The hydration of binder is an exothermic reaction, which results in an obvious temperature increase in concrete in the early hydration period. The shrinkage of hardened concrete due to the temperature drop is a main cracking trigger of concrete. A kind of temperature rising inhibitor is developed to decrease the hydration heat of binder in early hydration age, which can reduce the cracking risk of concrete. Cyclodextrin is a main functional composition of the temperature rising inhibitor. C₃A is an important clinker mineral influencing the early exothermal characteristics of Portland cement. In this paper, the effect of cyclodextrin on the hydration of tricalcium aluminate-gypsum (C₃A-CaSO₄·2H₂O) was investigated. This work could favor understanding the action mechanism of the temperature rising inhibitor to reduce the cracking risk of concrete structures.

Pure C₃A was calcined, the chemical pure gypsum and cyclodextrin was used. The hydration exothermal curves of C₃A-CaSO₄·2H₂O pastes containing different dosages of cyclodextrin were measured. The hydration products of C₃A-CaSO₄·2H₂O pastes containing different dosages of cyclodextrin in different ages were in-situ determined by quantitative X-ray diffraction (QXRD). The morphology of hydration products on the surface of C₃A particles immersed in different solutions was characterized by scanning electron microscopy (SEM). The etching situation on the surface of C₃A particles washed by different solutions was determined by three-dimensional white light interferometric surface profilometry.

The beginning time of second hydration of C₃A moves up and its exothermic rate decreases, but its reacting time prolongs with the increase of cyclodextrin dosage. The heat output of C₃A during its second hydration stage varies little. The consumption of C₃A and CaSO₄·2H₂O increases continuously and the exhausting time of gypsum reduces with the increase of cyclodextrin dosage. The forming quantity of ettringate in the paste containing cyclodextrin is greater than that in controlling paste. The transformation of ettringate to AFm is suppressed after the exhaust of gypsum. Cyclodextrin can expedite the dissolution of C₃A in CaSO₄ solution to form more deeper etch pits on the surface of C₃A particles, which speeds up the hydration of C₃A. Needle-like ettringite changes to stick-like one, and the transformation of ettringite to AFm restrains when cyclodextrin exists in pastes.

Cyclodextrin could promote the initial hydration of C₃A to move up the beginning of the second hydration of C₃A. The consumption of C₃A and CaSO₄·2H₂O increased continuously in the first hydration stage and the exhausting time of gypsum reduced with the increase of cyclodextrin dosage. Cyclodextrin reduced the reaction speed and prolonged reacting time of C₃A during its second hydration stage, its heat output changed little. Cyclodextrin could enhance the dissolution of C₃A in gypsum solution to form more deep etch pits on the surface of C₃A particles, speeding up the hydration of C₃A. Cyclodextrin could change needle-like ettringate to stick-like one and suppress the transformation of ettringate to AFm.

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