Preparation And Properties Of Polypyrrole/Carbon Nanotubes/Cement Composites

Cement based materials are the most widely used structural materials in the field of civil engineering and construction.With the development of science and technology,these traditional building materials are also facing new challenges.Under the harsh service environment and long-term fatigue stress,the internal cement structure is easy to be damaged and destroyed,which leads to the decline of cement structural performance and may cause accidents.In order to avoid this kind of situation,it is the most effective method to monitor the safety of the structure in real time.In this context,cement based self-sensing composite materials emerge as the times require.While ensuring the mechanical properties of cement-based materials,the self-sensing characteristics of materials are given.The changes of internal structure of cement based materials can be perceived by monitoring the changes of electrical properties,so as to achieve the effect of non-destructive testing and on-line health monitoring.In the current research,many scholars choose to add conductive fillers in the cement matrix to achieve self-sensing effect.The conductive fillers used are mainly metal materials and conductive inorganic materials,and the research on conductive polymer conductive fillers is rarely reported.As one of the most representative conductive polymer materials,polypyrrole（PPy）has been widely used in many fields due to its excellent conductivity,simple preparation process and excellent environmental stability.Carbon nanotubes（CNTs）are also a promising new material because of their unique one-dimensional nano structure,excellent mechanical and electrical properties and thermal stability.In this paper,waterborne polypyrrole（WPPy）was prepared by doping hydrophilic agent,and WPPy/CNTs composites were prepared by in-situ polymerization on the surface of CNTs.Then,WPPy and WPPy/CNTs composites were added to cement paste to prepare two kinds of cement based composites.（1）Preparation and characterization of WPPy:The intrinsic PPy is insoluble and has poor processability,and its dispersibility is poor due to its nanometer size.In this paper,five hydrophilic agents:sodium lignosulfonate,sodium dodecyl sulfonate,sodium dodecyl sulfate,sodium benzene sulfonate and sodium dodecyl benzene sulfonate were selected to modify PPy.The effects of the type and content of hydrophilic agents on the conductivity and contact angle of WPPy were studied.When the content of sodium lignosulfonate is 40wt%,the conductivity of WPPy is 15.06times that of the intrinsic PPy,and the contact angle is 69.95%lower than that of the intrinsic PPy.Thermogravimetric analysis showed that the thermal stability of the five WPPy with 40wt%hydrophilic agent was slightly lower than that of the intrinsic state.SEM analysis showed that the amount of hydrophilic agent had a great influence on the particle size of WPPy.The shape and particle size of WPPy could be controlled by adjusting the amount of hydrophilic agent.With the increase of the amount of sodium lignosulfonate from 10wt% to 40wt%and 60wt%,the average diameter of WPPy particles decreased from 200nm to 100nm and 50nm.（2）Preparation and characterization of WPPy/Cement composites:WPPy/Cement composites with different contents were prepared by compounding WPPy and cement with 40wt%sodium lignosulfonate.The effects of WPPy on the compressive strength and resistivity of WPPy/Cement composites with different moisture content were studied.When the content of WPPy is 1.00wt%,the compressive strength of WPPy/Cement composite is 51.15MPa,which is 22.18%lower than that of cement paste.When the content of cement paste is 1.25wt%,the resistivity is 402kΩ.cm,which is 95.61%lower than that of cement paste.The water content has a great influence on the resistivity of WPPy/Cement composites,and the saturated water content is about 8wt%.The influence of WPPy on the hydration process of cement composite was analyzed by TG and SEM.The morphology of hydration products of the two materials was different.The formation of Ca（OH）₂ was inhibited by WPPy in the early hydration process and the hydration process of cement was slowed down.Based on the test results,it was found that the best performance was obtained when the content of WPPy was 1.00wt%.（3）Preparation and characterization of WPPy/CNTs composites:WPPy/CNTs composites were synthesized by in-situ polymerization by adjusting the ratio of Py to CNTs.The results show that the WPPy/CNTs composite has the best properties when the content CNTs is 40wt%.The conductivity is 297.62s/cm,which is 101.92%higher than that without CNTs,and the contact angle is 28.73°which is 29.13%lower than that without CNTs.SEM showed that WPPy was coated on the surface of CNTs,and the composite structure was perfect.Thermogravimetric analysis showed that the thermal stability of wppy was better than that of CNTs,and the synthesis of WPPy/CNTs composite was confirmed by FTIR and XRD.（4）Preparation and characterization of WPPy/CNTs/Cement composites:WPPy/CNTs/Cement composites with different contents were prepared by compounding WPPy/CNTs composites with CNTs content of 40wt%.The effects of WPPy/CNTs composites on the compressive strength and resistivity of WPPy/CNTs/Cement composites were studied.When the content is 1.00wt%,the maximum compressive strength is 73.4MPa,which is 11.72%higher than that of cement paste;the resistivity under dry state is 456kΩ.cm,which is 95.02%lower than that of blank cement paste.The water content also has a great influence on the electrical conductivity of WPPy/CNTs/Cement composites,and the saturated water content is about 8wt%.The effect of WPPy/CNTs on the hydration process of cement-based composites was analyzed by TG and SEM,the hydration process of WPPy/CNTs cement composite is slower than that of cement paste at the same age.At all ages,the decomposition region of Ca（OH）₂crystal was not found by thermogravimetric analysis.It is speculated that the formation of Ca（OH）₂ is inhibited by the presence of WPPy/CNTs composite.