The Dispersibility Of Nano SiO₂ And The Preparation And Properties Investigation Of Modified Layered Cement-based Materials Of Nano SiO₂

In recent years,utilization of nanomaterials in cement concrete has been gaining widespread attention as it exhibits great potential of accelerating the hydration of cement,improving the mechanical property and thus contributes to an improved durability of concrete.Among all the nanomaterials,nanoSiO₂ is the most widely used owing to its ultra-high pozzolanic reactivity.Research reports showed that both the easiness of agglomeration and the difficulty of dispersion of nanomaterials hinder the property-gain of cement-based materials modified by nanomaterials.In addition,the high cost of nanomaterials can be a serious problem in its application.Based on the above mentioned problems,the dispersibility of nanoSiO₂ in cementitous materials were investigated and a modified dispersion method of nanoSiO₂ was studied and compared.Then the properties of modified layered cement-based materials using nanomaterials were investigated with the hope of adding value to the commercial application of nanomaterials in cementitious materials.Firstly,the effects of surfactant?superplasticizer?and physical dispersion of nanoSiO₂ on the setting and hardening process were tailored.The results revealed that some commercial superplasticizers cannot further disperse the nanoSiO₂ through ultrasonic dispersion.And two groups that in which superplasticizers were added to both nanoSiO₂ and cement mixture have a comparable effect on mortar fluidity and compressive strength.Results showed that although the physical dispersion of high intensity can improve the dispersion of nanoSiO₂ in the aqueous solution,the nanoSiO₂ agglomerated together again when this solution was added into the pore solution with much ions.Hence,the common dispersion methods cannot be beneficial for a homogeneous distribution of nanoSiO₂ in the cementitous materials.Secondly,the effects of the adsorption dispersion on the properties of fresh and hardened cement-based materials were investigated based on the adsorption behavior of nanoSiO₂ on the surface of silica fume and fly ash,for example,the rheological property and the hydration of cement at later age.The results showed that the adsorption of nanoSiO₂ on silica fume followed the Freundlich adsorption model,and a multi-layered adsorption characteristics were found.And the ability of the adsorption of nanoSiO₂ on silica fume was better than that of the adsorption of nanoSiO₂ on fly ash.In comparison with the samples with an adsorption of nanoSiO₂ on fly ash,the adsorption of nanoSiO₂ on silica fume can improve the rheological property of mortar,which showed that adsorption dispersion had a significant effect on the dispersion of nanoSiO₂ in cement matrix.And the setting and hardening process of cement also can be more significantly improved by the pre-adsorbed nanoSiO₂,which resulted in improved mechanical properties.Furthermore,the hindrance effects of nanoSiO₂ on cement hydration at later age could be mitigated by the pre-adsorption of nanoSiO₂ on the surface of silica fume,although it cannot be fully avoided: a higher content of unhydrated cement was seen when compared with the control sample,which can contribute to a improved compressive strength and a denser structure at later age.Finally,the thesis discuss the effects of modified layer of nanoSiO₂ on the properties of cementitous materials.From the flexural strength and morphology of interfacial transition zone?ITZ?between cement matrix and modified layer of nanoSiO₂,nanoSiO₂ can enhance the flexural strength of ITZ at early age.But the flexural strength of ITZ decreased with increasing of curing age,which can be attributed to the more prominent weak zone of ITZ due to the increase of strength of both sides with increasing of curing age.Moreover,the structure with modified layer of nanoSiO₂ can lower the transmission of chloride,and the thickness of modified layer of nanoSiO₂ can be tailored to meet requirement of different degree of transmission of chloride.Under the condition of carbonation,when the amount of nanoSiO₂ in cement mortar increased,the carbonation of C-S-H gel in the layered cement mortar modified by nanoSiO₂ was obvious,which can have a negative effect on the stability of the structure of C-S-H gel.The carbonation properties of cementbased materials with modified layer of nanoSiO₂ need further research.