Study On Early Age Cement Hydration Behavior Using Electrodless Cement And Concrete Resistivity Tests

Behavior of early age hydration of cement and concrete research is of special significance, but its complexity make researchers feel arduous to do it well, so new methods are continually invented. Electrodless Cement and Concrete Resistivity Tests (abbr. CCR) is a new method invented in recent years, it has virtues of NDT (Non-Destructive Test), and can observe and research the early age cement and concrete not only from a physical standpoint but also from a chemical standpoint. In this paper, firstly, CCR was used to research the cement hydration and the resistivity curve was expatiated; then with the results of the hydration research, setting time determination and cement strength prediction were conducted; finally, CCR was also used to research the interaction of cement and plasticizer.Results show that the resistivity curve can describe the hydration course well, based on the resistivity curve and derivate curve of resistivity, the hydration course of Portland cement can be divided into four periods, they are the dissolution period , induction period, accelerating period and decelerating period.XRD and DSC/TG and other methods were used to analysis the mineral composing and Ca(OH)₂ content. Results show that resistivity and content of Ca(OH)₂ have a linear relationship after entered into induction period, so the resistivity reflects the hydration degree. The peak of derivate curve of resistivity appearing in the accelerating period is caused by the transformation of AFt to AFm. Induction period begins with a sharp rise of resistivity and a sudden decrease of Ca²⁺ concentration, DSC/TG analysis indicated that some substances were absorbing heat and losing weight at temperature of 411℃, encapsulation effect of hydration products may caused the induction period. It is clear that the derivate curve of resistivity have a good relationship with the curve of hydration heat rate, for both of them belong to the rate category.Results also show thatρ_min, minimum of resistivity, and T_m in, time of minimum resistivity, can be used for testing cement setting time, and the setting time tested by CCR method have a relationship with the setting time tested by Vicat method, the pertinence coefficient is higher than 0.9. Various kinds and classes of cements'resistivity hydrated for 24 hours and their 28-day compressive strength were tested. Results show good linear relationship between 24-hour resistivity and 28-day compressive strength, and CCR method can thus be used for accelerated strength testing.When a naphthalene-based super-plasticizer FDN was used to research the interaction of plasticizer and cement, it is found that the curve of initial resistivity versus dosage was compounded by a"L"type curve and a"S"type curve, the"L"type curve reflects the adsorption effect and the"S"type curve reflects the dissolving and ionization effects. The fluidity experiments proved that the best fluidity of slurry with FDN were caused by both adsorption effect and dissolving effect. It is amazing that when the Calcium Lignin Sulfonate was used to be a plasticizer, the dosage of the largest initial resistivity is equal to dosage of the best fluidity. This result means that the resistivity method can be used to test the saturated dosage of Calcium Lignin Sulfonate. It is also found that when the dosage is larger than the dosage of the largest initial resistivity, the resistivity curve of hydration was fundamentally changed, it may because when the plasticizer dissolved in solution, it may affects the hydration products'solubility, nuclear formation and crystallization.