Ultrasound monitoring of setting and hardening process of ultra-high performance cementitious materials

A novel specially designed ultrasonic monitoring apparatus (UMA) for in-situ continuous studying of the early age hydration process of cementitious materials was developed. Utilizing UMA, the early microstructure formation process was systematically investigated for ultra-high performance cementitious materials made with various mineral admixtures (fly ash, slag and silica fume), fine and coarse aggregates, different volume fraction of steel fiber (0, 1%, 2% and 3%). The influence of curing temperature (20, 40, 60, 80 and 90 °C) was also studied. The results show that four stages can be clearly identified during the microstructure formation process of ultra-high performance cementitious materials with sand, aggregate or steel fiber: pre-dormant stage, dormant stage, acceleration stage, and deceleration stage, while only the last three stages occur for the one without fillers and reinforcement. Curing temperature, mineral admixtures, fillers and reinforcement addition have great impact on microstructure formation process. Hydration reaction rate is obviously promoted with an increase in curing temperature. Silica fume addition also accelerates the microstructure formation, while the reverse phenomenon is observed when fly ash and slag are incorporated. Steel fiber addition retards the microstructure formation, especially in high volume fraction of fiber.