# Study on Preparation and Rheological Properties of 3D Printed Pre-Foaming Concrete
## 1 Introduction  
Additive manufacturing in construction is one of the latest development trends in the concrete prefabrication industry [1; 2]. This technology does not require formwork construction, greatly reduces labor costs, shortens construction time and brings extensive benefits to the construction industry [3; 4]. At the same time, 3D-printed concrete based on extrusion molding is developing rapidly. Academic researchers and industry practitioners have developed printing concrete with good thixotropy; that is, materials maintain high yield stress in a static state and low viscosity after shear [5; 6]. However, due to the high self weight of traditional 3D printing materials, there are huge costs in raw materials, transportation and labor. In addition, the dense wall with high thermal conductivity accelerates the indoor and outdoor heat loss, which is not conducive to the sustainable development of the environment. Therefore, it is urgent to develop an environment-friendly 3D printing material with light weight, porous and excellent thermal insulation effects [7]. One of the most significant sources of greenhouse gas emissions of carbon dioxide is the construction industry, which emits more than 40% of global emissions over its entire life cycle. With the development of the global concept of \"carbon neutral\", many scholars began to devote themselves to the development of low-density and energy-saving building materials [8]. Based on a surfactant and prefabricated foaming method, pre-foaming concrete has been recognized by practitioners in the industry [9; 10]. Foamed concrete is a kind of lightweight porous material, which is formed by mixing a foaming agent with high-speed mechanical mixing and then pouring and curing foam into the mixedmortar [11; 12]. Due to the honeycomb porous structure, foam concrete has low-density, high fluidity, self-compacting and perfect thermal performance. Among them, the density can range from 300 to 1800 kg/m\({}^{3}\), and the thermal conductivity can range from 0.131 to 0.52 W/(m\(\cdot\)K) which is only 10-50% of that of ordinary dense concrete [13; 14; 15]. It can be easily pumped to the construction position, which brings significant convenience to the construction industry. At present, the digital construction wall adopts the method of multi-layer printing and superposition to strengthen the bearing and improve the effect of thermal insulation and sound insulation. This means that the strength requirements for inner materials are not so high, which provides the possibility for the development and application of lightweight foams. If energy-saving materials with low thermal conductivity are used in inner thermal insulation walls, the thermal insulation capacity of the building can be greatly improved [16]. Extrusion-based 3D printing molding technology consists of four main stages, i.e., mixing, pumping, extrusion and construction [17]. Due to this special processing, the rheological properties of the printing ink are required differently in different stages. Low yield stress and apparent viscosity are required in the pumping and extrusion stages [18; 19]. However, when the paste is extruded, high yield strength is required to resist the load from the subsequent build-up layer. In order to meet this challenge, many researchers have made a series of studies. [PERSON] et al. prepared 3D printable foam fly ash-based geopolymer matrices with surfactant [20]. The porosity of extruded geopolymeric foams was in the range of 55 to 75% for densities from 0.6 to 1.0 kg/m\({}^{3}\). Compared with the cast concrete in the same density range, these properties are very similar. However, the thermal conductivity of the foaming matrix is still very high, ranging from 0.15 W/(m\(\cdot\)K) to 0.25 W/(m\(\cdot\)K), which is due to the small pore in the foam and the large spacing between the printing paths. [PERSON] et al.