Study On The Properties Of Cement Hydration Using Electrical Resistivity Method

Cement-based material is the largest building material in 21th century, and its later strength is the most important performance index. It will bring us large convenience if we can deduce the later strength according to early performance, which is the purpose of this thesis. At the same time, the concept of high performance concrete has been widely accepted by people, and mineral admixture and chemical admixture have been the primary components of cement-based material already. Their incorporation will change the hydration process which has been extremely complex, and particularly will have a great influence on the early hydration process. Study on the hydration mechanism of cement can provide theoretical basis for cement production, use and development of new types of cement-based materials and even development of chemical admixture, which are of important significance, and this subject was put forward in the background above.In this thesis, Electrodeless Resistivity Meter was used to study the following contents, namely the development properties of pastes electrical risistivity curves with different grade of cements and different ratios of water/cement; influences on curves of electrical resistivity development of pastes incorporated with different replacement amounts of fly ash, microsilica powder, steel slag and different addition amounts of calcium lignosulphonate, polycarboxylate at the same ratio of cement/solid. And during the test, Vicat apparatus, SEM, ratio of strength and so on were employed to study hydration mechanism of various conditions as auxiliary methods.The results show that, electrical resistivity method exhibits great superiority in studying early hydration process of cement-based material. Based on the curve of electrical resistivity of paste, we can divide early hydration process of composite Portland cement into four stages: dissolving period, induction-forming and induction period, setting period and hardening period. During the period of dissolving period and induction-forming and induction period, the electrical resistivity of paste is determined by ions concentration of solution and paste porosity; during setting and hardening period, the change of paste electrical resistivity is closely related to development of pore structure, and curves of electrical resistivity development can well reflect the change process of paste microstructure. When the paste is not incorporated with admixture, the electrical resistivity at 1d and ratio of water/cement have power function relationship, and if either of them is known, we can deduce the other; there is a good linear relation between the electrical resistivity at 1d and corresponding compressive strength of 2d, and there are good logarithmic relations between the electrical resistivity at 1d and corresponding compressive strengths of 7d and 28d, therefore, we can deduce later strength rapidly according to the electrical resistivity at 1d.When the replacement amount of fly ash is not less than 40%, there is an abnormal extremum point before the minimum value on the electrical resistivity curve of paste whose ratio of water/binder is 0.5, which is related to the adsorption of fly ash particles and un-burnt carbon powder, and the adsorption mainly occurs at early stage. For mortar, Fly ash's improvement on flexural strength was more than its improvement on compressive strength, and this kind of microsilica can not increase the strength of paste, and it increases the strength of mortar or concrete by improving the interfacial structure between pastes and aggregates.The decrease of cement paste electrical resistivity when incorporated with steel slag is not caused by the conductivity of high-Fe admixture but the decrease of hydration products. The use of high-Fe admixture will not reduce the resistance between cathode and anode of concrete excessively and aggravate corrosion process, and we can use it normally.Electrical resistivity method can be used to identify the activity level of mineral admixtures and determine the proper incorporating amount of chemical admixtures, and it also can be used to study the admixture's mechanism of action when the admixture is incorporated in the cement.