# Study on Preparation and Rheological Properties of 3D Printed Pre-Foaming Concrete
## 4 Discussion  
In foam concrete 3D printing, the rheological properties of fresh foam mortar are crucial for printability and buildability. This is consistent with previous research reports [7; 42]. From the results shown in Section 3, when 0.1% HPMC is added to the foam mortar, the yield stress and apparent viscosity of 3 DFCs change significantly. This is because the addition of HPMC agglomerates the cement particles together and increases the proportion of large flocculates, thus increasing the yield stress and apparent viscosity of foam mortar [43]. Accordingly, the fluidity and mini-slump of foam mortar also change obviously. Through the actual buildability experiments, it is proved that 0.1% HPMC does make 3 DFC have good printability and construction height. This also confirms that fluidity test and mini-slump test can be used as a macroscopic test method to verify the printability of mortar before actual printing [44].  
The content of surfactant directly determines the bubble content in fresh foam mortar and the pore structure of hardened mortar, which is very important to the rheological and hardening properties of 3 DFCs. At low surfactant concentration, 3 DFCs showed higher static yield stress and apparent viscosity (especially in low shear state). Accordingly, it shows higher mini-slump and excellent constructability. With the increase of the dosage of surfactant, the rheological properties of fresh foam mortar change obviously. The static yield stress and apparent viscosity decreased obviously, and macroscopically, it also showed relatively low mini-slump and high fluidity. This change is closely related to the bubble content in foam mortar. In foam mortar, the hydrophilicity and surface charge of cement particles make them adsorb around the bubbles with opposite charge. When the bubble content in 3 DFCs is low, the distance between the solid particles is also close. At this time, the particles have a strong van der Waals force and a weak intermolecular repulsion force, which promotes the adsorption between particles and the formation of flocculation structure [7]. With the increase of bubble volume fraction in foam mortar, the connection and penetration of bubbles increase the bubble size, so the interaction force between particles is significantly reduced; thus, the yield stress of foam mortar is reduced [45]. In addition, it can be seen from the buildability experiments that the number of continuous printing layers of 3 DFCs decreases obviously with the increase of bubble volume fraction in foam mortar, which may be due to the decrease of yield stress leading to the decrease of load-bearing capacity of 3 DFCs [46].  
Obviously, as shown in Figure 12, when the amount of surfactant increases from 0 to 2%, the volume density of 3 DFCs decreases significantly, accompanied by a sharp decrease in compressive strength. This is related to the development of pore structure in hardened foam mortar [47]. It can be observed from the MIP experiment that the cumulative number of pores in hardened foam mortar increases obviously with the increase of the dosage of surfactant in 3 DFCs. It is worth noting that the incorporation of foam significantly increased the number of 10-100 \(\upmu\)m macropores in 3 DFCs. In addition, the large pores of 100-500 \(\upmu\)m in the hardening foam mortar were also observed with SEM experiments. It can be observed that compared with the sample 3 DFC1, the pore size and the number of large pores in the sample 3 DFC2 are significantly larger. In general, with the increase of surfactant dosage, the total pore number and pore size in hardened foam mortar also increase. The development of pore structure not only causes the volume density change of foam mortar but also contributes to the decrease of thermal conductivity of 3 DFCs, which is very beneficial to the practical application of 3D printing lightweight thermal insulation mortar [48; 49].