Reinforcing effect of CNT on the microstructure and creep properties of high-strength lightweight concrete

Demand for precast high-strength lightweight concrete (HSLC) has recently increased for cost benefits. This study aims to explore the reinforcing effect of carbon nanotube (CNT) on the microstructure and creep properties of HSLC. A total of four types of HSLC mixture were applied by securing two target properties (a compressive strength of 120 MPa and a density of 2000 kg/m3; a compressive strength of 80 MPa and a density of 1600 kg/m3) through steam curing with high temperature (90℃), considering a combination of lightweight materials and CNT utilization as variables. Lightweight aggregates operated internal curing (IC) more effectively than hollow microsphere, improving the pore structure of HSLC and leading to the formation of hydration products, especially calcium silicate hydrate (C-S-H). The efficiency of lightweight aggregate on IC was proven through nanoindentation testing, identifying that no unhydrated particles were observed in the specimen incorporating only lightweight aggregate as lightweight material, while unhydrated particles made up 8–19% of the total phase volume with hollow microsphere incorporation. The utilization of CNT enhanced the microstructure of the cement matrix by filling pores and inducing formation of ultra-high-density (UHD) C-S-H, improving the creep resistance of HSLC. Furthermore, CNT bridging suppressed redistribution of C-S-H caused by C-S-H slip, resulting in approximately 49% of the total specific creep compared to conventional high-strength concrete (HSC). The fédération internationale du béton (fib) model is considered to derive the creep properties of HSLC, since it indirectly applies the impact of lightweight material utilizing its oven-dried density.