Hydration of Cement

1. University Of SalahaddinCollege Of EngineeringCivil Engineering Dept. ADVANCED CONCRETETECHNOLOGYMSc STRUCTURES 2018 -2019 SEMESTER 1

2. In cement chemistry, the term 'hydration' denotes the totality of the changes that occur when anhydrous cement, or one of its constituent phases, is mixed with water. (H. F. W. Taylor) Portland cement hydration • Introduction The hydration of Portland cement involves the reaction of the anhydrous calcium silicate and aluminate phases with water to form hydrated phases. These solid hydrates occupy more space than the anhydrous particles and the result is a rigid interlocking mass whose porosity is a function of the ratio of water to cement (w/c) in the original mix. Provided the mix has sufficient plasticity to be fully compacted, the lower the w/c, the higher will be the compressive strength of the hydrated cement paste/mortar/concrete and the higher the resistance to penetration by potentially deleterious substances from the environment. Cement hydration is complex and it is appropriate to consider the reactions of the silicate phases (C3S and C2S) and the aluminate phases (C3A and C4AF) separately. Hydration of Cement

3. Upon the addition of water, calcium silicate reacts to release calcium ions, hydroxide ions, and a large amount of heat. The pH quickly raises to over 12 because of the release of alkaline hydroxide (OH-) ions. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved. The C-S-H produced is the principal binding phase in Portland cements and is quantitatively the most significant hydration product. The equation for the hydration of calcium silicate is given by: Calcium silicate + Water Calcium silicate hydrate + Calcium hydroxide + heat 2C3S + 6H2O C3S2H3 + 3Ca(OH)2 2C2S + 4H2O C3S2H3 + Ca(OH)2 Hydration of silicates (C3S and C2S)

4. So both silicates required approximately the same amount of water, but C3S produces more than twice as much Ca(OH)2 as is formed by the hydration of C2S. C3S & C2S are the main constituents that occupying cement therefore, the physical properties of the calcium silicates hydrate are of interest in connection with the setting and hardening properties of cement. The behavior of each type of cement depends on the content of these components. The hydration reactions of C3S and C2S are summarized in the following table: (Hydration of calcium silicates)

5. Monosulphate Ettringite ettringite A B C D E C3A C3A • Hydration of tricalcium aluminate (C3A) The C3A present is comparatively small but its effect and behavior is important. The stable form of the calcium aluminate hydrate is probably the cubic crystal C3AH6, but it is possible that hexagonal C4AH12 crystallizes out first and later changes to cubic form. Pure C3A reaction with water is very violent and leads to immediate stiffening of the paste known as flash set. Gypsum (CaSO4.2H2O) is added to cement clinker, thus, to retard the reaction. Hydration of C3A occurs with sulfate ions supplied by dissolved gypsum. The result of the reaction is calcium sulfoaluminate hydrate, called "ettringite" after a naturally occurring mineral. C3A + 3CaSO4 + 26H2O 3CaO.Al2O3.3CaSO4.32H2O Ettringite 3CaO.Al2O3. CaSO4.12H2O Monosulphate C3A C3A (A schematic description of retarded setting caused by sulphate)

6. Thin layer of ettringite is formed around C3A. • Formation of ettringite directly on C3A grains producing crystallization pressure. • Bursting of ettringite • Burst section is sealed by newly formed etteringite • Insufficient sulphate ions to allow formation of ettringite, on further hydration of C3A, Ettringite converts to monosulphate. If all gypsum is consumed before all the C3A, the direct hydrate, C3AH6 is formed. C3A + 6H2O C3AH6 + heat There is some evidence that the hydration of C3A is retarded by Ca(OH)2 liberated by the hydration of C3S. This occurs due to the fact that Ca(OH)2 react with C3A and water to form C4AH19, which form a protective coating on the surface of unhydrated grains of C3A. The presence of C3A in cement is undesirable because of: • It contributes little or nothing to the strength of cement except at early age. • When hardened cement paste is attacked by sulphate, expansion due to the formation of calcium sulphoaluminate from C3A may result in a disruption of the hardened paste.

7. However, C3A acts as a flux and thus reduces the temperature of burning of clinker and facilitates the combination of lime and silica, for those reasons, C3A is useful in the manufacture of cement. • Hydration of tetracalcium aluminate ferrite (C4AF) Ferrite Phase (C4AF) acts as a flux and during hydration forms Calcium Sulphoferrite and calcium sulphoalominate, and its presence may accelerate the hydration of silicates. C4AF + Ca(OH)2 + CaSO4.2H2O C3(A, F).3CaSO4.NH2O in the presence of gypsum C3 (A,F).CaSO4, (OH)2 after depletion of gypsum The hydration reactions of C3A and C4AF are summarized in the table below: (Hydrationof aluminates) (Hydrationof aluminates)

8. Note: where in cement chemists’ notation C represents CaO, S represents SiO2, A represents Al2O3, F represents Fe2O3 S represents SO3, and H represents H2O. The rate of hydration of C3A, C3S, C4AF & C2S in a pure state differ considerably as shown in the Figure below. Reactions for even identical compounds may vary due to: 1) fineness, 2) rate of cooling of clinker, and 3) impurities. (Development of hydration of pure compounds) The hydration of Portland cement is rather more complex than that of the individual constituent minerals. A simplified illustration of the development of hydrate structure in cement paste is given in the Figure below:

9. (Simplified illustration of hydration of cement paste)

10. The following illustrative table shows the chemical composition of cement main compounds before and after hydration and their percentages: Cement Hydration products • Heat of Hydration The heat of hydration is the heat generated when water and Portland cement react.  Heat of hydration is most influenced by the proportion of C3S and C3A in the cement, but is also influenced by water-cement ratio, fineness and curing temperature.  As each one of these factors is increased, heat of hydration increases. 

11. In large mass concrete structures such as gravity dams, hydration heat is produced significantly faster than it can be dissipated (especially in the center of large concrete masses), which can create high temperatures in the center of these large concrete masses that, in turn, may cause undesirable stresses as the concrete cools to ambient temperature.  Conversely, the heat of hydration may prevent freezing of the water in the capillaries of freshly placed concrete in cold weather. The heat of hydration is the quantity of heat, in joules per gram of unhydrated cement, evolved upon complete hydration at a given temperature. The rate of heat evolution depends on: • Temperature at which hydration take place. • Cement composition, as different compounds hydrate at different rate, the rate and the total heat will be composition dependent (C3A & C3S particularly). • Type of cement. • Fineness speeds up hydration reactions and consequently the rate evolution but does not affect the total amount of heat. • Richness of the mix affects the total heat developing.

12. 300 200 100 0 Heat evolved J/gm 300 200 100 0 62% C3S content Average Heat of hydration (J/gm) 47% Heat evolved J/gm 20% C3A content 16% 11% 600 500 400 300 200 100 0 Type III Type I 0% Type II Type V Type IV 4 8 12 16 20 24 Time (hrs) 4 8 12 16 20 24 Time (hrs) 3 7 28 3 1 6.5 day month years Age (log scale) (Development of heat of hydration of different cements cured at 21 oC) (Influence of C3S & C3A content on heat evolution)

13. Gel particles C Gel pores C C C Capillary pores • Structure of Hydrated Cement Mechanical properties depend so much on the chemical composition of the hydrated cement as on the physical structure of the product of hydration. Fresh cement is a plastic network of particles of cement in water. When set the volume of the paste remains approximately constant. • At any stage of hydration the hardened paste consists of: • Gel which is hydrates of various compounds insoluble with cementing properties. The actual source of strength of gel is not fully understood but it may be arise from two kinds of cohesive bonds: • Physical attraction between solid surfaces separated only by small gel pores, i.e. Van der Waal's force. • Chemical bonds, as swelling is limited it seems that gel particles are cross-linked by chemical forces. • Unhydrated cement • crystals of Ca(OH)2 & minor components • Pores Gel pores within the gel • Capillary pores (residue of water-filled spaces)

14. Water is absorbed and causes an increase in the surface area. Water existed in several forms: • Free water (pore water) • Gel water (adsorbed water) • Chemical water The progress of hydration of cement can be determined by: • The amount of Ca(OH)2 in the paste • The heat evolved by hydration • The specific gravity of the paste • The amount of chemically combined water • The amount of unhydrated cement present • Also indirectly from the strength of hydrated paste. • Volume of Product of Hydration Cement hydration results in solid product (solid gel) and gel pores. Gel pores: represent a fraction of the total hydration product which accommodates gel water. The gel pores related to the porosity of the cement paste.

15. Capillary pores: represent the part of the gross volume which has not been filled by the product of hydration. It is reduced with the progress of hydration. The porosity depends on the w/c ratio and the degree of hydration. It is generally agreed that above 0.38 w/c ratio capillary pores will left. Further hydration blocks more capillary pores leading to reduced permeability (function of capillary pores interconnection). The absence of continuous capillaries is due to suitable w/c ratio and curing.

16. Techniques used to study hydration The following techniques are used to study the hydration of cement: • Thermal analysis • X-ray diffraction • Scanning electron microscopy In order to study hydrated cement it is necessary to remove free water and arrest hydration. This is normally done by immersing the sample in acetone and drying it at low temperature (<50°C) in a partial vacuum. Thermal analysis The most commonly applied technique is thermo-gravimetric analysis. A small sample of hydrated cement is placed in a the rmobalance and the change in weight recorded as the sample is heated at a controlled rate from ambient to ~1000°C. The technique enables the proportion of certain hydrates present, such as ettringite and Ca(OH)2 to be determined quantitatively. X-ray diffraction This technique is rapid but provides limited information as many of the hydrates present, notably C–S–H gel and calcium aluminate monosulfate are poorly crystalline and give ill-defined diffraction patterns.

17. Scanning electron microscopy (SEM) This is a powerful technique, particularly when the microscope is equipped with a microprobe analyser. It involves techniques akin to X-ray fluorescence to determine the chemical composition of hydrates in the field of view. The high resolution of the SEM enables the microstructure of the hydrated cement paste in concrete or mortar to be studied. However, caution must be exercised when interpreting the images as specimen preparation and the vacuum required by most microscopes can generate features, which are not present in the moist paste.