Tag Archives: Clinker

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.

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.

Brief introduction of limestone cement

Limestone has been utilized as a mineral addition in cement industry for a long time. The latest European Standard (EN 197-1-2000) allows up to 5% limestone as a minor additional constituent, and also identifies four types of Portland limestone cement containing 6-20% limestone (types II/A-L and II/A-LL) and 21-35% limestone (types II/B-L and II/B-LL), respectively. The ASTM Standard (C150-04) allows up to 5% of limestone filler. Since the early 1980s, the Canadian cement standard has allowed the inclusion of up to 5% limestone addition to Portland limestone cement.

Similar trend could also be found in Latin-American countries such as Argentina, Brazil, and Mexico (Menendez). According the review Cement Standards of the World (CEMBUR-EAU 1997), more than 25 countries allow the use of between 1% and 5% limestone in their Portland cement. Many countries even allow up to 35% replacement in Portland composite.

limestone-cement-composition
Compositions analysis of several typical limestone

The main composition of limestone is calcite (CaCO3), with smaller proportion of quartz or amorphous silica and sometimes of dolomite. Table above is composition analysis of several typical limestones. Though the LOI content of limestone is up to 42.5% or even more, they must be low in clay minerals and organic matter to guarantee the water demand and setting time. As for the quality requirement, according to ASTM Standard (ASTM C-150-4), the calcium carbonate (CaCO3) content calculated from the calcium oxide content should be at least 70% by mass.

Limestone is softer than cement clinkers, thus could be grind into finer particles. When it interground with clinkers, for an overall specific surface area (Blaine) of 420 m2/kg, 50% of the limestone can be below 700nm, compared with 3 µm for the corresponding clinker. Addition of finer limestone particles to clinker completes the fine fraction in the granulometric curve of cement without an increment on water demand, and improves the cement packing and blocks the capillary pores.