Preparation Of Nano-C-S-H By Polymer Coprecipitation And Its Effect On Cement Hydration Mechanism

The development of green cement and the reduction of cement content in concrete to reduce carbon dioxide emissions often lead to the reduction of hydration activity and strength,especially in the early stage,which is in conflict with economic benefits and technological requirements.In addition to nano silica and other nanoparticles with pozzolanic properties,calcium silicate hydrate（C-S-H）nanoparticles have significant ability to accelerate cement hydration without affecting the long-term strength of cement,which has become a research hotspot in recent years.Although the accelerating effect of C-S-H is still controversial,it is found that its chemical and physical properties have a great influence on its accelerating performance.The preparation methods of C-S-H include sol-gel method,pozzolanic method,hydrothermal method,etc.The co-precipitation method can overcome the problem that the samples synthesized by the above methods are easy to agglomerate.The influence of two water-soluble polymers with carboxylic acid（PCE）and sulfonic acid（PSE）functional groups on the physical and chemical properties of semi-crystalline hydrated calcium silicates is studied by co-precipitation method.And the correlation between C-S-H and their application as cement early strength agent is also studied.The hydration process of cement is the process of dissolution and precipitation of various mineral phases.The study of the preparation process of C-S-H can provide a theoretical basis for the hydration process of cement.The influence of C-S-H-polymer on the hydration process of cement and C₃S is studied.And its hydration kinetics are simulated.The effect of inorganic C-S-H and C-S-H-polymer nanoparticles on the hydration of C₃S is compared by in-situ FT-IR and in-situ XRD.The separated hydration method provides a direct evidence that C-S-H precipitates preferentially grow on nano-C-S-H particles.Some natural and low-cost montmorillonites have been successful in accelerating cement hydration.But at the early stage,the effect is either very small or not shown at all,especially at a relatively low dose of less than 2 wt%.In order to make up for this deficiency,nano-montmorillonite（OMMt）is synthesized with surfactant modifier as the main synthesis factor,which effectively controlls the particle size and stability of montmorillonite suspension.Their influences on the hydration properties of Portland cement are also discussed.The main conclusions of this paper are as follows:I Co-growth mechanism of C-S-H and polymer in the preparation of C-S-H-PCE nanocomposites（1）In XRD analysis,the characteristic peaks at 2θ=17°,29°,32°,50°and 55°all exist in the synthesized C-S-H-polymer samples.The incorporation of different polymer molecules result in the disappearance of the peak at 2θof 7°where the d₀₀₂ basal spacing reflection of 1.2 nm.This can be explained in two ways.The addition of comb-like PCE or PSE copolymer completely distorts the structure of C-S-H,reduces the crystallinity of C-S-H,and lead to the disappearance of reflexes angle corresponding to d₀₀₂ of C-S-PCE/PSE.Another possibility is that the polymer is inserted into the C-S-H structure,which may increase the layer spacing of d₀₀₂.Therefore,the peak of C-S-H-PCE/PSE sample at 2θ=7°shifts towards a smaller angle,which is beyond the limit of detection range.（2）Combining the TEM-EDS of CSH-PCE-6 and CSH-PSE-6,the Ca/Si ratio（0.6～0.8）is lower.There should be quite a long silicon chain.According to the results of ²⁹Si NMR,the MCL of CSH-PCE-6 and CSH-PSE-6 does not reach the same value with average chain length（MCL=9.5）of inorganic C-S-H under the same calcium silicon ratio.Because the increase of Q₁ represents the increase of terminal silicate units,which indicates that there will be more defects on the nanostructure and the possibility of polymer grafting increases.（3）Both PCE molecules show similar complexation.The complexation of calcium ions is mainly related to the number of anionic groups in the polymer.PCE-6 is more inclined to complex more Ca²⁺from solution than PCE-2.Compared with sulfonate superplasticizer（PSE）,Ca²⁺has more affinity to the carboxyl functional group of PCE polymer.（4）The effect of polymer molecules on the thermodynamics of nucleation and crystallization in the co-precipitation process is analyzed by isothermal calorimeter.Compared with inorganic C-S-H,the heat release time of C-S-H-polymer composites is lagged.The crystallization rate of C-S-H-polymer is much slower than that of inorganic C-S-H.The addition of polymer hinders the crystal growth of C-S-H-polymer and effectively controlls the growth rate of C-S-H-polymer.Through electrostatic interaction and complexation with calcium ions,polymer molecules reduce the probability of collision with silicates,thus reducing the growth rate of crystal nucleus.At the same time,polymers with a certain side chain length also play a role of dispersion through steric hindrance.（5）For the synthesis process of inorganic C-S-H,the concentration of calcium and silicon ions has been reduced to the lowest level and the crystal growth of C-S-H has ended at 5 mins of reaction time.However,in CSH-PCE-6 system,when the reaction is at 10 minutes,the concentration of calcium and silicon ions is decreased to the minimum,and the crystal growth process is stopped.The addition of PCE-6 polymer delays the growth of hydrated calcium silicate.II Effect of C-S-H-PCE nanoparticles on cement hydration（1）The hydration heat of cement paste with different C-S-H-polymers is evaluated by isothermal calorimeter.The addition of C-S-H composite shortens the induction period and increases the peak height corresponding to the maximum heat release rate of hydration.The acceleration effect of PCE typed C-S-H composite is more obvious than that of PSE typed C-S-H composite.（2）With the increase of total surface area introduced by C-S-H seeds,the peak corresponding to maximum heat release rate of hydration(R_peak)is increased significantly,and the induction period（t_i）is shortened.When the surface area of C-S-H seed exceeds0.3 m²/g cement,the hydration process of cement is controlled not only by the precipitation of hydration products,but also by the dissolution process of unhydrated clinker.The shoulder peak appears after the main peak and is considered to be the so-called peak of sulfate consumption.In addition to the silicate reaction,the addition of nano-C-S-H also greatly accelerates the aluminate reaction in the cement hydration process.（3）The C-S-H suspension and the serum of the suspension obtained by centrifugation of the C-S-H suspension are respectively mixed into the cement slurry.The separated C-S-H suspension shows a strong accelerating effect on the hydration of C₃S again,while the serum of C-S-H suspension even showed a slight delay effect,which is probably due to the presence of PCE in serum.（4）Based on the K-D model,the geometric growth index n is increased from 1.7 to3.2 with the addition of C-S-H-PCE.This shows that the addition of nano-C-S-H has a great influence on the geometric growth process of hydration products.The addition of nano-C-S-H makes the nucleation process of cement hydration change from non-uniform nucleation to uniform nucleation.In Avrami nucleation and growth model,when the hydration product layer reaches a certain critical thickness,it can be controled by diffusion kinetics.（5）In situ FT-IR,at the same hydration time,the characteristic peaks corresponding to Q₁ and Q₂ in the structure of hydrate product C-S-H in C₃S system doped with CSH-PCE-6 nanocomposites are stronger than those in C₃S system.In C₃S with CSH-PCE-6nanoparticles,the position of Q₂ in the structure of C-S-H moves to a higher wavenumber.The larger the Ca/Si ratio in C-S-H is,the higher the wavenumber is,and the more mature and dense the C-S-H is.The in-situ XRD test shows that it takes 8 hours for hydrates products greatly to start to form in C₃S hydration process.When C-S-H-polymer with larger specific surface area is added,the time is shortened to 4 hours.This shows that the addition of C-S-H-polymer nanoparticles greatly accelerates the hydration of alite.（6）In the separated hydration experiment,the mass weight changes in the dialysis bag with inorganic C-S-H and C-S-H-polymer are compared at 44 h.The mass weight increase of the former reached the maximum value at first,while the substance weight of the latter in the dialysis bag at 44 h still increases.This is because the total surface area provided by inorganic nano-C-S-H is smaller than that of C-S-H-polymer at the same dosage.Therefore,the surface of inorganic nano-C-S-H is completely covered by further growth of hydrates product earlier,then there is no redundant growth site.Combined with XRD results,the structure of hydration products grown on the surface of inorganic nano-C-S-H is more mature and dense with the increase of hydration time.III Effect of organically modified montmorillonite on rheological properties,hydration properties and pore structure of cement paste（1）Through organically modification,the nanometer OMMt composite with good dispersion and stability is prepared.FT-IR,XRD,TGA,BET and TEM analysis results show that the surfactant polymer can be intercalated or grafted into the structure of Mt.Compared with the original Mt,the organically modified OMMt nanocomposites have larger specific surface area.The results of FT-IR and TGA show that there is surfactant polymer in the modified OMMt composites,and the polymer content accounts for about50 wt%of the OMMt composites,which is consistent with the original design.（2）Based on the rheological property of cement paste,the addition of OMMt will not affect the workability of cement based materials system.When OMMt nanocomposites are added,the hydration process of cement particles can be promoted and more hydrate products can be increased.This behavior may attribute to nucleation effect.The total pore volume of cement matrix with OMMt is analyzed.The addition of these nanocomposites makes more pore structures move towards the gel pores.And more hydrates are distributed on the pore structure.Combined with the calorimetric curve and MIP results,this study provides a clear mechanism diagram to explain the role of OMMt nanoparticles in the process of cement hydration.