Sulfobelite cements

Table 4.1 shows the phase composition of sulfobelite cements as developed by different investigators. The amounts of the individual phases in the clinker may vary in the following ranges ... 

Table 4.1 Composition of sulfobelite cements developed by different investigators.

Sulfobelite cements may perform either as non-expansive high early strength cements, or as expansive cements, depending on the proportion of the individual phases in the cUnker and the amount of interground calcium sulfate. 

The terminology of various sulfobelite cements is rather confused. Tentatively the following designations are being suggested ... 

Belite (dicalcium silicate C2S) is usuahy the most abundant phase in sulfobelite cements. Its amount in sulfobelitic clinkers may vary over a wide range. It is usuahy present as -C2S, but may sometimes also be present in its a modification. Behte contributes only little to the strength of the cement after short hydration times, but is the phase mainly responsible for the ultimate strength. 

To obtain sulfobelite cement the clinker that is produced must be ground to a specific surface area similar to that of ordinary Portland cement. Gypsum or anhydrite must be added only if the amount of anhydrite in the clinker is insufficient. The grindability of sulfobelitic clinkers is better than that of typical Portland cement clinkers. 

At equal starting water/cement ratios and degrees of hydration the porosity of pastes made from sulfobelite cements is distinctly lower than that of ordinary Portland cement pastes. This is due to the higher amount of water bound within the crystalline lattice of the solid phases formed in the hydration of the former cement, especially in that of the AFt phase. 

The development of strength and the extent of expansion will depend on the composition of the cement and the proportions of the phases involved. Within reasonable limits and at appropriate calcium sulfate contents the early strength will increase with increasing amounts of the sulfoaluminate phase in the cement and— to a lesser extent— with increasing content of the ferrite phase. However, the belite phase will affect maiidy the final strength of the cement. Table 4.2 shows the effect of cement composition on the compressive strength development of a series of sulfobelite cements. 

Table 4.2 Effect of composition of sulfobelite cements on compressive strength development.

It has been claimed (Huo et al., 1996) that the compressive strength of sulfobelite cement may be increased by up to 35% by exposing the mixing water to a magnetic field before mixing, but this claim has not so far been confirmed from other sources. 

Non-expansive sulfobelite cements exhibit good chemical resistance in contact with... 

Heat liberation in the hydration of sulfobelite cements is characterized by two peaks within the first few hours, attributed to the formation of gypsum from anhydrite and the precipitation of ettringite (Sudoh et al, 1980 Sahu et al, 1993). In spite of rapid heat liberation at the beginning of hydration, due mainly to the formation of ettringite, the total amount of heat liberated in the hydration of sulfobelite cements is lower than that of Portland cement, and amounts to about 200 kJ/kg (Su et al, 1997). This is due to the absence of C3S and C3A in this cement, which are phases whose hydration is highly exothermic. The overall heat of hydration of sulfobelite cement will decline with increasing belite content. 

Table 4.3 Strength properties of blended high-iron sulfobelite cements...

Toreanu et al. (1986) produced a white sulfobelite cement by omitting Fe203 completely from the raw meal, and by increasing its MgO content to 10% to increase its bumability. 

Sulfobelite cement is more sensitive to carbonation than ordinary Portland cement In addition to carbonation as it exists in ordinary Portland cement, which involves the poitlandite and the C-S-H phases, the ettringite present in the hardened paste may also react with the COj in the air, yielding calcite or aragonite, gypsum and alumina gel (Beretka et a/., 1992, 1997 Sherman et a/., 1995) ... 

Sulfobelite cements do not meet the requirements of most specifications for Portland cement, because their SO3 content is higher than permitted. However, special standards for sulfobelite cements were issued in China, and about a million tons of this type of cement are produced annually in that countiy (Zhang et al, 1999). 

Portland cement Belite cement Sulfobelite cement Sulfoalite cement Phase composition of clinker (%)... 

The ettringite that is formed exhibits distinet eementing properties, but may cause expansion at a different rate and to a different degree, depending on the Al source involved in the reaction. Cements that eontain significant amounts of calcium sulfate include expansive cements (see seetion 21), regulated set cement (see section 5.2), supersulfated cement (see section 8.4), sulfobelite cement (see section 4.2), and sulfoalite cement (see section 4.3). Limited amounts of ealeium sulfate are also present in Portland cement (see section 2) and blended eements (see section 7). 

Unlike belite eements, sulfobelite cements tend to exhibit a fast initial rate of hydration heat release, owing to a fast hydration of the C4A3 S phase, and do not usually qualify as low-heat cements. 

Sulfobelite cements contain belite ( -028), calcium sulfoaluminate and calcium... 

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