Chemical reaction of limestone with C3S and C3A
The hydration rate of C3S is accelerated in the presence of limestone probably because of the dilute effect and the multiplied nucleation sites. Jean Pera et al. reported that the total heat resulting from pure C3S hydration was 145 joules while that of the mixture (50%C3S+50%CaCO3) reached 260 joules. Ramachandran’s investigation showed that some calcium carbonate was consumed as the hydration of Portland cement proceeded. The product of the reaction of limestone and C3S is calcium carbosilicate, whose exact type cannot be identified by X-ray diffraction probably due to its amorphous property and small amount of formation.
Besides the reaction with C3S, limestone can reacts with C3A as well. According the investigation of Ingram et al., for a combination of 2% gypsum, 6% limestone, and 92% clinker, CaCO3 reacts with C3A in the clinker. The reaction begins with a C3A·CaCO3·12H2O product, then forms compounds containing a molar ratio of C3A to CaCO3 between 0.5 and 0.25. Later, the product appears to stabilize as C3A·xCaCO3·11H2O, where x ranges from 0.5 to 0.25. Other researchers reported similar results.
Bensted found that in the absence of gypsum CaCO3 in limestone reacts with C3A to form both “hexagonal prism” phase tricarbonate C3A·3CaCO3·30H2O and hexagonal plate phase monocarbonate C3A·CaCO3·11H2O, but the former one tricarbonate is much less stable than AFt at ambient temperature, thus it would not be formed in cement hydration.
The transformation of ettringite to monosulfate in the presence of limestone is delayed and reduced due to the formation of monocarboaluminate. Bentz proposed a possible chemical reaction of this process as follows,
3(CaO)3(Al2O3)·CaSO4·12H2O+2CaCO3 +18H2O →
2(CaO)3(Al2O3)·CaCO3·11H2O + (CaO)3(Al2O3)·3CaSO4·32H2O
G. Kakali et al. concluded that in pastes containing CaCO3, either as a chemical reagent or as a limestone constituent, the ettringite’s transformation to monosulfate is delayed, while calcium aluminate monocarbonate is preferably formed instead of monosulfate even at early ages.
It is should be noted that the formation of ettringite is accelerated by the addition of limestone at the very beginning of hydration (e.g. 30 min) in the presence of gypsum, corresponding with the accelerated conversion of ettringite to monosulphoaluminate due to the incorporation of limestone.