In-situ Synthesis Of Carbon Nanotubes And Its Effect On The Piezoresistive And Mechanical Properties Of Cement Mortar

With the development of nanotechnology,carbon nanotube reinforced cement-based composites(CNTRCC)have made significant progress over the worldwide.Compared with ordinary cement-based materials,CNTRCC have high strength,high durability and great multifunction.High durability can reduce the economic loss and the consumption of energy and natural resources caused by the structural failure,which is consistent with the sustainable development strategy.As another advantage of CNTRCC,multifunction is considered a feasible way to solve the incompatibility between the structures and the sensors in the structural health monitoring system,and would make the sensors to certain advantages,such as low cost,high durability,large sensing volume and no mechanical property loss.Therefore,the modification of cement-based materials by carbon nanotubes conforms to the requirements of high-performance cement-based composites and represents the development direction of cement-based composites.However,the difficult dispersion of carbon nanotubes(CNTs)restricts the development of the CNTRCC.The current dispersion techniques are susceptible to cause CNT damages or structural defects.Here we tried the in-situ growth of CNTs on the surface of fly ash(FA)to improve the dispersion of CNTs without introducing a detrimental effect on CNT properties.To verify the advantage of CNT-coated FA(CNT@FA)in dispersion,we investigated the effect of CNT@FA on the piezoresistive behaviors and mechanical properties of CNTRCC in detail and compared the results with cement-based composites incorporated commercial CNTs(C-MWCNT).The reinforcement mechanism of CNTRCC was also investigated by the analysis of microstructure.CNT@FA were first successfully synthesized by one-spot microwave heating method in40 s.Upon microwave irradiation,CNTs grew rapidly on the surface of fly ash in a domestic microwave oven at room temperature without expensive equipment and any inert gas protection.Effects of different parameters on the growth of CNTs were thoroughly investigated.Results showed that the addition of conductive materials during microwave irradiation significantly enhanced the growth of CNT.The synthesized CNTs accounted for about 33%of the total mass of CNT@FA,and the diameter ranges from 30 nm to 100 nm.CNT@FA had a better dispersion state in aqueous solution than C-MWCNT.After ultrasonication,the quantitative analysis based optical microscopy inspection showed that the CNT@FA homogenously dispersed in water with the 54.28?m~2 of average area of agglomerates while the C-MWCNTs re-agglomerated into bundles and large aggregates with average area of 1367.25?m~2.With great dispersion,CNT@FA was proved to be able to create highly sensitive CNT@FA cement sensors.When the CNT concentration was 2.0 wt.%and the amplitude of compressive loading was 10 MPa,the CNT@FA cement sensor achieved the highest gauge factor of 7438.77(0–100??),the highest average gauge factor of 1333.74(0–600??)and the highest resistivity change of 77%,which were all much higher than the highest resistivity change of 4.23%and the highest gauge factor of 163.24 that the C-MWCNT cement sensor obtained.These piezoresistive parameter values(resistivity change and gauge factor)that the CNT@FA mortar obtained are also much higher than the values currently reported for CNTRCC.Moreover,the piezoresistivity of CNT@FA cement sensor was stable upon cyclic compressive loading.The resistivity of the CNT@FA cement sensor varied regularly and stably upon cyclic compressive loading and showed a high signal to noise ratio.The great piezoresistivity of the CNT@FA cement sensor was explained by two mechanisms:1)the high possibility of the breakup/formation of CNT conductive paths provided by the unique morphology of CNT@FA;2)high ratio of tunneling resistance with respect to the total resistance caused by the good dispersion of CNTs.However,the incorporation of CNT@FA and nano carbon black resulted in the decrease of piezoresistive sensitivity of the mortar specimen.This phenomenon may be related to the amount of added nano carbon black and the poor dispersion of nano carbon black.The effect of CNT@FA on the mechanical properties of cement mortar with low,medium and high strength level was systematically investigated and results were compared with C-MWCNT modified cement mortar at the same condition to highlight the advantage of CNT@FA in the improvement of mechanical properties for cement mortar.The improvement mechanism was also investigated.Experimental results showed CNT@FA improved the macro-mechanics properties of cement mortars more than C-MWCNT even at a smaller concentration.When the CNT concentration was 0.2 wt.%,0.05 wt.%and 0.05 wt.%,the CNT@FA reinforced M30,M50 and M80 mortars achieved the highest compressive and flexural strength values.For 0.2wt.%CNT@FA reinforced M30 mortar,0.05 wt.%CNT@FA reinforced M50 mortar and 0.05wt.%CNT@FA reinforced M80 mortar,there was an increase of 46%,92%and 28%in the 7d compressive strength,an increase of 26%,40%and 21%in the 28 d compressive strength,an increase of 39.8%,56.6%and 53%in the 7 d flexural strength and an increase of 8.8%,68.5%and 32.3%in the 28 d flexural strength with respect to the plain mortars respectively.In terms of the elastic modulus,the 0.05 wt.%,1.0 wt.%and 0.2 wt.%of CNT additions respectively caused a 75.6%,62.8%and 42.2%increase in the 28 d elastic modulus of the CNT@FA reinforced M30,M50 and M80 cement mortars.It was observed that the increase of elastic modulus of the CNT@FA reinforced mortar was not associated with increased compressive strength.Therefore,it would not introduce the high brittleness and high propensity of autogenous shrinkage cracking to the modified cement-based materials like conventional ways to increase elastic modulus.The investigation on the microstructure of cement mortars revealed that the great dispersion of CNT@FA in mortar matrix may be the main reason for the significantly improved macro-mechanics properties of CNT@FA modified cement mortars.With the homogeneous dispersion of CNT@FA,the synergistic effect of the nucleation effect,filling effect and bridging effect of carbon nanotubes effectively reduced the porosity of cement mortars,refined the pore structures and improved the matrix compactness.The strong bonding between the CNTs and hydration products and the bridging effect of CNTs guaranteed the load transfer from matrix to CNTs.Certainly,the mechanical properties were enhanced.Therefore,the in-situ growth of CNTs provides an alternative to improve the dispersion of CNTs in cement-based materials.Due to the great dispersion of in-situ CNTs,CNT@FA significantly improved the piezoresistive and mechanical properties of cement nanocomposites,which would substantially promote the application of cement-based materials in structural health monitoring and the development of high performance of cement-based composites.