Effects Of PC Modified GO On The Reinforcement Of Cement Composites

Cement-based material retains its popularity as the construction material in the field of engineering all over the world due to the abundant raw materials,low cost,high strength,and simple production technology.It is widely used in civil buildings,roads,bridges,airports,ports,nuclear power projects,and water projects.However,the major drawbacks of cement-based composites are heterogeneity,porosity,and brittleness.Stress concentrates at the flaws,which leads to crack propagation and thus brittle rupture,limiting the durability and increasing the maintenance cost.Recently,a variety of reinforcing materials have been incorporated to address the aforementioned shortcomings,such as nanoparticles（0D）,micro/nano-fibers（1D）,and nano-sheets（2D）.As a graphene derivative,graphene oxide（GO）,which consists of a mono-layer of sp²-hybridized carbon atoms bearing many oxygen-containing functional groups on its surface,is an attractive candidate for the use as a nano-reinforcement in cement composites due to the excellent mechanical properties,high specific surface area,and better dispersibility in aqueous solution.However,GO aggregates in cement pore solution(containing Ca²⁺,K⁺,and Na⁺ions)due to the fact that Ca²⁺could complex with-COO^-groups of GO,which restricts the reinforcing effects of GO in cement composites.To fulfill the excellent properties of GO,it is necessary to find out adequate dispersion and modification methods to make GO disperse in cement pore solution.In this research,modification of GO by water reducing agents of concrete,lignosulfonate（LS）,β-naphthalene sulfonate formaldehyde（PNS）,and polycarboxylate superplasticizer（PC）was first carried out.Then the dispersion states of modified GO in cement pore solution were characterized.PC was determined as the optimal modification of GO and a PC to GO mass ratio of 4 was chosen for subsequent preparation of cement composites.The influences and mechanisms of PC modified GO（PC@GO）on mechanical properties,transport properties,cement hydration,pore structure,micro-cracks,interfacial transition zone（ITZ）and hydrated calcium silicate（C-S-H）gels were systematically investigated.Moreover,a hybrid structure of PC@GO nanosheets and SiO₂ nanoparticles（SiO₂ NPs）was prepared and its co-effects on cement composites were investigated.The main results are as follows:1.The selection of modification agent of GO and the dispersion mechanismsThe dispersion states of LS,PNS,and PC modified GO in cement pore solution were investigated by using a combination of visual observation,optical microscopy,and transmission electron microscopy（TEM）.The results indicated that with the mass ratio of water reducing agents to GO fixing at 5:1,GO agglomeration occurred immediately when LS or PNS modified GO was dropped into cement pore solution.Increasing the content of LS or PNS did not prevent the agglomeration of GO.While PC@GO presented the best overall macroscopic dispersion without identifiable agglomerates in cement pore solution up to 24 h.Although GO agglomerates at the micro-level gradually formed with time,the existence of dispersed GO nanosheets could be observed by TEM after PC@GO（PC:GO=4:1）was mixed with cement pore solution for 2 h.In the alkaline environment,LS and PNS failed to adsorb onto the surface of GO due to the higher electrostatic repulsion forces between the deprotonated GO and negatively charged surfactants.While PC could adsorb onto the surface of GO successfully even in the strong alkaline environment due to the higher entropy gain of the comb-like molecules of PC.The dispersion mechanisms of PC@GO are considered to be a combination of the adsorption of PC onto the surface of GO driven by entropy,the lowered free Ca²⁺concentration due to the strong complexation with the-COO^-groups on the main chain of PC and impeding the cross-linking of Ca²⁺through steric hindrance effects of PC side chains.Based on the experimental results,PC was selected as the optimum modification agent and was employed in the fabrication of GO reinforced cement composites with the mass ratio of PC:GO=4:1.2.The effects of PC@GO on the mechanical and transport properties of cement compositesPC@GO with various dosages was incorporated in cement composites and subjected to mechanical and chloride transport tests.The results showed that with the addition of 0.022 wt%GO（0.22 wt%PC）（wt%was based on the mass of cement,the solid content of PC was 40%,based on the liquid content of PC,hearafter）,the 3 d compressive and flexural strengths were increased by28.41%and 16.67%,the 28 d compressive and flexural strengths were increased by 17.27%,and12.51%,Young’s modulus and flexural toughness were increased by 16.33%and 33.81%,respectively,suggesting the delayed propagation of cracks in cement composites containing PC@GO.Small fractions of GO addition can achieve high levels of reinforcement,indicating that GO is a potential nano-reinforcement of concrete.However,increasing PC@GO contents led to slight decreases in mechanical strengths,which may result from the side-effects of PC at high dosages,such as the increased viscosity,air entrainment,and bleeding.Moreover,A very low fraction of GO（0.011 wt%,0.11 wt%PC）could significantly reduce the content of chloride in cement mortars and the average chloride depth was dropped by 28.6%compared to the plain sample,suggesting that PC@GO can effectively resist the transport of aggressive agents.The results indicate that GO has the potential to enhance the concrete durability and increase the life-span of cement based structures.3.The reinforcing mechanisms of PC@GO on cement composites.The effects of PC@GO on the kinetics of cement hydration,pore structure,micro-cracks,ITZ and C-S-H gels were systematically investigated by a variety of techniques,such as isothermal calorimeter,thermal gravity analysis（TG）,X-ray diffraction（XRD）,mercury intrusion porosimetry（MIP）,nitrogen isotherm adsorption analysis,scanning electron microscope（SEM）,X-ray photoelectron spectroscopy（XPS）and nuclear magnetic resonance（NMR）,to investigate the reinforcing mechanisms of PC@GO on cement composites.The results showed that GO could accelerate the cement hydration,decrease the porosity of macropores and increase the polymerization degree of C-S-H gels due to the nucleation effects of GO.The dense hydration products prohibited the initiation and propagation of micro-cracks.The internal curing effects of GO could maintain the continuous cement hydration and lower the local water to cement ratio within ITZ,which prevented the growth of lamellar Ca（OH）₂ and need-like ettringite.Ultimately,the ITZ was filled with compact hydration products.GO could intercalate into the interlayer space of C-S-H gels through ionic bonding with Ca²⁺and fill in the gel pores,creating a 3D network structure.The refined microstructure of cement composites with the presence of PC@GO is deemed as the fundamental source for the improved mechanical strengths and higher resistance against chloride ingress.4.Synergistic effects of PC@GO and SiO₂ NPs on cement compositesTo further increase the strength of cement composites,a hybrid structure of PC@GO and SiO₂NPs（PC@GO/SiO₂ NPs）was prepared and incorporated in cement composites to investigate the synergistic effects and mechanisms of PC@GO/SiO₂ NPs.The results showed that the hybrid structure of PC@GO and SiO₂ NPs increased the dispersion of SiO₂ NPs,which tended to better disperse on GO surface.The addition of 0.022 wt%GO and 1.67 wt%SiO₂ NPs resulted in 38.56%,44.80%,and 38.87%enhancement of compressive strength,37.94%,30.04%,and 25.88%enhancement of flexural strength at 3,7 and 28 days,respectively.The mechanical properties with the addition of PC@GO/SiO₂ NPs were significantly higher than those of cement composites with PC@GO or SiO₂ NPs at the same dosage,indicating that the synergistic effects of PC@GO/SiO₂NPs were achieved.Based on the results of isothermal calorimetry,TG,XRD,and SEM,it could be concluded that PC@GO/SiO₂ NPs not only accelerated the cement hydration but also promoted the pozzolanic reaction between SiO₂ NPs and Ca（OH）_2.As a result,the content of Ca（OH）₂ crystals was decreased and the compact C-S-H gels were generated.The synergistic effects of PC@GO/SiO₂ NPs are considered to be benefited from the better dispersion of SiO₂ NPs and a higher degree of pozzolanic reaction.