Chemistry is truly relevant for concrete because chemistry controls the life/durability of concrete. It explains why cement hardens and the interaction between cement and its environment. Dr. S.B.Hegde at Udaipur Cement Works, highlights the basic inorganic chemistry of cement and concrete under service conditions.
Chemistry and concrete
When cement is exposed to atmosphere it begins to deteriorate is a chemical reaction. When it comes in contact with water it forms calcium silicate hydrate that gives strength to concrete. This is one aspect of chemistry, but chemistry is not only thermodynamics, it is also kinetics. In other words, concrete has the potential to change, but how fast will that happen? Concrete made carefully with the right materials/proportions will develop good optimum microstructure by adequate curing can last for so many years.
Concrete is inherently reactive subject to exposure conditions that reactivity will result in either excellent durability or poor durability. In both the long and short term, it is the chemistry that makes the difference in concrete’s performance. Examples of typical exposure agents that affect durability are:
- Moisture and ground water
- Temperature cycles
- Marine environments
- Pollutants like CO2, NOx, SOx
Water to cement ratio (w/c) dictates the strength of concrete. Although the size and grades of concrete and the quantity of cement influence the quantity of water in a mixture controls the strength of concrete. Therefore, one has to use smallest quantity of water that will produce plastic or workable concrete. Water may depend on quality, nature of aggregates used and concrete curing history, cause such deleterious effects such as “alkali-silicate-reactivity” (ASR) or “delayed ettringite formation” (DEF). Water may also act as a transport medium for ingress of destructive agents like sulphates to enter into the system.
Temperature affects the rate of chemical reactions and a general rule of thumb is that rate of chemical reaction doubles for every 10°C in temperature. Thus, temperature influences the rate of both concrete setting and hardening. Curing concrete above certain critical temperature may lead to expansion and cracking associated with delayed ettringite formation (DEF).
Several external environmental factors may initiate destructive chemical reactions in concrete, particularly, concrete with more open porosity (due to high w/c). Some of these factors include chlorides etc. or tidal exposure to sea water and sulphate containing soils or ground water. Chlorides can slowly diffuse into concrete in presence of moisture and oxygen will initiate corrosion of reinforcing steel. The oxidation of iron to produce iron oxide is a chemical process that gives large volumes of oxidation products, which makes the structure weak as it creates localized pressure that can cause severe cracking of concrete. Once cracking begins more surface of concrete surface is exposed to further chemical attack.
Most vulnerable part of concrete is the cement paste. Although, concrete is comprised of 10-15 % by mass of cement, it becomes the focus point for aggressive chemical agents like atmosphere, carbon dioxide etc. that dissolve in moisture. Cement paste is highly alkaline with pH of around 12.5-13.5. This high pH is due to the presence of calcium hydroxide and hydration products and the lesser amounts of alkali salts. Under ideal carbonation conditions (50-70 % relative humidity and exposed surface of cement paste) the hydrated lime constituents reacts with carbon dioxide to form calcium carbonate. As this process slowly progresses perhaps at even rates of only a mm or less per year, the pH is gradually lowered, and finally crystalline calcium carbonate replaces the hydration products.
During designing of a concrete mix, chemistry should be considered. For instance, if the concrete is going to be placed where it has to expose to aggressive environments such as chlorides and sulphates in that situation, only particular type of cement to be used which must be resistant to such aggressive environments.
It is true that not only mix design but also the curing of concrete is very important. Curing of concrete is a process that provides sufficient moisture and thermal energy to promote the hydration phenomena. Curing conditions provide the strength development and thermal cracking: therefore, they have significant impact on durability of concrete. The classic example showing importance of curing and temperature control is concrete deterioration from delayed ettringite formation (DEF). Ettringite formation is normal and useful event as cement begins to set. If its formation is greatly delayed (days or months of concrete hardening) however, it can cause serious durability problems.
Chemical reactions of cement
Portland cement contains calcium silicates and calcium aluminates formed by sequence of thermal and chemical processes including decomposition of limestone: reactions with additives such as clay, iron ore, bauxite etc; fusion of these ingredients and finally the formation of hard, rounded nodules called clinker. This will be formed at a temperature of 1350-1450°C in a rotary kiln. After cooling, clinker is ground together with approximately 5% gypsum.
At this stage we will review some elementary cement chemistry. The present knowledge of chemical composition of Portland cement and what happens to it when it is mixed with water was first disclosed in 1887 by French chemist by name Henry Le chatalier. In his doctoral thesis he correctly identified the major cement minerals. In 1915 scientists at Geo physical laboratory, Washington D.C. was studying the high temperature phase relationships of ternary system CaO-SiO2-Al2O3. Among the mineral phases investigated were-C3S, C2S, C3A, C4AF. The abbreviation used were tricalcium silicate (Ca3SiO5) or 3CaO.SiO2; C=CaO, S=SiO2, A=Al2O3. Accordingly, 3CaO.SiO2 could be written as C3S, similarly, C2S,C3A and C4AF. The abbreviations for oxides are Fe2O3=F, MgO=M, H2O=H, Na2O=N, K2O=K and S=SO3. Tricalcium silicate and dicalcium silicates with impurities are called alite and belite respectively. Other phases like magnesia is called periclase, calcium hydroxide is called Portlandite and ettringite for calcium almunio silicate hydrate.
Figure 1: Micrograph of SEM of ettringite formationNormal hydration reactions
The most rapid reaction that occurs when mixing cement and water is the hydration of C3A.Entirely by itself C3A and water will quickly form Calcium aluminates hydrates such C4AH13 and C12AH8. This can occur so rapidly that the cement/concrete may set within minutes and became entirely unworkable. This condition is called flash set. Chemically the C3A, gypsum and water would form a protective coating of calcium sulpho aluminate hydrate (ettringite) over exposed tricalcium aluminate surfaces that would remain for several hours. C3A hydration reactivates as the initial strength begins consuming sulphates and forming ettringite If C3A contains more than 8% which is much greater percentage than sulphate from some of the ettringite (trisulphate) to form another stable calcium sulphoaluminate compound called monosulphate C3A.CSH12. A fourth major mineral in cement are the ferrite phases or tetra calcium alunino ferrite (C4AF) also hydrates although much more slowly to form chemically similar trisulphates and mono sulphate compounds in which iron replaces a portion of the alumina, cement chemists generally called these Aft (aluminate ferrite trisubstituted) and Afm (aluminate-ferrite-monosubstitued) phase respectively.
C3A + 3CS H2 + 26 H = C6AS3 H32 (Ettringite) - (1)
2 C3A + C6AS3 H32 + 4H = 3C4ASH12 (monosulphoaluminate) - (2)
The major strength development of concrete however results from the hydration of Ca-silicate phases. Both of these Ca-silicates combine with water to form gel like silicate hydrate or C-S-H
2 C3S + 6 H = C-S-H + 3 H - (3)
2 C2S + 4 H = C-S-H + CH - (4)
The most common type of chemical attack on concrete results from exposure to soils or ground water containing higher SO4 contents. The mechanism of SO4 attack is simple. If cement contains higher C3A a substantial amount of Ca-monosulphate (Afm) phase will form during hydration. This substance is reactive and if additional SO4 from an outside source such as soil or groundwater penetrates the concrete, the monosulphate will readily react with it and convert back to Aft phase or ettringite. The conversion of monosulphate phase into ettringite will result in a significant volumetric increase and be disruptive to the concrete. If this is allowed to continue the concrete will eventually be destroyed.
Chemistry and cracking
Cracks in concrete may develop for physical reasons such as drying shrinkage or mechanical loading. Local chemical reactions in the concrete, however, may also result in expansion; buildup of internal pressure and then cracking. Concrete is a brittle material and therefore can only expand to a limited degree before cracking. It is not possible to determine the cause of expansion and cracking from the appearance of the crack pattern on the surface of the concrete. Interior samples should be examined microscopically, chemically or both to determine the root cause of the internal expansion. There are two modes of concrete expansion:
- The aggregate can expand relative to the cement paste
- The cement paste can expand relative to the aggregate.
Shrinkage of cement paste is a common phenomenon related to hydration, and from a cracking point of view, is equivalent to the expansion of aggregate particles.
Figure 2: Alkali aggregate reaction in concreteAlkali silica reactivity (ASR)
Chemical reaction called ASR takes place between the highly alkaline (very high pH) pore solution and reactive siliceous portions of some aggregate particles. The large amount of hydroxyl (OH) ions present in the pore solution due to high alkali concentration dissolve the reactive silica in aggregate surface to form an alkali silicate gel. Although any form of silica can react with alkali hydroxides, theoretically. It is the siliceous rocks such as opal, graywacks, chert and glassy volcanic materials that appear to be most reactive. Reactive siliceous aggregates will form alkali-silica gel starting at the surface of the aggregate and moving inwards.
Tensile stresses build up during the reactions causing aggregate particles and surrounding paste to crack; the paste between cracks maintains its composition and strength. Hard, poly crystalline rocks like granite will react with much more slowly. The chemical reactions occur at those heterogeneous areas of grain boundaries. In such cases only minimum degree of reactions may be needed to cause cracking, but only meager amount of gel will form. In ASR distress since in each internal fracture of concrete creates an empty space that the alkali silica reactions cause corresponding increase in volume. The resulting visual evidence of the reaction is observed on the concrete surface.
Figure 3: Delayed ettringite formation in concreteDelayed ettringite formation (DEF)
ASR is an example of chemical reaction in which aggregate portion of the concrete plays a role in deterioration of mechanism. Reactions related to sulphates are a group of reactions that involve only the cement paste. Delayed ettringite formation (DEF) reactions that are associated with concrete exposure to high temperatures during curing.
In DEF affected concrete, the aluminate ferrite tri substituted (Aft) phase or ettringite is usually observed. But ettringite in concrete is not unique to DEF. Ettringite is normal hydration product formed by the chemical reaction between aluminate phases of cement, water and calcium sulphate (gypsum). The formation of ettringite takes place in the paste and is uniformly distributed. Within mature concrete exposed to moist conditions, ettringite is usually found in pores and cracks. This is not indication of damage but rather the result of a normal recrystallization process known as “Oswald ripening”. This means small crystals have higher solubility than large crystals, when concrete becomes water saturated to contain degree the smaller crystals within the
Figure 4: SEM of calcium sulphoaluminatePaste dissolves in the pore liquid and subsequently recrystallizes as large crystals in any available spaces such as cracks and pores. Oswald ripening is a general chemical principle and calcium hydroxide crystallization behaves in a similar fashion. Regarding DEF, a high concrete temperature at an early age is very important parameter. At certain temperatures generally above 70°C and more frequently above 80°C ettringite becomes unstable because its solubility increases. This temperature is strongly dependant upon the alkali content and other compositional factors of cement that are less understood. Where the components of ettringite go after its decomposition is not clear. Portion of ettringite may be consumed by the C-S-H or may stay in solution. This issue is of great interest for further research. One sign of paste expansion is the presence of voids or cracks around the aggregate particles. Usually, ettringite fills these gaps. Above certain temperatures ettringite is unstable and the primary hydrated aluminate phase is calcium mono sulphoaluminate (Afm).
Therefore, after cooling to room temperature following heat treatment, concrete will contain anhydrous aluminates particles with Afm and Afm phases. During the passage of time and moist curing of such concrete these particles will continue to react with sulphate in pore solution. Both ettringite and Afm phases form depending upon the composition of pore solution. The sulphate liberated from the C-S-H that initially absorbed it during heat treatment maintains the sulphate concentration of pore solution.
The hardened paste confines the reacting particles and the volumes of mono sulpho aluminate (Afm) and ettringite formed will result in development of localized pressure. Crystals under pressure will have higher solubility than crystals were not under pressure. When more and more Aft and Afm phases are formed on the reacting particles, the pressure will act locally on the particles, the pressure will act locally on the particle and surroundings. In this way reacting particles can act as a local pressure center. This will cause stress to build up in surrounding paste as sort of “sphere influence” around the particle. If pressure created is larger than the tensile strength of the paste, the paste will create or yield. If reacting particles are close to each other mass volumetric expansion will result.
Conclusion
Chemistry is very important because concrete composition and performance are based on variety of chemical reactions that ranges from the original setting and hardening of Portland cement constituent to the ultimately desired engineering properties. The durability of concrete depends on chemical processes developing out of cement and aggregate compositional factor, curing conditions, and exposure to a variety of environmental effects. The chemical reactions that occur during the hydration of chemical minerals determine the concrete microstructure. The hardened concrete is chemically reactive given the right conditions. Therefore, it is essential to design concrete mixes properly and erect structures in a way to control on chemical activity/corrosion.
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