Portland cement

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.

Portland cement

Physical effects of limestone on the hydration of cement clinkers

Effects of limestone addition on the hydration of limestone blended cement can be classified into two aspects, viz. physical effects and chemical effects.

Numerous researchers who investigated Portland limestone cement with high level of calcite addition regard it as inert filler, e. g. by means of both thermal and BSE image analysis, it was investigated by G. Ye et al. that limestone powder does not take part in the chemical reaction at all.

The low reactivity of limestone due to its low solubility is the main cause leading to the inert property; however, even acting as filler, limestone in blended cement causes an apparent hydration acceleration of cement clinker especially at early ages.

On the one hand, the effective w/c ratio in limestone blended system is increased significantly due to the low reactivity of limestone powder, altering the hydration characteristics of cements and providing more water and space for the hydration of clinkers.

On the other hand, limestone is softer than clinker which could achieve a finer particle size when interground with Portland cement clinkers, producing an improved particle size distribution and improving particle packing, e.g., for an overall specific Blaine surface area of 420 m2/ kg, 50% of the limestone particles can be below 700nm, compared with 3µm for the clinkers;

Therefore, a large number of tiny limestone particles act as nucleation sites for the hydration of C3S (and also C2S ), which then markedly decrease the concentration of Ca and Si ions in the solution phases promoting the transformation of C3S phase to the solution phase, thus the hydration of C3S being accelerated.

Blended cement

Advantage of ternary blended cement

The binary cement, e.g. Portland cement blends with slag or fly ash or limestone, has gained great popularity in many countries. However, each kind of binary cement has its own shortfalls, e.g. the addition limestone can reduce the later strength due to its dilution effect, while the reaction of slag is relatively slow in the early ages.

To utilize materials more effectively and reduce the cost in construction industry, ternary cement that made with Portland clinkers and other two admixtures may be a better option because it presents several advantages over binary cements. With the development of separate grinding and mixing technology in cement industry, it is becoming more convenient to produce these so-called market-oriented or tailor-made cements.

The combination of limestone and slag with Portland cement can help to produce a new cement-based material which has advantage over binary blended cement. An obvious advantage is that limestone contributes to the early strength while slag increases the long-term strength, this leads to an adequate development of strength compared with either slag cement or limestone cement.

To find the strength development of ternary cement with limestone filler and blast-furnace slag, Menendez et al. tested mortar prisms in which Portland cement was replaced by up to 20% limestone and 35% blast-furnace slag. The test at 1, 3, 7, 28 and 90 days shows that the contribution of limestone to the hydration degree of Portland cement at 1 and 3 days increases the early strength of ternary system containing about 5-15% limestone and 0-20% blast-furnace slag. The later hydration of blast-furnace slag is very effective in producing ternary cements with similar or higher compressive strength than Portland cement at the ages of 28 days and 90 days.