Study On Effect Of Polycarboxylate Superplasticizer With Different Structures In Cement-based Cementitious System

The flowability of fresh cementitious paste is a crucial factor in the construction process and is often used to guide the design of concrete,pumping,3D printing,and other construction processes.Comb-shaped polycarboxylate ester/ether（PCE）superplasticizers can effectively improve the flowability of concrete without reducing the water-cement ratio,making them a key component of high-performance concrete.The dispersing capability of PCE in cement paste is generally proportional to its adsorption capacity on the surface of cement particles.PCEs with high rigid skeleton,short main chain,large molecular weight,or long side chain have a high adsorption capacity and exhibit excellent dispersion in cement paste.PCEs with high charge density also demonstrate high adsorption amounts and good initial dispersion capability.However,the impact of PCEs with different locations and distributions of–COO^–groups on the performance of cementitious paste remains unknown.Moreover,the effectiveness of PCE in cement paste can be severely affected by their fineness and mineralogical composition,particularly the C ₃A content,sulfate ions released from cement,and clays,which are common contaminants in concrete aggregates and sand.These factors pose significant barriers to improving the robustness and facilitating the application of PCEs in a wide range of high-performance concrete.To meet the demands of higher-performance and more environmentally friendly constructions in the future,it is essential to explore the effect of PCEs with different molecular structures on the properties of fresh cementitious paste and the corresponding mechanisms.This thesis focused on investigating the effects of PCEs with different densities and local configurations of–COO^–groups on the properties of normal water binder ratio of cement paste,low water binder ratio of cement,and cement-silica fume pastes.Additionally,it aimed to clarify the interaction between PCE and montmorillonite under different PCE and montmorillonite ratios,and reveal the mechanism of PCE with different functional groups in cement-montmorillonite paste.The study also examined the effects of PCE dosage and crystallization environment on the early growth of ettringite.Based on the research results,the following conclusions were drawn:（1）Intermolecular chelation between–COO^–groups in FA-PCE and Ca²⁺dominates,while intramolecular chelation is the main coordination mode between–COO^–in MAH-PCE and Ca²⁺.Compared with MAH-PCE,FA-PCE exhibits a larger adsorption amount and thicker adsorption layer.Consequently,FA-PCE demonstrates stronger dispersion efficiency and a cement hydration-retarding effect,resulting in excellent flowability and rheological properties of cement pastes.（2）The adsorbed amount of PCE in cement-silica fume pastes is higher than that in pure cement paste,while the flowability of pure cement paste is higher than that of cement-silica fume paste.The Ca²⁺binding capacities of the PCEs follow the order of FA-PCE>MAH-PCE>IA-PCE.FA-PCE exhibits a greater adsorption amount and thicker adsorption layer on cement particles than MAH-PCE and IA-PCE,resulting in excellent flowability and rheological properties of the corresponding paste.（3）The adsorbed amount of PCE linearly increases with the dosage of mo-ntmorillonite,while the spread flow of cementitious pastes linearly decreases with the dosage of montmorillonite.X-ray diffraction results reveal that elect-rostatic attraction plays a significant role at low PCE/montmorillonite ratios,w hereas side chain intercalation dominates at high PCE/montmorillonite ratios.The reduction in flowability caused by PCE is primarily attributed to the comp etitive adsorption of montmorillonite in cement paste.（4）PCEs with cationic groups or bulky aryl groups cannot effectively inhibit the intercalation of PCE side chains into the montmorillonite layers.The dispersing effect of FA-TPEG reduces linearly as montmorillonite content increases,whereas FA-DMC-TPEG and FA-SSS-TPEG barely decrease when montmorillonite percentage is less than 1.5%and 1%,respectively.Among the tested PCEs,FA-SSS-TPEG showed excellent flowability due to its high affinity for cement particles and strong steric hindrance effect.（5）PCE had no effect on the types of products,but its adsorption changed the proportion,stability,and morphology of ettringite.Sulfate ions and PCE were predominantly adsorbed on the face of rectangular prism-shaped ettringite crystals,which limited their growth in the{10（?）0}faces and resulted in their longitudinal growth.The inhibition effect of PCE on the growth of ettringite crystals was proportional to the adsorption amount on ettringite and weakened as the crystallization time increased.The adsorption kinetics and isotherms of PCE on the surface of ettringite followed the pseudo-second-order and Freundlich equations,respectively.