# Overview of the Development of 3D-Printing Concrete: A Review
## 4 Key Preparation Technology
### Cementitious Materials  
The types of 3D-printing concrete cementing materials are wide, mainly including Portland cement, sulphoaluminate cement, resin, geopolymer, and so on. Cementitious materials have a certain adjustment effect on the setting time, strength, bonding performance, and stability of 3D printed concrete structures. Portland cement is the most common coagulation material in the construction industry, but studies have found that ordinary Portland cement cannot bend moments, and it has disadvantageous shear forces, bonding ability, and setting time between layers, which makes it difficult to meet the needs of 3D-printing [33; 51]. It is necessary to improve the fineness and mineral composition of cement. [PERSON] et al. [52] modified the properties of Portland cement by adding polymer and other emulsions to reduce shrinkage and improve adhesion. It is also possible to consider mixing ordinary Portland cement and sulfoaluminate cement into 3D printed concrete to adjust the early hydration speed and early strength of the concrete. Sulphoaluminate cement has the characteristics of early strength, and the initial setting time is about 6 min. Its early strength is relatively high, applying it to the proportion of printed concrete can get a significant quick setting effect [53]. [PERSON] et al. [54] found that magnesium oxysulfide cement does not harden in water, and has the advantages of light weight, fast setting speed, early strength, and strong adhesion. The compressive strength and flexural strength of magnesium phosphate cement are higher than other types of cement. The setting and hardening time is 1-10 min, which can meet the requirements of the bearing capacity of 3D printed concrete structures. Resin-based cementitious materials can greatly improve the bearing capacity of components. [PERSON] et al. [55] used epoxy resin to repair concrete cracks. Due to the existence of cracks, the compressive strength of the cube decreased by 40.9%, and only 8% after repair. [PERSON] et al. [56] injected epoxy resin into the flexural-shear cracks to strengthen the cracked reinforced concrete members, which increased the bearing capacity by 49%. Geopolymer cementitious materials can repair cracks, improving the mechanical properties of components. The strength of components can also be improved by increasing the ratio of geopolymers. [PERSON] et al. [57] used polymer-modified cement mortar to repair  
Figure 10: Mechanical properties at elevated temperature. (**a**) Temperature versus compressive strength. (**b**) Temperature versus flexural strength. Reprinted with permission from ref. [49]. Copyright 2018 Elsevier. the reinforced concrete column surface layer, and the axial stiffness of the repaired column was only reduced by 13%. [PERSON] et al. [58] used geopolymer modified cement mortar to repair the tension zone of the beam, and the load value of the first crack increased by 30%. [PERSON] et al. [59] added 7.5% styrene-butadiene rubber polymer to cement-based materials, forming polymer membranes to better combine the cement hydration products which effectively reduced the adverse effects caused by the interlayer tensile strength of the printed specimens. [PERSON] et al. [45] mixed sand aggregate and carbon-sulfur polymer in a ratio of 3:2 to prepare carbon-sulfur polymer modified cement mortar which can enhance the inter-layer tensile properties of the cement mortar. The tensile strength is 100% higher than that of non-adhesive bonded specimens. [PERSON] et al. [60] mixed cement and styrene-butadiene emulsion in a ratio of 3:2 to prepare a styrene-butadiene emulsion modified cement paste to enhance the interlayer tensile properties of the concrete. The stretched strength of styrene-butadiene emulsion bonded specimens 7 d and 28 d interlayer increased by 144% and 96% respectively. [PERSON] et al.