Effect Of Polycarboxylate Ether Superplasticizers On Hydration Of Cement In Induction Period

Cement hydration is a very complex and heterogeneous reaction process,and the hydration in the induction period has an important influence on the performance of concrete,which is also one of the fundamental mechanisms of cement hydration.Polycarboxylate ether（PCE）superplasticizer has been widely used in concrete and plays a crucial role in the configuration of high-performance concrete.The interaction between cement and PCE in the early hydration greatly affects the workability of cement paste.Hence it is very important to understand the influences of the PCE on cement dissolution,water film thickness（WFT）,the growth processes of hydration products,and the microstructure of cement paste.Therefore,the effect of PCE on cement hydration in induction period were multi-level studied in this paper by using the knowledge of cement chemistry,interface chemistry and micromorphology.Experiment using the typical ether polycarboxylic superplasticizers（ether PC）,ester polycarboxylic superplasticizers（ester PC）and a standard cement,the molecular weight of this two types of the PCEs,based on the analysis of the determination of molecular structure and performance.First of all,the influence of these two types of PCEs on the dissolution process of cement particles and the later growth process of hydration products was studied immediately after contacting with water.Based on the analysis of the interaction between the PCEs and cement brings new insights into the origin of setting retardation in the presence of the PCEs.Secondly,the influence of the PCEs on the WFT,zeta potential,hydration products and rheological properties of cement paste at different water to cement（w/c）ratios was studied to reveal the effect of the PCEs changing the flocculating water on the working performance of cement paste.Thirdly,the early microstructure of cement-water-PCE system was studied by using backscattering scanning electron microscopy（BSE）and ternary technique,and then the effect of the PCEs on the average distance between cement particles was calculated.Finally,the effects of the PCE on the packing density and macroscopic working performance of cement paste mixed with fly ash and mineral powder were studied,which provided guidance for rational and efficient use of PCE and its preparation of high-performance concrete.Based on the experimental and theoretical work conducted in this study,a few specific conclusions can be drawn as follows:（1）In view of the current research,the researches on cement dissolution process mostly uses calcium hydroxide as the solvent and lacks the initial period of cement just contacting water.Therefore,the automatic cement hydration heat tester and automatic hydration heat tester were used to study this period.Using pure water as the solvent,the effects of the PCEs on dispersion degree,conductivity,hydration ions,hydration products and hydration process of cement suspensions at different w/c ratios were studied.The results showed that the adsorption of the PCEs on cement improves the dispersion degree of cement particles,increases the reactive sites on the surface of cement particles,accelerates the dissolution rate,and increases the total amount of hydration ions.However,the conductivity of the cement suspension solution decreased because of the complexation between the PCEs and Ca²⁺ions,which means that the total number of ions that can participate in the hydration reaction decreased.In addition,the amount of the ettringite（AFt）forming on the surface of cement particles was reduced but the nano particles dispersed well in the interstitial pore space in the presence of the PCEs during induction period.Furthermore,the increased Al³⁺ions in the solution with the PCE also showed that the dissolution rate of C₃S was inhibited.The above three factors are responsible for the retardation effect of cement hydration.Moreover,by comparing the influence of ether PC and ester PC on the initial dispersion degree of cement,it is found that the ether PC on the dispersion degree of cement particles is greater than the ester PC,cement particles release more hydration ions,and this kind of PCE leads to more obvious retardation effect of cement hydration.（2）The effects of the PCEs on the dispersing properties and initial hydration of cement particles with various w/c ratios was investigated,including the WFT,rheology,fluidity,adsorption of the PCEs,zeta potential,degree of hydration and hydration products.The experimental results demonstrate that at a lower w/c the addition of the PCE had more obviously influence on rheology,flow parameters,WFT,the amount of hydration products and zeta potential than at a higher w/c.This might be attributed to the more adsorption amount of the PCE on cement particles at a lower w/c than at a higher w/c.By fitting the correlation between the microscopic parameters and the macroscopic rheological properties,the WFT and the yield stress,apparent viscosity and fluidity of cement paste can all be established exponential function,the correlation coefficient is above 0.90,WFT can be used as a new measurement parameter to judge the rheological properties of cement paste.（3）Most prior studies focused on the calculation of the hydration degree of cement by using BSE-IA,but lack of research on cement particle spacing.In this paper,the average distance between cement particles is quantititvely calculated by using ternary technique and the nearest neighbor function,and the influence of PCE on the microstructure formation characteristics of cement paste in the erarly setting process is explored.Without the PCEs,the distribution of cement particles is not uniform,the size of cement particles is obviously different and the cement paste structure is relatively loose.In this regard,numerous hydration products may gather around small cement particles and then adsorb other unhydrated particles,forming the continuous area.By contrast,the particles possessing relatively large size are surrounded by noticeable pores,which interrupt the continuity of the hydration products region.When introducing the PCEs,the agglomeration of cement particles is reduced,the cement paste structure is more compact,the continuous hydration area is formed around the large particles,and the number of large pores is reduced.The results show that PCE can effectively restrain the hydration degree of cement paste and reduce the average distance between cement particles and the porosity of cement paste.（4）The influence of the PCE,fly ash and mineral powder on the WFT of cementitious material and the correlation study on the workability of cement paste.The PCE breaks the flocculating structure of cement paste through its dispersion characteristics,increases the packing density of cementitious material,enlarges the WFT of cementitious material,decreases the cohesiveness of cement paste,and increases the fluidity and flow rate.Compared with mineral powder,fly ash has a excellent filling effect with cement particles,which significantly improves the packing density of cementitious materials,increases the WFT,and increases the fluidity of paste.However,the cohesiveness and flow rate of cement paste decrease because fly ash has micro-aggregate effect.And mineral powder and cement particle cannot fill well with each other because each other shape is irregular,and cementitious material packing density increased slightly,but because of mineral powder’s specific surface area is greater than the surface of cement particles,so the WFT of cementing material is reduced.The deflocculating effect of mineral powder make the fluidity of cement paste,cohesion reduced,velocity increase.By analyzing the relationship between the WFT and cohesiveness,fluidity,flow rate and compressive strength,it is found that the WFT is an important factor affecting the macroscopic working properties of cement paste.