Reactivity of Alite and Belite towards Sustainability of Concrete

Dr. S.B.Hegde, Professor, Pennsylvania State University, USA
Dr. S.B.Hegde
Professor, Pennsylvania State University, USA

Alite and belite are the predominant phases of Portland cement formulation. Alite is impure tricalcium silicate (C3S) and belite is impure dicalcium silicate (C2S). The impurities are an integral part as cement is manufactured from natural raw materials (and because alumina and iron are included to form liquids at kiln temperatures to improve the efficiency of kiln reactions). However, the word impurity may have a negative connotation, in this case, impurities are typically a benefit, as they stabilize clinker phases at lower kiln temperatures (reducing kiln energy requirements and related CO2 emissions) and can make the cement phases more reactive when mixed with water.

A range of these impurities are found naturally in cement raw materials and they form clinker phases with a range of compositions and reactivities. Alite and belite account for about 75% by weight of modern cements and dominate the development of properties such as setting time and strength development. Although both are calcium silicates, alite reacts much more quickly than belite when mixed with water. Both alite and belite react to form calcium silicate hydrate (known in cement chemistry shorthand as C-S-H), which is the primary binding material in cement paste, mortar, and concrete.

A Review of Research
The total amount of alite and belite in cements has not changed appreciably in many decades and is limited by process considerations; however, modern cements are produced in kilns with better control of burning conditions allowing higher percentage of alite in cements, which respond to market demands for higher early strength gains permitted by its relatively rapid reactions.

The overall reaction of alite and belite can be described chemically (approximately) as:

C3S + 5.3 +ĺ&1.7-S-H4 + 1.3 CH
C2S + 4.3 +ĺC1.7-S-H4+ 0.3 CH

where H is water (H2O) and CH is calcium hydroxide. Note that the reaction products of both silicates are C-S-H and CH. C-S-H is the main binding phase in Portland cement-based systems and primarily responsible for the strength and durability of concrete. CH is a by-product of the hydration reaction that does not contribute much directly to the strength of concrete (it is a solid, and thus reduces porosity, which of course does improve strength).

Calcium hydroxide is beneficial however as it helps maintain a pH at which embedded reinforcing steel is maintained in a passivated state thereby preventing corrosion. CH also reacts with silicates in supplementary cementitious materials (SCMs) like fly ash to produce additional C-S-H via a pozzolanic reaction. Reducing the quantity of CH resulting from hydration of Portland cements, however, can improve the potential strength and durability of concrete. Belite produces about ¼ as much CH as alite and thus relatively more C-S-H. It might be expected that cement would therefore be composed of as much belite as practical.

However, the early reaction of belite is much slower than alite and therefore a problem for efficient construction cycles. As well, the higher CH contents of cements with higher alite contents may favour use of higher levels of SCMs, which can significantly reduce concrete’s environmental footprint.

Some researchers have suggested that the increased amount of calcium hydroxide (CH) resulting from the hydration of modern cements could lead to reduced durability. Calcium hydroxide is relatively soluble in water and can react with sulfate ions to form gypsum in an expansive reaction, which is a secondary sulfate attack mechanism. Since CH provides relatively little contribution to strength compared to C-S-H, stronger, more durable concrete in some applications might be obtained if the quantity of belite is increased relative to the percentage of alite. However, the slow reaction rate of belite makes it less suitable for use as the primary silicate in cement from a practical standpoint. If the reaction of belite could be accelerated, several benefits might be achievable:
  1. Relatively C-S-H, perhaps increasing concrete sustainability as less cement might be needed to achieve necessary strength and durability.
  2. Since belite forms at lower kiln temperatures and requires less limestone as a raw ingredient in cement manufacture, perhaps 10% to 15% lower CO2 emissions from calcination and fuel burning, as well as lower energy consumption might be achieved.
  3. Improved knowledge about reaction mechanisms could also lead to more reactive alites, which could also improve sustainability. In contrast, several potentially negative effects of higher belite contents in cements might be observed: 1. Relatively less CH would be formed, limiting the use of SCMs, like fly ash and slag, which partially react with CH to form additional C-S-H (although at a slower rate compared to clinker silicates), and 2. Grinding energy may be increased for cements rich in belite. Although belite is slightly “softer,” alite is more brittle. The net result is that alite is generally slightly easier to grind; however, a number of other factors also impact clinker grindability.
Reactivity of alite and belite
The term “reactivity” has different meanings. To a cement chemist, it is a measure of how fast the reactions of cement with water occur. This might be measured in overall terms by techniques such as measuring the heat generated when a paste reacts with water (reactions of the primary cement phases with water are exothermic) or by tracking the concentrations of various ions in solution. It might be determined microscopically by measuring the amount of hydration products that form over time or by determining how much clinker has dissolved after a certain amount of time.

To a concrete technologist, reactivity may refer to how quickly a cement paste, mortar or concrete reaches initial set or gains strength. All of these definitions are related, because they all depend on the multiple simultaneous chemical reactions that occur when cement comes into contact with water.

A general calorimetry curve for alite displays some of the complex characteristics of the reactions of these phases. Stage 0 is usually attributed to wetting of the particles and Stage I is an initial brief reaction period in which perhaps 1% to 2% of alite is reacted. Stage II is referred to the induction period, a time of very low reactivity shortly after mixing with water. This period is characteristic not only of alite, but also Portland cements. The induction period is useful because it provides sufficient working time for concrete or mortar to be placed and consolidated before setting and gaining appreciable strength, which for alite typically lasts a few hours, while for belite it may last a few days.

Stage III is the period when the hydration reactions begin in earnest and setting and strength development take place. During Stage IV the reactions slow while strength development continues. Strength and hydration reactions continue in a slow and decreasing rate in Stage V, provided sufficient water and space for hydration products remain. Each of these stages of reaction can be better understood, and this understanding could lead to improved control and optimization of cement-based materials.

Polymorphism of Alite and Belite
Alite has two common polymorphs (different crystal structures for the same phase): M1-C3S and M3-C3S, both are monoclinic (hence the “M”), which describes the shape of the crystal. These two monoclinic structures are the same crystal shape, but the M1 and M3 forms of alite have different spacing of the atoms in the crystals, possibly due to different types or levels of “impurity” atoms in the crystals making those crystal forms stable. Likewise, Belite also has two, is also monoclinic in shape, while the other, (ɲH’-C2S), is triclinic. Impurities in clinker are often beneficial as they can make the crystals more reactive (and conversely, some can make the crystals less reactive). Thus, impurities can impact alite and belite reactivity in two ways: by stabilizing polymorphs that are more (or less) reactive and by making the crystals of specific polymorphs less (or more) stable, and therefore more reactive.

Other advantages may result from the detailed study of silicate reactivity. Several theories have been postulated to account for the induction period, the period when concrete can be placed and formed/finished before hardening and strength development. Detailed studies of the reaction mechanisms can provide a fundamental understanding of the induction period, and potentially new avenues for the control of this critical period.

Conclusion
Alite reacts relatively rapidly with water and is responsible for most of the early strength development of concretes. Belite is less reactive at early ages, but can contribute appreciably to strength at later ages.

The manufacture of cements with higher alite contents may have a larger environmental impact. Compositionally, increased limestone quantities in the raw meal are required to form alite, which results in higher CO2 emissions from calcination.

If cements with higher belite contents could be developed with strength gain characteristics similar to current cements, cements could be manufactured with a reduced environmental impact, which would result in a lower environmental footprint for concrete as a construction material.

In contrast to this approach is the concept that, because of high initial strength development, cements with higher alite contents can be used in concretes with higher amounts of secondary cementitious materials like fly ash, slag etc. This also has the potential to reduce the environmental impact of cement manufacturing and concrete production. Silicate phases also impact finish grinding energy requirements of cements. More reactive alite and belite could lead to grinding energy savings as coarser cements may be able to be used with no loss of early strength development or other performance characteristics.
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