CHEMISTRY AND QUALITY CONTROL FORMULAS IN THE CEMENT INDUSTRY
From a technical and commercial point of view, Portland cement is by far the most important of the so-called “hydraulic” cements. Initially named because of its ability to set and develop strength under water, hydraulic cements are mineral materials that, in the form of a fine powder, react with water, generate heat and form a dense and strong mass of exceptional low solubility.
Portland cement is the result of a very fundamental technology and where a complex combination of different raw materials is applied. It consists of heating the pulverized raw material to 1450 °C, considering a very specific composition to produce an intermediate product called Portland cement clinker or simply clinker. The clinker is composed of 4 minerals that react with water to create strong mechanical bonds. These 4 minerals are: tricalcium silicate (50 to 60%), dicalcium silicate (20 to 25%), tricalcium aluminate (6 to 10%) and tetracalcium ferroaluminate (6 to 10%). Some gypsum is added to the clinker before final grinding to control the hydration of tricalcium aluminate. The morphology and fineness of the cement particles represent very important characteristics.
Portland cement is manufactured by crushing, grinding and dosing different types of materials:
- Lime or calcium oxide, CaO: limestone, calcium carbonate, shale or rock calcareous
- Silica, SiO2: sand, old bottles, clay or clay rock
- Alumina, Al2O3: bauxite, recycled aluminum, clay
- Iron, Fe2O3: clay, iron ore, scrap and fly ash
- Gypsum, CaSO4.2H20: found together with limestone
Main active compounds in Portland cement:
Only representative weights. Actual weight varies with the type of cement.
Due to the complex chemical nature of cement, an abbreviated form is used to denote chemical compounds.
The Portland cement clinker must be ground until a fine powder is obtained before it can be used as a building material. When mixed with water, a series of chemical reactions occur that convert the four clinker minerals present into hydrated minerals. Hydrated minerals occupy a larger volume than clinker minerals and they interlock to form a dense mass which bonds sand and aggregate particles together. A mixture of cement and sand, of the type used to bond bricks, is known as mortar, and a mixture of cement, sand, and aggregates (crushed rock) is known as concrete.
The properties of cement and mortar or hardened concrete are influenced by the fineness of grinding and the proportions of the different clinker minerals in the cement.
Clinker mineral hydration
Table 1 summarizes the different reactivities and strength properties of the four clinker minerals. C3S is the main strength mineral in Portland cement, and most Portland cements manufactured in developed parts of the world will contain more than 45 % of this mineral. The main product of the reaction with water is a non-crystalline material known as calcium silicate hydrate. This gel-like material covers sand particles and aggregates and is the “glue” that binds the mortar and concrete. The ratio of calcium to silicon in C-S-H is lower than in C3S (1.8:1 compared to 3:1). Consequently, when C3S is hydrated, it releases excess lime in the form of calcium hydroxide (Ca(OH)2). Unlike C-S-H, Ca(OH)2 appears as relatively large crystals that do not contribute significantly to strength.
CLINKER MINERAL HYDRATION
The lower coated silicate, C2S is much less reactive, but contributes significantly to resistance beyond 14 days under moist curing conditions.
The aluminate phase, C3A, is extremely reactive, and its reaction rate must be controlled by the addition of calcium sulfate during cement grinding, usually in the form of gypsum (CaSO42H2O). The soluble calcium sulfate supplied by gypsum results in the formation of a coating of a hydrated mineral known as ettringite, on the surface of C3A particles. This protective layer reduces the reaction rate of the C3A and allows the mortar and concrete to be placed before setting occurs. Calcium sulfate, therefore, acts as a retarder. The form of calcium sulfate in cement can have a marked influence on the properties of the cement when it is first mixed with water
The ferrite phase,C4AF, is quite unreactive, but ultimately forms hydrates that are similar to those formed by C3A.
Figure 1 schematically illustrates the sequence of reactions that ultimately results in the formation of dense hardened concrete.
The water content of a mortar or concrete is typically considered in terms of the ratio of water to cement (w/c). The normal range for concretes is 0.4 to 0.7. The higher the w/c ratio, the more porous the mortar or hydrated concrete will be, and the lower the resistance. Low w/c ratios require a high cement content (~ 350 kg of cement per m³ of concrete) or the use of chemical additives to disperse cement particles and lubricate the mixture.
PORTLAND CEMENT HYDRATION
CONCRETE MIXED WITH WATER
CONCRETE STARTS TO SET
Figure 2 illustrates the relative reaction rates of the four clinker minerals. The reaction percentages are only approximate and are strongly influenced in practice by the fineness of the grinding of the cement and the presence of impurities in the minerals, which can alter its crystalline form and its reactivity.
The reaction of cement with water represents an exothermic chemical reaction, that is, it releases heat. This can be an advantage in cold climates by placing concrete in moderately sized spills, but it presents potential problems in large concrete spills, especially in hot climates.
Figure 3 illustrates the influence of the concrete size that is poured on the temperature at the heart of the concrete. In very large pores with high cement contents temperatures of up to 80 °C can be reached. The phenomenon results in lower final resistance and can also cause thermal cracking, as a result of the temperature difference and, therefore, of expansion, between the center and the exterior of the concrete pouring.
ROLE OF GYPSUM AND FORMS OF CALCIUM SULFATE IN CEMENT
As described in the previous section, calcium sulfate is interground with clinker to control the initial hydration reactions of C3A.. If the clinker is ground without gypsum, or if not enough gypsum is added, the cement will flash set. The C3A. reacts quickly, releasing heat, and the concrete or mortar mixture hardens. More water must be added to recover the workability of the mixture, and the result is poor quality concrete.
The gypsum used by the cement industry is normally not pure CaS04.2H2O. The anhydrous form of CaSO4, known as anhydrite, is normally present, along with impurities such as clay and limestone (calcium carbonate).
Cement grinding in ball mills represents a relatively inefficient process, and most of the electrical energy that enters the mill motor is converted into heat within the mill. Although the mills are cooled, either externally or internally by water, temperatures above 1OO °C are easily reached. Under these conditions, the aggregate dihydrate gypsum loses water and progressively becomes hemi (half) hydrate and soluble anhydrite. The sequence of changes is illustrated in Figure 4.
Dehydrated forms of gypsum dissolve in water faster than dihydrate. This can be both advantageous and disadvantageous. The advantage is that the ready supply of soluble calcium sulfate can control the reaction rate of finely ground C3A reactive forms. The disadvantage is that, if there is too much dehydrated gypsum present, a supersaturated solution is produced, from which gypsum crystals can be formed that produce a so-called false setting. The different solubilities of the different forms of calcium sulfate are illustrated in Figure 5. The formation of solids in the experiment with an initial concentration of 50 mmol l-1 of CaSO4 and equilibrated at 12 ° C; (I): formation of small primary entities / dispersers as evidenced by the change in I (q) ∞q – 1 for q> 1 nm – 1 and I (q) ∞q0 for q <1 nm – 1 (pink and orange dotted lines); (II) development of a structure factor; (III) the formation and development of large dispersion characteristics evidenced by the increase in intensity at q <1 nm-1; this change followed a dependency on I (q) ∞q – 3> a> −4 (where a is the exponent: green dashed line); (IV) the growth of the primary species is manifested aq> 1 nm – 1 by the displacement of the dispersion curves towards lower q values and the gradual decrease of the intensity towards I (q) ∞q – 4 (blue dashed line ); The box shows a selected dispersion curve and indicates the meaning of the dependence I (q) of the dispersion exponents q (dashed pink, orange and blue lines) that indicate the characteristic features of the dispersion as described in Figure 5.
In order to obtain the optimum properties of the cement with a minimum demand for water and a controlled setting behavior, it is necessary to match the level of easily soluble (dehydrated) calcium sulfate with the reactivity of the clinker. This approach is illustrated in Figure 6. The SO3 present in the cement as hemihydrate (CaS041/2H20) and the soluble anhydrite are called D.SO3.
INFLUENCE OF ADDITIVES
Most of the concrete produced contains at least one, and sometimes a combination of chemical additives. The main types of additives are,
- The normal range of water reducers (plasticizers)
- High-end water reducers (superplasticizers))
- Strength development
- Air entrainment agents
It is not practical for the cement producer to test the cement in combination with all the different chemicals that can be added to the concrete or mortar mixer. The mixtures modify the initial hydration reactions and, sometimes, it can happen that the cement that shows a well-controlled initial rheology in standard laboratory tests undergoes rapid stiffness when used with a certain type of mixture.
This problem of additive incompatibility seems to occur more frequently when a cement with a fairly low level of DSO3 is used with a water reducing agent.
Flash set: this is due to the lack (or absence) of adding calcium sulfate (gypsum/anhydrite) to the clinker. This addition is useful for “diverting” the natural hydration pathway of the aluminate phases of the hydrogarnets and other hydroaluminates towards ettringite. The hydroaluminatos induce the adjustment of the flash: a fast and exothermic hardening in a matter of minutes. Ettringite and its counterparts induce a much smoother hydration path, easier to handle in terms of workability.
False set: this is usually due to the nature of the added calcium sulfates, specifically too much calcium sulfate hemihydrate (bassanite or, more commonly, gypsum). Calcium sulfates come in various degrees of hydration: dihydrate (gypsum), hemihydrate (bassanite), anhydrous (anhydrite). When an excessively high amount of hemihydrate is present, it simply follows its own hydration path to the precipitation of the gypsum, which leads to an early setting of the cement. The hemihydrate can be formed in the hot spots of the mill during the joint grinding of clinker- gypsum. 120-140 °C is sufficient.
HYDRATES PRESENT IN HARDENED CONCRETE
When there is an abundant supply of CaSO4, since, during the first stages of hydration of a well retarded cement, the C3A phase reacts to form the C C3A,CaSO.32H2O ettringite hydrate. In most types of cement that have C3A contents in the normal range of 7-10 %, the supply of gypsum or anhydrite CaS4O is depleted in 1-2 days and the C3ACaSO4.12H2O hydrate monosulfate is formed as others C3A hydrates. Normally, in about 7 days, all the ettringite will have become monosulfate.
Figure 7 describes the components of a typical hardened cement paste. It can be seen that the largest volume is occupied by the C-S-H gel, followed by approximately equal volumes of calcium hydroxide and monosulfate.
This combination produces a strong and durable concrete that should work well for hundreds of years. However, if additional sulfate ions enter the concrete from the external environment, for example, from groundwater, the monosulfate can again become ettringite. Conversion is associated with expansion, and concrete weakens severely as a result.
QUAILITY CONTROL FORMULAS
H F W Taylor was for many years Professor of Inorganic Chemistry at he University of Aberdeen, Scotland. Since 1948, his main research interest has been the chemistry of cement. His early work laid the foundations of our understanding of the structure at the nanometre level of C-S-H, the principal product formed when cement is mixed with water, and the one mainly reponsible for its hardening. Subsequent studies took him into many additional aspects of the chemistry and materials science of cement and concrete. His work has been recognized by Fellowships and by other honours and awards
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