Effect Of Chemical Additives On Formation And Structures Of Synthetic Ettringite,portlandite And C-S-H

Chemical additives have become a necessary component in modern cement and concrete production,which have significant impact on the hydration process of cement.Cement hydration is the key factor determining most properties of cementitious materials,including workability,setting and hardening,strength development,and even long-term durability.A deep understanding of the influence mechanism of chemical additives on the formation and structure of hydration products is of great significance for the development of cement and concrete additives and for the proper use of the additives to regulate the properties of cement and concrete.Triethanolamine（TEA）is an important component of cement grinding aid.Polycarboxylic superplasticizer（PCE）has become the most widely used concrete water reducer.In this paper,the effects of TEA and PCEs with varied polymer architecture on formation and final structure of ettringite（AFt）,calcium hydroxide（CH）and C-S-H were studied.AFt,CH and C-S-H were synthesised by solution coprecipitation method.The effects of TEA and PCEs on the crystallization and precipitation process,structure and morphology of the three main hydration products were systematically investigited by means of liquid phase analysis by measurements of solution conductivity,ion concentration,and solid phase analysis involving techniques of XRD,TGA and BET.The main conclusions are as follows:（1）TEA has no obvious effect on the crystallization process of synthetic AFt and a small amount of TEA adsorbed on the surface of AFt dose not significantly change the crystal structure and morphology of ettringite.TEA strongly inhibits the nucleation of CH and participates in the crystallization process of CH.High concentration TEA leads to the formation of"organic-inorganic complex"with brand-new crystal structure and morphology.TEA interacts with C-S-H through surface adsorption and grafting at silicon chain vacancy,resulting in the increase of C-S-H specific surface area.（2）The complexation of PCE with calcium ions has negligible effect on the ion composition in liquid phase.A large number of PCE molecules are adsorbed on the surface of crystalline particles,which significantly affects the crystallization process and morphology of AFt and CH.PCE greatly increase the saturation index of CH and C-S-H in whole nucleation and stable growth process,which means that the nucleation and growth process of precipitation are inhibited.The adsorption of PCE on the crystal surface leads to the reduction of crystal size and the increase of specific surface area of the final obtained precipitates.In addition,due to the special crystallite structure of C-S-H,the interaction mode between PCE and C-S-H includes intercalation and grafting,besides absorption that is the major interaction mode of PCE with AFt and CH.As a consequence,the interlayer spacing and specific surface area of C-S-H are visibly increased,and the particle size is decreased by intercalation and grafting of PCE in C-S-H structure.（3）The adsorption capacity of PCE is closely related to its molecular archtechture.PCE with higher carboxyl density and shorter side chain presents stronger adsorption capacity,resulting in a more significant impact on the abovementioned three hydration products.（4）The above results successfully explain the formation of a large number of nano-scale AFt and C-S-H suspending particles formed in the early hydration process of fresh cement pastes in the presence of PCE,and provide a new basis for proving that“precipitation control”is the main mechanism of retarded cement hydration by PCE.（5）The addition of TEA results in in the formation of CH with much smaller sized,larger specific surface area and even amorphous structure,which exhibit much faster carbonation rate upon contact with CO₂.Such accelerated carbonation process may successfully explain the accelerated the self-healing of mortar cracks under water immersion,which is a new finding of this study.