# Overview of the Development of 3D-Printing Concrete: A Review
## 4 Key Preparation Technology
### Mineral Admixture  
The active ingredients of mineral admixtures can greatly increase the strength of 3D printed concrete components, the density of the structure, and improve the durability of the material and the service life of the structure.  
The incorporation of fly ash can effectively improve the working performance, mechanical properties, and durability of printed concrete. Fly ash is the main admixture for preparing high-performance 3D printed concrete. However, the incorporation of a large  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Cementitious Material Name** & **Effect** & **Reference** \\ \hline \multirow{2}{*}{Portland cement} & The most commonly used condensing material, with high early strength & [33; 51] \\ \hline \multirow{2}{*}{Sulphoaluminate cement} & High early strength, good quick-setting effect, high compressive and flexural strength & [53; 54] \\ \hline \multirow{2}{*}{Resin} & Significantly improve component bearing capacity & [55; 56] \\ \hline \multirow{2}{*}{Geopolymer} & It can repair cracks with good repair effect and improve the strength of components & [53; 57; 58; 59; 60; 61] \\ \hline \hline \end{tabular}
\end{table}
Table 2: The effect of different cementitious materials on components.  
Figure 12: (**a**) Effect of fineness modulus on Fluidity. (**b**) The Mechanical Properties of Mortar. Reprinted with permission from ref. [63]. Copyright 2017 IPO.  
amount of fly ash causes the slow development of the early strength of the printed concrete, and the obvious reduction of carbonization resistance and frost resistance. Therefore, fly ash is generally mixed with other admixtures. [PERSON] et al. [64] mixed fly ash and GH ore powder to prepare high-strength and high-performance concrete. [PERSON] et al. [65] observed that the addition of fly ash improved the fluidity of cement-based materials, but reduced the shape stability of the material. The cement-based material obtained by mixing sintered clay and fly ash has good fluidity and shape stability.  
Silica fume has high activity. As an admixture for concrete, it not only increases the initial strength of the printed concrete, but also improves the later flexural strength of the printed concrete. The silica fume particles are very small, which can be used to fill the pores around the aggregate, improving the bonding performance of the concrete, and enhancing the transition zone between the cement slurry and the aggregate interface [66]. [PERSON] et al. [67] added silica fume, which accounts for 10% of the cementitious material, to improve the adhesion and extrusion properties of the printing material. [PERSON] et al. [68] found that the addition of silica fume can optimize the particle size distribution of the printing material. After adding silicon powder, the actual and ideal particle size distribution can be more closely matched, as shown in Figure 13, increasing its yield stress, and improving the buildability of the printing material. However, it will reduce the fluidity of the material. [PERSON] et al. [69] added 2% of the cement mass of nano-clay and silica fume into concrete to prepare a concrete material with high thixotropy.