Potential Effect Of Surface Modified Nano-SiO₂ On The Cement Paste Hydration In Early

Concrete,the most used manmade material,primarily depends on the cement industry.In the cement manufacturing process,it is responsible for about 5-8%of the worldwide manmade carbon dioxide?CO₂?emissions because of the burning fuel and chemical decomposition of materials.The cement industry becomes the third largest source of worldwide CO₂ emissions.On the other hand,the discovery of nanoscale calcium-silicate-hydrate?C-S-H?gel in a hardened cement paste,which was made by Taylor in 1986,opened new prospects in the modification of cement-based materials in nano scale.Nanoparticles mainly include nano silica?NS?,nano alumina,nano graphene oxide,nano-titanium oxid and carbon nanotube,among other nanoparticles,NS has been widely used due to its pozzolanic,seeding and filler effects.However,NS still exists as natural aggregates,because of its very high specific surface area and surface energy,even in the colloidal suspensions.Thus,to disperse NS in an aqueous solution,proper chemical admixtures or much more water should be added.However,there was an incompatibility between current commercially useable PCE and NS.In addition,the cement pore solution exhibits a high pH and very high ionic strength.Being applied to cement-based materials,the colloidal suspension interferes immediately with the cement paste resulting in the formation of gels,thus reducing the cement paste fluidity.Based on the reason above,NS/APCS and NS/KPCE were obtained after amphoteric polycarboxylate superplasticizer?APCS?and polycarboxylate ether-based superplasticizer containing silane coupling agent?KPCE?were grafted onto the NS,and the effect of NS/APCS and NS/KPCE on the property of cement pastes was investigated in this paper.The schematic of NS surface functionalization with APCS was proposed.The mechanism about improving the the distribution of NS was investigated by the Zeta potential.The adsorption conformation of APCS and KPCE on NS was investigated by Dynamic Light Scattering.The effect of NS/APCS and NS/KPCE on the fluidity and hydration in early of cement pastes were investigated.Inaddition,afterPoly-?diallyldimethylammonium??PDDA?and2-methacryloyloxyethyltrimethylammoniumchloride?DMC?were grafted onto NS,the effect of formed NS/PDDA and NS/DMC on the cement paste hydration in early were investigated,too.Currently the mechanics of the interaction between modified NS and cement pastes are not well understood.So,the four means of modifying NS can provide fundamental basis for the further research about the application of modified NS in the cement-based materials.The conclusions were as follows:?1?Possible schematic drawing of the adsorption of NS/PDDA on cement particle surfaces was proposed.When the dosage of NS/PDDA was 0.3%,the cement hydration was controlled by PDDA,which delayed the exothermic peaks of cement paste hydration.While the dosage of NS/PDDA was 1%,the cement hydration was controlled by NS,which accelerated the exothermic peaks of cement paste hydration.However,NS existed as aggregates at the moment and the cement hydration was controlled by aggregates.When the dosage of NS/PDDA was 0.3%,acceleration to the cement hydration was not obvious due to more serious aggregates.?2?Short range interactions were achieved by addition of DMC.When the dosage of NS/PDDA was 0.3%,1%and 3%?mass ratio to cement?,respectively,the cement hydration was controlled by DMC,the hydrophobicity of DMC?the shell of NS/DMC?delayed the exothermic peaks of cement paste hydration.While the dosage of NS/PDDA was 1%,NS existed as aggregates at the moment,which reduced the delaying effect.When the dosage of NS/PDDA was 3%,further delaying effect to the cement hydration was found due to more serious aggregates.?3?The different interactions resulted in the different effect of NS/PDDA and NS/DMC on cement hydration in early.?4?Schematic of NS surface functionalization with APCS via electrostatic self-assembly was proposed,and the method ensured the formation of a core-shell NS/APCS.Schematic of NS surface functionalization with KPCE was proposed,and the method ensured the formation of a core-shell-shell NS/KPCE.Moreover,after the surface modification,due to the APCS?or KPCE?core,an improved compatibility between NS and APCS?or KPCE?and a higher dispersion stability of NS in the saturated calcium hydroxide solution were attained.?5?The mean particle size of APCS?or KPCE?and NS/APCS?or NS/KPCE?indicated they existed as partially stretched conformation with AA,DMC?or KH570?and TGA.?6?For NS/APCS,the APCS shell exhibited enhanced electrostatic and steric hindrance repulsion properties for the dispersion stability of NS with AA?polymerization temperature,TGA and H₂O_2.When the molar ratio of DMC to TPEG was less than 1.2,electrostatic and steric hindrance repulsion controlled the dispersion stability of NS,more than 1.2,steric hindrance repulsion controlled manily the dispersion stability.For NS/KPCE,the KPCE shell exhibited enhanced electrostatic and steric hindrance repulsion properties for the dispersion stability of NS with AA?KH570?polymerization temperature,TGA and H₂O_2.?7?The total share of APCS?or KPCE??from NS/APCS?or NS/KPCE?and additional APCS?or KPCE??was 0.2 wt%of the cement weight.Due to the improvement in the compatibility between APCS?or KPCE?and NS and stability of NS in the saturated calcium hydroxide solution,more APCS?or KPCE?molecules could be involved in the dispersion of cement particles,in comparison with pure NS,leading to an increase in fluidity.?8?When the dosage of NS/APCS?or NS/KPCE?was 1%of the cement weight,all the samples delayed the exothermic peaks of cement paste hydration,however,it did not affect obviously extent of cement hydration at the age of 3d.The compressive strength of cement pastes having higher fluidity at the age of 3d was increased due to the increase in degree of density.