Study On The Structure-property Relationships Of Polycarboxylate Superplasticizer,viscosity Modifying Agent And Nucleation Accelerator

In the concrete industry,the workability of fresh concrete,which is the characteristics describing the ability of fresh concrete to move and to be compacted,is one of the essential properties among all properties of concrete such as mechanical properties,durability etc.The appropriate workability of fresh concrete is the key for successful construction of concrete structures,which ensures the transportation and casting of fresh concrete mixture,and is also benefical to the long-term performances of concrete during service.The required workability for casting concrete depends on the type of construction technique,e.g.placement and consolidation methods,such as pumping,self-compacting concrete?SCC?,underwater concrete,shotcrete or roller compacted concrete.Theoretically,concrete workability is characterized by its rheological properties.The rheology behavior of fresh concrete is usually defined as a Bingham fluid,characterized by parameters of yield stress and plastic viscosity.Water to cement ratio,optimization of binder system and concrete formuation are the traditional tools to adjust the workability of fresh concrete,which however is limited by the strength requirement of the final concrete.The considerabe progress of chemical admixtures technology in the last decades,such as the polycarboxylate superplasticizers?PCEs?and viscosity modifying admixtures?VMAs?,provides a more powerful tool allowing us to adjust the workability of fresh concrete in a much wider range without negatively affecting the long-term performances of final concrete.Despite the vast practical application of the various chemical admixtures,the fundamental understanding of the structure-performance relationship of those chemical admixtures is still unsatisfactory.In addition,more application of PCE such as in oil well cementing project requires more fundamental investigation due to the extremely severe condition such as the high environmental temperature.Therefore,one of the main tasks of this thesis is to provide more fundamental insights into the structure-performance relationship of the two types of typical chemical admixtures,PCEs and VMAs.On the other hand,the incorporation of the PCEs or VMAs brings great influences on the cement hydration process,usually retardation effect,which in ture delay the setting and strength growth of concrete.The updated understanding of the retardation mechanisms of those chemical admixures is the so-called nucleation poisoning effect.That is to say,the nucleation process of hydration products,C-S-H is significantly inhibited by the addition of the PCEs and VMAs.In recent years,a novel approach to accelerate cement hydration has been proposed by adding external C-S-H nuclei into a hydrating cementitious system,which is called seeding technology.It is believed that such seeding technology is a promising technique to compensate the retardation effects of the PCEs or VMAs.The preparation method and specific working mechanism of C-S-H seeds are unfortunately still not very much documented.Therefore,another task of this thesis is to prepare the C-S-H seed and to explor the working mechanism of C-S-H seed.In this thesis,a series of PCEs with varied molecular structure are prepared by radical polymerization and fully characterized.The dispersion morphology of cement grains in fresh cement pastes containing PCEs was observed by Morphologi G3microscope.Particular attention was paid to the interaction between the PCEs and the ions,especially Ca²⁺ions in the interstitial solution of fresh cement paste by measurement of the average hydrodynamic radius?R_h?of PCEs using dynamic laser scattering?DLS?technique.In the cement pastes,the effect of bivalent cation and non-absorbed polymer on the dispersion of PCEs was investigated.Results showed that the functions of side chain in the comb-like PCE not only generate steric hindrance effect but also hinder the bridging effect of ion once they are adsorbed on surface of cement grains.In fresh cement pastes,the bivalent cations would reduce the PCE's dispersion effect.Longer side chain and higher side chain density is beneficial to the hindrance of the Ca²⁺bridging effect.A series of VMAs?or Fluid loss additives,used to reduce water loss from the cement slurry in oil well cementing?with varied structures are prepared.The bleeding?or water loss?were tested to compare the performance of the obtained polymers as VMAs or FLAs in cement pastes.The rheology of fresh cement pastes containing the polymers is also investigated by Brookfield rheolometer.The adsorption of polymers on cement was measured by TOC.The hydrodynamic radius of the polymers was calculated with Einstein viscosity equation.Results showed that the VMA polymers could significantly decrease the bleeding tendency or water loss of the cement pastes.On the other hand,the VMA polymers also increased the plastic viscosity and the yield stress of fresh cement pastes.Further experiments showed that their performances were related with their adsorption capacity on surface of cement grains and their hydrodynamic radius in pore solution.Therefore,it is concluded that the anchoring groups and the long hydrated polymer backbone are the two necessary components of VAM polymers.By sol-gel method nano-sized C-S-H particles?Nano-C-S-H?were prepared and fully characterized using techniques of FTIR,XRD,TEM,DLS etc.The Nano-C-S-H particles were added as admixture into Portland cement?OPC?systems.The working mechanism of the C-S-H particles in accelerating cement hydration was systematically investigated by using techniques of isothermal calorimetry,XRD,SEM and ICP-OES.Strength development of the mortar with inclusion of Nano-C-S-H was followed in the age of 28d.Results showed that the prepared nano-sized C-S-H particles significantly enhanced the early strength development for cement mortars without visible reduction of the late strength.The cement hydration was significantly accelerated by the addition of the nano-sized C-S-H particles,which was majorly attributed to the advanced and enhanced nucleation process of C-S-H during cement hydration.