Research On Control Mechanism Of Chemical Admixtures On Cement Hydration And Initial Structure Formation

This project was from of the national basic research plan ("973" project No. 2001CB 610704-3, named basic research of the production and application of high performance cement). Based on the key technology subject of it, the preparation of the high active and content C3S cement (abbr. HCC), the differences of the compatibility and control mechanism between chemical admixtures with the cement made by the "973" group and the traditional cement were discussed. Influences and mechanisms of different chemical admixtures on HCC initial hydration process and the structure forming state were studied systematically. The main achievements are listed as follows.1. Synthetically evaluating methods on the initial hydration process of cement.1) (1) To establish the initial structure forming model to describe the hydration process of cement. The instantaneous electrical property of cement hydration is evaluated through online examination and the instantaneous state of initial structure formation was characterized by resistivity method. The initial structure forming model of cement includes three stages, i.e. solution-solution equilibrium stage, structure forming stage and structure stabilizing stage.2) To establish the Heat-Electric Model (abbr. H-E model) of the initial cement hydration for the first time. This model is built through the combination of the initial structure forming model and the setting heat model. The H-E model provides the thermal parameter and the structure forming parameter of cement initial hydration process, which combines the chemical reaction course and physical state of the structure forming.3) To establish the rheological property model of initial cement hydration. By the way of short-swing surge, the storage modulus model of cement behaves as "S". There was a sudden change on structure forming when the hydration of cement turns into the inducing period.2. The effects of typical water reducers on initial hydration of HCC were studied by the H-E model.Results show that the compatibility problems of chemical admixture and HCC are more obvious than those of the ordinary cement. For HCC, the special cement of'973' is better than Taiheiyo cement on compatibility with chemical admixtures and the strength, especially early strength. For different water reducers, the compatibility between polycarboxylate superplasticizer and HCC is the best among of them.3. Synthesis study of polycarboxylate superplasticizer.The product, named as PC-23, is synthesized successfully. The rate of water reduced is equal to that of the X404, the best product in the world, when used at the lower dosage. The examination results of H-E property of hydration process show that the PC-23 is better than X404.4. Influence of setting adjustment agents on initial hydration of cement.1) Action of electrolyte on initial hydration of cement.The combining style and the absorption to electricity in solution of cation and anion of electrolyte are complicated, which makes the cement hydration very complicated. For sulfate, the kind of cation influences the control on hydration process greatly. Along with the increasing of atomic number, the action on cement hydration turns from setting promoting and early strength enhancing to setting retarding.For the same cation, different acid radical makes the action on cement hydration different. Both ZnCb and ZnSC>4 retard the hydration process of cement, Cl~ may promote the setting retarding function of Zn2+, while the effect of ZnSC>4 is much smaller and shows the critical effect with the change of mixing mass.For sodium salts with different phosphate, the effect on setting retarding is different greatly according to different acid radical. The influence mechanisms of Na3PC?4, Na2HPC?4 and NaH2PC>4 are accordant, i.e. forming the calcium phosphate and retarding hydration. For NasPaOlo, the complex is formed by absorption with Ca2+ and makes the retarding function strongest among of them.2) Effects of organic setting retarders.The influence regulation of organic retarders on cement hydration heat process can be described as a saddle dual-peak phenomenon and becomes more obvious with the increasing of mixing amount. Cumulating quantity of heat at 1-day hydration age decreases at first time and increases at later, demonstrating the accelerating hydration process. Resistivity curve shows that structure formation becomes worse with the increasing of mixing amount, which is consistent with the strength development trend.There exists dual-critical effect of organic retarders on cement hydration. First, cement hydration is delayed when the amount is low, while accelerated when the amount is comparatively high. But paste is not hardened even after a long time under the latter situation. Secondly, there exists a critical amount of setting retarding. When it is lower than that,retarding effect increases with the enhancement of amount. While it is higher, retarding effect decreases with the enhancement of amount. This effect is more prominent for high C3S content cement, and the amount range of organic retarders is narrower than that in traditional cement.3) Control technology of HCC-chemical material system.Results show that the adoption of current technic method by application of HCC material will lead the decrease on damp and increase of the loss of damp with time elapsing. It can be resolved the methods as follows: ?composite of chemical admixtures, (2) increasing the dosage of chemical admixtures(0.05%-0.1%) because of the excursion of the saturation, (Dusing the chemical admixtures with extensive applicability.5. To establish the counter-percolation model of cement initial hydration for the first time.1) The counter-percolation process of cement initial hydration.It can be considered that two qualities leaps (or call it as segment) exist in the cement hydration process by onling examination of the change of resistivity and storage modulus along with it, i.e. a) Change from fluidity to plasticity; b) Change from plasticity to harden solid.Through the above two mutations along with it, the cement hydration process can be described as the process from existing of percolation passage to being filled little by little. Two critical thresholds appeare in the filling process, i.e. G'threshoid according to pmjn (not completely consistent on time) and puireshoid (may be more reasonable expressed as the critical segment). Therefore cement hydration process is the counter-percolation process, and a gradating process from fluidity to harden solid2) Relationship between the eigenvalue of resistivity and strength is built.Results show that all data can be classified by p24 and pmjn. The relation between p24 and strength is logarithmic. At 1-day age of hydration, the structure of cement stone has been formed (except for the super retarding situation); while the characteristic of microstructure at this time is closely relative to the physical-mechanical property and durability for long term. Therefore P24 expresses a better relativity with strength.The data group can be classified as two subset (subset A and subset B) by pmjn. Data in subset A are mainly chemical admixtures with setting retarding function, and the dosage in cement is relatively bigger. Data in subset B are mainly inorganic chemical admixtures, and organic admixtures in it are the results at lower mass.The relation between resistivity eigenvalue and strength is linearity in subset A. the action mechanism on cement hydration of them has common characteristic, which plays an important role in the structure forming process. The two critical thresholds express better relativity with strength by analysis of data in subset B. The structure forming state from adding water to pmin and from the end of solution - solution equili- brium stage (induce period in thermal model) to pthreshoid is closely relative to the physical- mechanical property of cement. The instantaneous state of resistivity in the cement hydration process gained by non-contacting electrical resistivity device can describe this course very efficaciously, which proves the counter-percolation process of cement hydration process along with the mutation of resistivity.