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The reactions involved when cement is mixed with water are complex. It is important to understand these reactions or the cementing operation may not be successful.
Stages of cement hydration reactions
Each phase hydrates by a different reaction mechanism and at different rates (Fig. 1). The reactions, however, are not independent of each other because of the composite nature of the cement particle and proximity of the phases.
In all, five distinct stages have been identified:
- Stage 1: Pre-induction
- Stage 2: “Dormant” (induction) period
- Stage 3: Acceleration
- Stage 4: Deceleration
- Stage 5: Steady state
In cementing operations, the most important of these are Stages 1 through 3. Stage 1 dictates the initial mixability of the cement, and is attributed primarily to the aluminate and ferrite phase reactions. Stage 2 relates to the pumpability time, while Stage 3 gives an indication on setting properties and gel-strength development.
Hydration of pure mineral phases
During hydration, the cement forms four major crystalline phases.
Tricalcium silicate (3CaO•SiO2 = C3S)
C3S on reaction with water produces C-S-H and calcium hydroxide, CH, (also known as Portlandite). The hyphens used in the C-S-H formula are to depict its variable composition: CSH would imply a fixed composition of CaO.SiO2.H2O. C/S ratios in C-S-H vary from 1.2 to 2.0, and H/S ratios vary between 1.3 and 2.1.
Dicalcium silicate (2CaO•SiO2 = C2S)
The kinetics and hydration mechanism for C2S are similar to those of C3S, except that the rate of reaction is much slower. The hydration products are the same except that the proportion of CH produced is about one-third of that obtained on hydration of C3S.
Tricalcium aluminate (3CaO.Al2O3 = C3A)
The initial reaction of C3A with water in the absence of gypsum is vigorous, and can lead to “flash set” caused by the rapid production of the hexagonal crystal phases, C2AH8 (H = H2O) and C4AH19. Sufficient strength is developed to prevent continued mixing. The C2AH8 and C4AH19 subsequently convert to cubic C3AH6 (hydrogarnet), which is the thermodynamically stable phase at ambient temperature. Typically, gypsum is added to retard this reaction, though other chemical additives can be used.
The reaction products formed on reaction of C3A in the presence of gypsum depend primarily on the supply of sulfate ions available from the dissolution of gypsum. The primary phase formed is ettringite (C6AS3H32) (S = SO3). Ettringite is the stable phase only as long as there is an adequate supply of soluble sulfate. A second reaction takes place if all of the soluble sulfate is consumed before the C3A has completely reacted. In this reaction, the ettringite formed initially reacts with the remaining C3A to form a tetracalcium aluminate monosulfate-12- hydrate known as monosulfate or monosulfoaluminate (C4A SH12).
Tetracalcium aluminoferrite (4CaO•Al2O3•Fe2O3 = C4AF)
Hydration of C4AF gives hydration products that are similar in many respects to those formed from C3A under comparable conditions, though typically they contain Fe3+ as well as Al3+. An iron (III) hydroxide gel and calcium ferrite gel are also possible products of C4AF hydration. The reactivity of the pure C4AF is, in general, much slower than that of the C3A.
Hydration of cement phases
Although the basic reaction mechanisms and theories on the hydration of the pure phases pertain to the phases in cement, there are some significant differences. A schematic of the initial hydration reactions up to the time of set is illustrated in Fig. 2.
The alkalis, primarily sodium and potassium, are impurities that arise from shales, clays, or the fuel used in the manufacture of the cement. Although present in small amounts, < 1%, they have a significant effect on the hydration. Typically, they are present as sulfates, in the form of K2SO4 , Na2SO4, Na2SO4•3K2O (aphthitalite), and/or 2CaSO4•K2SO4 (calcium langbeinite), and they are usually deposited on the surface of the cement particles. The alkali sulfates dissolve almost immediately on contact with water, and alkalis can also be present as impurities in the cement phases, with sodium preferentially in the aluminate (C3A) phase and potassium more widely distributed in both calcium and aluminate phases. API Spec. 10A for Class G and H cements limits the alkali to 0.75% as Na2O4 to allow adequate thickening times to be achieved downhole.
In cements high in K2SO4, reaction between K2SO4 and gypsum in the presence of water can produce syngenite, . This can cause lumpiness on storage of the dry cement powder under high-humidity conditions (> 90% relative humidity) because the acts as an effective binder to the dry cement particles. Precipitation of during cement hydration can cause false or even flash setting.
Gypsum is added to the cement primarily to retard the hydration of the aluminate and ferrite phases. The effectiveness of the gypsum depends on the rate at which the relevant ionic species dissolve and come in contact with each other. Thus, interground gypsum is far more effective than interblending the same proportion because intergrinding brings the gypsum particles into closer contact with the cement particles and produces a shorter diffusion distance between the two. Temperature and humidity in the grinding mill can cause the gypsum to dehydrate, resulting in the formation of hemihydrate and/or soluble anhydrite . Hemihydrate or soluble anhydrite can rehydrate to give “secondary” gypsum, causing a rapid set, known as “false set.” Pumpability can be regained on further mixing or addition of water, assuming the quantity of secondary gypsum is not too great.
The reactivity and performance of cement is a culmination of the effect of the different impurities on the number of defects and morphology of the crystal structure of the different phases. This is why cement can vary not only from one source to another, but also between batches from the same source.
Effects of temperature on hydration
The rate of hydration of the cement phases, however, will increase with increasing temperature, and the resulting thickening and setting times will decrease. Above 230°F (110°C), the hydration products formed differ considerably from those obtained at lower temperatures. Alite and belite phases hydrate to give crystalline α-C2SH rather than amorphous C-S-H. α-C2SH is a relatively dense crystalline material that is porous and weak and is deleterious in that it provides high permeability and low compressive strength. Formation of α-C2SH can be prevented, or minimized, by the addition of finely ground silica, such as silica flour, to the cement.
Normally, in oilwell cementing, ~ 35% silica in the form of silica flour is used to prevent strength retrogression that can occur at temperatures above ~ 248°F (120°C). This percentage of added silica gives an effective C/S ratio in the cement blend of approximately 1.0. Generally, over time, the permeability increases slightly, and the compressive strength decreases as the phases increase in crystallinity.
Fly ash has often been considered as a potential source of silica for hydrothermal systems. There is considerable variability in the alumina/silica ratio of fly ashes from different sources, as well as in the reactivity of the aluminosilicate glass, and this clearly has an impact on the phases formed and their stability fields. The influence of this variability in composition and reactivity is that the fly ash, if used as a source of silica, can give properties that range from good to deleterious.
Sulfate attack is normally a problem only where Bottom Hole Statics Temperatures (BHSTs) are below approximately 60°C (140°F), where ettringite is present. Some formation waters contain high concentrations of sulfate. These sulfates attack the cement, and the cement will crumble with time.
Resistance to sulfate attack is increased by modifying the cement powder. Replacing the aluminate with ferrite reduces the amount of ettringite formed during hydration, and also lowers the amount of free lime. Addition of pozzolanic materials, such as fly ash, also reduce sulfate attack because they react with the CH in cement, and render it unavailable for reaction.
- API Spec. 10A, Specification for Cements and Materials for Well Cementing, 23rd edition. 2002. Washington, DC: API.