for all the dosages tested. Furthermore, the properties in the fresh and hardening state of concrete with 0%, 10%, 40%, or 60% replacement of cement by fly ash were determined. The ultrasonic pulse velocity changed in all mixes from 200-500 m/s to 4000-5000 m/s, but at a different rate. An increase in fly ash content resulted in a longer dormant period, decreasing values for the pulse velocity at early ages and an increase of the time necessary to asymptotically reach the maximum velocity (from 20 h for the reference to more than 47 h for the 60% fly ash mix). The final setting time, defined by an ultrasound velocity of 1500 m/s also increased significantly with increasing fly ash content. This point seemed to correspond to the point of inflection of the ultrasound velocity curve.
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