Effect of Cyclodextrin on the Hydration Process of Tricalcium Silicate

Tricalcium silicate (C₃S) is one of Portland cement clinker minerals whose proportion is over 50%. It hydrates quickly and releases lots of heat in early hydration period. The reaction rate and degree of C₃S in early hydration period are the chelloef constituent to determine the exothermal and mechanical performance of Portland cement; as well as the cracking risk of concrete. One of concrete additive, temperature rising inhibitor, was created to suppress the heat release of binder and decrease its exothermic rate. Cyclodextrin (CD) is the main functional composition of temperature rising inhibitor. It is studied that the effect of CD on the early hydration process of C₃S. This work is helpful to understand the action mechanism of temperature rising inhibitor.

Pure C₃S was calcined, chemical pure CD was used. C₃S paste with w/c= 0.5 was prepared. The CD dosage in paste is 0%, 0.05%, 0.10% and 0.20% separately. Dilute C₃S solution with the solid content of 10^–4 and containing 0, 2 mmol/L and 4 mmol/L CD solution was prepared to measure the dissolving rate of C₃S in the induction period of hydration. Pore solution of C₃S was obtained using centrifugal extraction for fresh paste or press method for hardened C₃S paste. The hydration exothermal curves of C₃S pastes containing different dosages of CD were measured using isothermal calorimetry. The time-depended concentration variation of Ca and Si in dilute C₃S solution and pore solution was measured by ICP-AES. SEM was used to investigate the formation of hydrate nucleus on the polished surface of C₃S particles etched by CD solution or pure water. The ion activity product of pore solution relative to C₃S was calculated.

There is not obvious induction period during the hydration of pure C₃S. The induction period of C₃S hydration prolongs with the increase of CD dosage in paste. The accumulate heat release of C₃S is proportional with its hydration degree. The hydration degree of C₃S paste containing CD is higher than blank one at the point of maximal exothermic rate. The accumulate heat release of C₃S paste with or without CD is almost same at the end of induction period. The addition of CD decreases the early hydration speed, but does not negatively influence the final hydration degree. CD inhibits momentarily the dissolution of C₃S in early hydration period, but does not affects the hydration of C₃S in long period. The Ca and Si concentration in pore solution increases with the increase of CD dosage in induction period of hydration. The ion activity product of pore solution relative to C₃S is changed when Cd is added into C₃S paste. CD is adsorbed at the etch pits on the surface of C₃S particles to increase the barrier Δ G_crit of the etch pit expansion model and decrease the growing speed of etch pit. There are lots of etch pit on the polished surface of C₃S particle after 1 h immersion in pure water. There are few etch pit on the polished surface of C₃S particle after 1 h immersion in 1 mmol/L CD solution.

CD prolongs the induction stage and the main reaction stage of hydration, decreases the reaction rate of C₃S. This effect is very intense when the dosage of CD increases to 0.2%. The hydration degree of C₃S paste increases in some extent at the peak reaction rate due to the prolongation of the main reaction period of C₃S paste containing CD. The ultimately hydration heat release of C₃S paste containing CD does not decrease due to the delay of hydration. CD is absorbed on the etch pit of C₃S particles in the initial hydration stage to elevate the energy barrier ΔG_crit of the etch pit expansion model. It decreases the dissolving rate of C₃S and prolongs the induction period of hydration. In the induction period the exist of CD results in an increase of Ca and Si concentration in pore solution and higher degree of supersaturation relative to Ca(OH)₂ and C-S-H. The formation of C-S-H nucleus is inhibited to hinder the acceleration of C₃S hydration