Study On Composition,Structure And Properties Of 3D Printed Magnesium Potassium Phosphate Cement-Based Materials

3D printing technology in construction industry can significantly improve the production efficiency and bring subversive changes for flexible design,energy conservation and environmentally friendly.Significant progress has been made in 3D printing concrete（3DPC）using traditional Portland cement and other cement-based materials.However,in view of the increasing requirements of cement-based products for styling design and aesthetic effect,the increasing demand for special-shaped functions,art landscape and color decorative structures,there is an urgent need for 3D printed cement-based materials with high performance and controllable rheology.Magnesium phosphate cement is a new type of 3D printable material developed in recent years.It has the advantages of high early strength,fast setting and hardening and high bonding strength,which can meet the needs of 3D printed art landscape materials.However,its main shortcoming is that the setting and hardening is too fast.In order to solve this problem,this paper systematically studied the setting time,rheological propery,printability and mechanical property of 3D printed magnesium potassium phosphate cement（MKPC）by regulating the mix proportion and type of phosphate and retarder.Three kinds of3D printed MKPC based on MgO-KH₂PO₄-K₂HPO₄,fast hardening and early strength MgO-SiO₂-KH₂PO₄and high strength and high water resistance MgO-SiO₂-K₂HPO₄were established.The high thixotropic property of 3D printed MKPC were characterized by rheological tests.The role of viscosity reducer and plasticizer of SF in MgO-SiO₂-KH₂PO₄,and the active reaction mechanism of SF and slag in MgO-SiO₂-K₂HPO₄were investigated.The main research results are as follows:（1）The composition of 3D printed magnesium potassium phosphate cement（MKPC）based on MgO-KH₂PO₄-K₂HPO₄using traditional retarder borax was established by regulating rheological property.The initial setting time of paste was higher than 25 min,3days and 28 days compressive strength of the hardened paste increased to 34.8 MPa and 44.8MPa when M/P mass ratio,fly ash content and borax content were 3.0,20%and 0.2,respectively.（2）The composition of fast hardened and high early strength 3D printed MKPC based on MgO-SiO₂-KH₂PO₄was established by regulating rheological property.The initial setting time of paste was higher than 25 min,1 day and 28 days compressive strength of the hardened paste increased to 48.47 MPa and 65.7 MPa when M/P mass ratio,water-cement ratio and compound retarder dosage were 3.0,0.14 and 0.06,respectively.（3）3D printing requires high thixotropic property of paste,which represents better printability.The thixotropic area of ordinary Portland cement（OPC）,white ordinary Portland cement（WPC）,calcium sulfoaluminate cement（CSAC）and MKPC are 0.976×10⁴Pa/s,0.289×10⁴Pa/s and 0.369×10⁴Pa/s and 9.366×10⁴Pa/s,respectively,and the structural parameters were 0.105,0.025,0.135 and 0.267,respectively.The apparent viscosity were17710.9 Pa·s,409.3 Pa·s,963.7 Pa·s and 59299.3 Pa·s after the same rest time of 60 s,respectively.The largest values of thixotropic area,structural parameter and apparent viscosity showed MKPC based on MgO-KH₂PO₄had the higher thixotropy for 3D printing.（4）The printability of 3D printed MKPC based on MgO-KH₂PO₄was significantly improved by silica fume（SF）,which can act as an effective viscosity-reducer and plasticizer.Different from the Ref.showed weaker layer stability and collapsed at 7 layers,the printed hollow cuboid of MKPC with 15%SF was 310 mm×250 mm×84 mm.Compared with the design size of 300 mm×250 mm×88 mm,there was no obvious structural deformation.The addition of SF can significantly reduce the pore volume and porosity,improving the compressive strength and water resistance of hardened paste.（5）The composition of high strength and high water resistance 3D printed MKPC based on MgO-SiO₂-K₂HPO₄was established by regulating rheological property.The initial setting time of paste was higher than 50 min,1 day,1 days and 28 days compressive strength of the hardened paste increased to 22.88 MPa,71.73 MPa and 115.95 MPa when M/P mass ratio,SF content and slag content were 3.0,20%and 10%,respectively.（6）The incorporation of SF can significantly reduce the deceleration period of hydration heat of paste（MgO-SiO₂-K₂HPO₄）,accelerating cement hydration and increasing the cumulative heat release.However,the cumulative heat release decreased when the SF content reached to 25%,indicating that excess SF is unfavorable to cement hydration and mechanical property.20%SF can significantly reduce the pore volume and porosity,improving the compressive strength and water resistance of the hardened paste.When SF content reached to 20%,the 28 days compressive strength of the hardened paste reached the maximum value of 133.89 MPa,which was 6 times higher than that of the Ref.of 19.22 MPa.With the soaking curing time increased from 1 day to 28 days,the strength retention rates were 93.4%,95.6%,97.3%and 96.0%,respectively,which remained above 93.0%.（7）The active reaction mechanism of SF and slag in 3D printed MKPC based on MgO-SiO₂-K₂HPO₄was revealed.When magnesium oxide reacted with K₂HPO₄·3H₂O,dissolution rate of magnesium oxide in a high alkali environment（p H>13）was slow and mostly existed in the form of Mg（OH）₂,which was difficult to be further hydrolyzed into Mg²⁺ions.However,a small amount of free Mg²⁺reacted with a mass of PO₄^3-and K⁺ions to participate a handful of K-struvite crystal.Thus,the mechanical property of hardened paste was poor.After adding SF and slag,SiO₂was first hydrolyzed to stable SiO₃^2-ions in alkaline environment.With the continuous consumption of OH^-ions,the dissolution of Mg（OH）₂was proceeded in a positive direction.More and more free Mg²⁺reacted with PO₄^3-and K⁺ions to participate more K-struvite crystal.At the same time,SiO₃^2-ion reacted with free Mg²⁺to participate Mg SiO₃and amorphous phase.The amount of hydration products increased and bonded with each other,resulted in the enhanced mechanical properties.20%SF significantly reduced the p H value of the cement paste and 28 days p H value decreased to 11.2 with a tendency to decrease continuously,representing continuous hydration and enhancement in strength.According to ²⁹Si NMR,Q³unit content in hardened paste decreased from 54.21%to 44.06%after adding 10%slag,and total Q¹and Q²（1Al）units increased from 8.75%to12.99%.The result indicated that Q⁰and Q¹（Slag）units participated in the formation of C-（A）-S-H gels.（8）3D printed MKPC based on MgO-SiO₂-KH₂PO₄and MgO-SiO₂-K₂HPO₄have the characteristics of rapid growth of static yield stress and high thixotropy at early stage,and the mechanism of high thixotropy was revealed.Contrary to the static yield stress of OPC increasing with hydration time,the static yield stress of this fast hardening cementitious material increased first and then decreased with hydration time,and the growth rate of A1_thixin stageⅠis significantly higher than that of A2_thixin stage II.The higher static yield stress in early stage is conducive to the buildability of the lower layer.In stageⅠ,the yield stress was dominated by a large amount of reversible gel forces（mainly van der Waals forces）formed rapidly in a short period of time,thus the paste showed more significant thixotropy in the early stage（20 min）than in the late stage.The yield stress of paste in stage II is dominated by nucleation rate,growth rate and solid volume fraction of K-struvite.As the reaction progressed,the particle spacing and interparticle force decreased,which alleviated the early flocculation colloid attraction of the paste,which was the reason of the A_thixdecreased in stage II.（9）The stress shear and frequency shear test results of PVA and PP fiber reinforced 3D printed MKPC based on MgO-SiO₂-KH₂PO₄and MgO-SiO₂-K₂HPO₄showed that with the increase of fiber content,the linear viscoelastic zone and elastic modulus of the paste increased,and the resistance to deformation increased.The 3D printed samples showed obvious mechanical anisotropy.Compared with 3DP-X and 3DP-Z directions,the printed samples showed the highest flexural strength in the 3DP-Y direction.