Enhancing performance and sustainability of ultra-high-performance concrete through solid calcium carbonate precipitation

Ultra-high-performance concrete (UHPC) exhibits high compressive strength and good durability. However, owing to the dense microstructure of UHPC, carbonation curing cannot be performed to capture and sequester carbon dioxide (CO₂). In this study, CO₂ was added to UHPC indirectly. Gaseous CO₂ was first converted into solid calcium carbonate (CaCO₃) using calcium hydroxide, and the converted CaCO₃ was then added to UHPC at 2, 4, and 6 wt% based on the cementitious material. The performance and sustainability of UHPC with indirect CO₂ addition were investigated through macroscopic and microscopic experiments. The experimental results showed that the method used did not negatively affect the performance of UHPC. Compared with the control group, the early strength, ultrasonic velocity, and resistivity of UHPC containing solid CO₂ improved to varying degrees. Microscopic experiments, such as heat of hydration and thermogravimetric analysis (TGA), demonstrated that adding captured CO₂ accelerated the hydration rate of the paste. Finally, the CO₂ emissions were normalized according to the compressive strength and resistivity at 28 days. The results indicated that the CO₂ emissions per unit compressive strength and unit resistivity of UHPC with CO₂ were lower than those of the control group.

Experimental study and theoretical prediction of mechanical properties of ultra-high-performance concrete incorporated with nanorice husk ash burning at different temperature treatments

This research aimed to investigate the effect of nanorice husk ash (NRHA) prepared using different thermal treatment methods on ultra-high-performance concrete (UHPC) behaviour. NRHA was prepared by two methods: (1) burning for 3 h at 300, 500, 700 and 900 °C and (2) burning for different durations (9, 7, 5 and 3 h) at 300, 500, 700 and 900 °C. NRHA was added to UHPC to make 25 mixtures with three dosages (1%, 3% and 5%). Density, compressive strength, tensile strength, flexure strength and ultrasonic pulse velocity tests were performed at the experimental level. Moreover, full microstructure analysis, including X-ray diffractometry, Brunauer-Emmett-Teller surface area analysis, thermogravimetric analysis, scanning electron microscopy and energy-dispersive X-ray spectroscopy, was performed. The best performances in in the first method (constant duration, different temperatures) were obtained by 1% NRHA burned at 900 °C with 12.5% compressive strength and 1% NRHA burned at 700 °C with increased ratio (10%). Moreover, the best performance in the second method (different burning durations and temperatures) was obtained by 3% NRHA with a ratio of 22.5% at 700 °C for 5 h. Burning rice husk ash improved the compressive strength. It also remarkably improved the splitting tensile strength and flexure strength by 32% and 47%, respectively, at 3% NRHA treated at 700 °C for 3 h. The microstructural analysis showed the efficient role of NRHA in the compactness of concrete sections. It improved the formation of new calcium silicate hydrate gel; decreased the cracks, voids, CaCO3 and Ca(OH)2; and increased the Ca/Si composition. The obtained experimental results were used to build an artificial neural network (ANN) to predict UHPC properties. The ANN model was used as a validation tool to determine the correlation between results. Results showed a remarkable improvement in the mechanical properties of UHPC incorporating NRHA for all mixtures. The ANN model indicated a reliable correlation between input and output variables. The R2 values for the training, validation and testing steps were all 0.99.