1.1 ?raw materials

The cement adopts P·Ⅱ 52.5 cement (PC) produced by Nanjing Onotian Cement Plant, hydroxypropyl methylcellulose, white powder, water content is 2.1%, pH value is 6.5 (1% aqueous solution, 25 ℃), viscosity is 95 Pa s (2% aqueous solution, 20 ℃), the dosage (calculated by cement mass) is 0%, 0.05%, 0.10%, 0.20%, 0.30%, respectively; the fine aggregate is quartz sand with a particle size of 0.212~0.425 mm .

1.2 ?experiment method

1.2.1 ??Material preparation

Using a mortar mixer of model JJ-5, firstly mix HPMC, cement and sand evenly, then add water and mix for 3 min (2 min at low speed and 1 min at high speed), and the performance test is carried out immediately after mixing.

1.2.2 ??Printable Performance Evaluation

The printability of mortar is mainly characterized by extrudability and stackability.

Good extrudability is the basis for realizing 3D printing, and the mortar is required to be smooth and not block the pipe during the extrusion process. Delivery requirements. Referring to GB/T 2419-2005 “Determination of Fluidity of Cement Mortar”, the fluidity of the mortar that was left standing for 0, 20, 40, and 60 min was tested by jumping table test.

Good stackability is the key to realizing 3D printing. It is required that the printed layer does not collapse or deform significantly under its own weight and the pressure of the upper layer. The shape retention rate and penetration resistance under its own weight can be used to comprehensively characterize the stackability of 3D printing mortar .

The shape retention rate under its own weight reflects the degree of deformation of the material under its own weight, which can be used to evaluate the stackability of 3D printing materials. The higher the shape retention rate, the smaller the deformation of the mortar under its own weight, which is more conducive to printing. Reference , put the mortar into a cylindrical mold with a diameter and a height of 100 mm, ram and vibrate 10 times, scrape the upper surface, and then lift the mold to test the retention height of the mortar, and the percentage of it with the initial height is the shape retention rate . The above method was used to test the shape retention rate of the mortar after standing for 0, 20, 40, and 60 min respectively.

The stackability of 3D printing mortar is directly related to the setting and hardening process of the material itself, so the penetration resistance method is used to obtain the stiffness development or structural construction behavior of cement-based materials during the setting process, so as to indirectly characterize the stackability. Refer to JGJ 70 – 2009 “Test method for basic performance of building mortar” to test the penetration resistance of mortar.

In addition, a gantry frame printer was used to extrude and print the outline of a single-layer cube with a side length of 200 mm, and the basic printing parameters such as the number of printing layers, the width of the top edge, and the width of the bottom edge were tested. The printing layer thickness is 8 mm, and the printer movement speed is 1 500 mm/min.

1.2.3 ??Rheological property testing

The rheological parameter is an important evaluation parameter to characterize the deformation and workability of the slurry, which can be used to predict the flow behavior of the 3D printing cement slurry. The apparent viscosity reflects the internal friction between the particles in the slurry and can evaluate the resistance of the slurry to deformation flow. The ability of HPMC to reflect the effect of HPMC on the extrudability of 3D printing mortar. Refer to the mixing ratio in Table 2 to prepare cement paste P-H0, P-H0.10, P-H0.20, P-H0.30, Use a Brookfield DVNEXT viscometer with an adapter to test its rheological properties. The test environment temperature is (20±2) °C. The pure slurry is pre-sheared for 10 s at 60.0 s?1 to make the slurry evenly distributed, and then paused for 10 s , and then the shear rate increases from 0.1 s?1 to 60.0 s?1 and then decreases to 0.1 s?1.

The Bingham model shown in Eq. (1) is used to linearly fit the shear stress-shear rate curve in the stable stage (shear rate is 10.0~50.0 s?1).

τ=τ0+μγ (1).

where τ is the shear stress; τ0 is the yield stress; μ is the plastic viscosity; γ is the shear rate.

When the cement-based material is in a static state, the plastic viscosity μ represents the degree of difficulty of the colloidal system failure, and the yield stress τ0 refers to the minimum stress required for the slurry to flow. The material only flows when the shear stress higher than τ0 occurs, so It can be used to reflect the influence of HPMC on the stackability of 3D printing mortar.

1.2.4 ??Mechanical property test

Referring to GB/T 17671-1999 “Testing method for the strength of cement mortar”, the mortar specimens with different HPMC contents were prepared according to the mixing ratio in Table 2, and their 28-day compressive and flexural strengths were tested.

There is no relevant standard for the test method of the bonding strength between layers of 3D printing mortar. In this study, the splitting method was used for the test. The 3D printing mortar specimen was cured for 28 d, and then cut into 3 parts, named A, B, C respectively. , as shown in Figure 2(a). The CMT-4204 universal testing machine (range 20 kN, accuracy class 1, loading rate 0.08 mm/min) was used to load the three-part interlayer junction to split failure stop, as shown in Figure 2(b).

The interlaminar bond strength Pb of the specimen is calculated according to the following formula:

Pb = 2FπA = 0.637 FA （2）

where F is the failure load of the specimen; A is the area of the split surface of the specimen.

1.2.5 ??Micromorphology

The microscopic morphology of the specimens at 3 d was observed with a Quanta 200 scanning electron microscope (SEM) from FEI Company, USA.