## 1 Introduction  
Estimates suggest that nearly 30 million housing units will be needed annually in the future to meet the increased demand due to population growth [1]. However, the traditional construction industry, mainly based on reinforced concrete (RC), faces significant challenges. Issues such as high accident rates, low efficiency, heavy reliance on labor, and high construction costs are prevalent. Template costs alone account for 60% of total costs, while the preparation time for templates consumes 50-70% of the total construction duration [2; 3; 4]. Therefore, it is necessary to seek new construction technologies that are safer, more efficient, and require fewer templates [5]. Concrete 3D printing technology is an emerging form of architectural automation and digital manufacturing. Compared to traditional construction methods, concrete 3D printing technology significantly reduces labor costs, saves construction time, mitigates construction accidents, and allows for the construction of complex structures [6; 7; 8; 9; 10; 11]. The geometric complexity offered by concrete 3D printing technology allows for the design of freely shaped architectural elements, paving the way for low-cost and highly customized designs [12; 13]. Lightweight concrete (LWC), as a historically proven functional building material [14], makes significant contributions to reducing component weight, saving costs, enhancing fire resistance, and reducing transportation and placement costs [15; 16; 17]. Buildings constructed with lightweight concrete also exhibit excellent thermal insulation and durability [18; 19]. Therefore, the application of lightweight concrete in the field of 3D printing for construction holds great promise. Particularly, the use of 3D-printed lightweight concrete in construction enables non-load-bearing modular lightweight units for light to medium structural loads, reducing the overall weight and rigging risks while improving engineering efficiency [20; 21]. Furthermore, 3D-printed lightweight concrete exhibits low thermal conductivity, as well as excellent thermal insulation and soundproofing performance, offering broad application prospects [22]. Research on 3D-printed lightweight concrete has primarily focused on foamed concrete and lightweight aggregate concrete. Foam concrete (FC) refers to the introduction of a large volume of foam into a fresh cement matrix [23], primarily achieved through mechanical foaming and chemical foaming methods. Studies have shown that 3D-printed foamed concrete (3 DFC) enhances mechanical performance in its fresh state compared to traditional foamed concrete, with minimal deformation observed. Additionally, when the mixing speed was increased from 1200 RPM to 3000 RPM, the compressive strength increased by 70% [24]. Recent studies on 3D-printed lightweight concrete have also achieved compression strengths exceeding 10 MPa within a density range of 1100-1580 kg/m\({}^{3}\)[25]. The main challenge of applying foam concrete (FC) in concrete 3D printing lies in its lower fresh-state yield stress, which affects its constructability. The fragility of foam during mixing and pumping exacerbates this issue, necessitating the use of a cementitious base with low yield stress to ensure foam stability [26]. To use foam concrete in concrete 3D printing, efforts have focused on improving the rheological properties of the mixture through additives. [PERSON] et al. [27] enhanced the mixture rheology by introducing nanoparticles and calcium sulfoaluminate cement, enabling the continuous printing of 15 layers without collapse. [PERSON] et al. [28] improved foam stability and adjusted rheological properties by adding hydroxypropyl methylcellulose and silica fume, achieving compressive strengths exceeding 20 MPa under conditions of 1815 kg/m\({}^{3}\) density. However, increased additives prolong setting times, and different types of foaming agents affect the fresh properties of the mixture. Therefore, low yield stress remains a critical factor limiting the application of foam concrete in 3D printing [26].