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.