Recycling coral waste into eco-friendly UHPC: Mechanical strength, microstructure, and environmental benefits.

On islands far away from the mainland, the raw materials for concrete production are often more difficult to obtain. Converting the coral waste generated during the island construction process into a marine ultra-high performance concrete (UHPC) mixture is an eco-friendly strategy. Coral powder (CP) is used to partially replace cement and silica fume (SF), and its mechanical strength, microstructure and environmental benefits are evaluated. Results show that using a small amount of CP (5%) to replace cement can improve the mechanical properties of UHPC, but the strength of UHPC decreases with the further increase of CP content. From the perspective of nanoindentation test, an appropriate amount of CP refines the pore structure of the UHPC matrix and increases the content of C-S-H, especially the proportion of high-density C-S-H. When 15% of SF is replaced by CP (SF15), the strength of UHPC decreases due to the decrease of C-S-H phase and the deterioration of microstructure. In terms of the width of the interface transition zone, the width of the C5 sample (CP replace 5% cement) is decreased by 16.7% compared with the control group, while the width of the SF15 group is increased by 38.9%. Compared with conventional UHPC, CP-based UHPC has lower carbon emission and non-renewable energy consumption, which effectively utilizes waste and promotes sustainability.

Microstructure of ultra high performance concrete containing lithium slag.

Lithium slag (LS) is discharged as a byproduct in the process of the lithium carbonate, and it is very urgent to explore an efficient way to recycle LS in order to protect the environments and save resources. Many available supplementary cementitious materials for partial replacement of cement and/or silica fume (SF) can be used to prepare ultra high performance concrete (UHPC). The effect of LS to replace SF partially by weight used as a supplementary cementitious material (0%, 5%, 10% and 15% of binder) on the compressive strengths and microstructure evolution of UHPC has experimentally been studied by multi-techniques including mercury intrusion porosimetry, scanning electron microscope and nanoindentation technique. The results show that the use of LS degrades the microstructure of UHPC at early ages, and however, the use of LS with the appropriate content improves microstructure of UHPC at later ages. The hydration products of UHPC are mainly dominated by ultra-high density calcium-silicate-hydrate (UHD C-S-H) and interfacial transition zone (ITZ) in UHPC has similar compact microstructure with the matrix. The use of LS improves the hydration degree of UHPC and increases the elastic modulus of ITZ in UHPC. LS is a promising substitute for SF for preparation UHPC.