## 1 Introduction
[16] combined silica fume into LWCC with FAC and succeeded in fabricating LWCC with compressive strength up to 69.4 MPa and flexural strength to 7.3 MPa. [PERSON] et al. [1] carried out experiments on LWCC containing both FAC and sintered fly ash, proposing that a composite with 50% FAC and 75% sintered fly ash met the requirements of M25 concrete, and a composite with 75% FAC and 50% sintered fly ash can be used as M20 grade concrete. Other attempts have also been made to enhance the brittle nature of lightweight cement-based materials, especially with fibre reinforcement to improve flexural strength and ductility. In this regard, [PERSON] et al. [17] introduced polyvinyl alcohol fibres into the fabrication of LWCC with FAC, iron ore tailings, and fly ash, concluding that FAC was the most effective material to reduce the density of cement composites and improve tensile ductility. [PERSON] et al. [18] compared the reinforcing effects of polyethene and steel fibres on the enhancement of flexural properties of LWCC containing FAC and silica fume. They addressed the fact that the development of flexural strength relied on the category of fibres as well as their bond with cement matrix, and steel fibres exhibited more significant effects than polyethene fibres. Due to the progress achieved in the field of nanotechnology, the introduction of nanomaterials with particle size finer than 100 nm in producing cement-based materials is identified as sustainable and effective in ameliorating their performance [19]. There is a wide range of nanomaterials used in manufacturing cement-based materials, and among them, nano silica (NS), as well as carbon nanotubes (CNTs), are the two most welcomed in research regarding nano-modification and nano-engineering of cement materials, due to their unique characteristics. The presence of CNTs can act as fibres and effectively bridge the micro-sized and nano-sized gaps in the cementitious matrix, preventing the propagation of micro cracks and improving pore structure [20; 21]. The filling effects of CNTs can significantly reduce the porosity and total volume of the cement matrix by filling up micro gaps and pores [22]. Further, CNTs can perform as nucleating sites for hydration reactions, which efficiently facilitate the formation of hydration products [23]. Similar to CNTs, the nucleation and filler effects of NS, due to its small particle size and great surface area, can contribute to the formation of a denser microstructure of cement materials [24]. In addition, NS can participate in pozzolanic reactions with calcium hydroxide (CH) on the silica surface, producing more calcium silicate hydrate (CSH) gel that fills up the tiny pores in the cement mixture [25]. Most of the studies on NS and CNTs mainly concentrate on their single effects on cement-based materials [21; 22; 26; 27; 28; 29]. Although there have been some attempts made to disclose the synergic influence of NS or CNTs combined with other modifiers such as glass powder, carbon fibres, and fly ash [30; 31; 32; 33; 34; 35], very few efforts have been made regarding the hybrid effects of CNTs combined with NS on cementitious materials. Only [PERSON] et al. [36] implemented compressive and calorimetry experiments on cement pastes reinforced by CNT/NS core-shell additives, showing that the incorporation of an NS shell improved the binding capability between cement matrices and CNTs. [PERSON] et al. [37] studied the synergic influence of CNTs and NS on the compressive, flexural strength and microstructure of cement pastes. They asserted that the addition of high percentages of NS can account for further improvement of mechanical properties of CNTs-cement composites. [PERSON] et al. [38] researched compressive behavior, water absorption, and microstructure of LWC containing lightweight expanded clay aggregates under the hybrid effect of CNTs and NS. They found that the incorporation of 1% NS and 2% CNTs led to the peak value of compressive strength after 28-day curing. [PERSON] et al.