Cement and Hydration. Outline: Cement and Hydration. Portland Cement. A modern material with an ancient history. Manufacture. What is Portland Cement?

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1 Outline: Cement and Hydration Cement and Hydration Historical perspective Manufacture Cement hydration Cement Types Structure of the hydrated cement paste Dr. Kimberly Kurtis School of Civil Engineering Georgia Institute of Technology Atlanta, Georgia Next topics: SCMs Fresh concrete properties Chemical admixtures Hardened concrete properties Durability A modern material with an ancient history 3500BC Egyptians used cement - calcined impure gypsum 500BC-400AD Greeks and Romans - calcined limestone - added broken tile, brick and brick dust or volcanic ash (pozzoli) AD Middle Ages - knowledge of calcining and pozzolans nearly lost Portland Cement 1756 John Smeaton first develops a portland-like cement - construction of Eddystone Lighthouse, UK 1824 Patents Issued - British patent 5022 issued to Joseph Aspdin - David Saylor patented a similar process using materials in Coplay, PA in 1871 What is Portland Cement? Synthetic, but it comes from the earth Raw materials: -CaCO 3 from limestone, chalk, or marl -SiO 2 from clay, shales (which also contain Al 2 O 3, Fe 2 O 3, Na 2 O and K 2 O) Raw materials are abundant worldwide Manufacture Crushing, grinding, proportioning and blending of raw materials Water is added to mixture (wet process only) Preheating (dry process only) Raw materials pass through inclined rotary kiln Clinker produced is cooled ~5% gypsum is added to clinker and mixture is ground to 1-100um

2 What happens in the kiln? What are the products? (alite) (belite) (aluminate) Calcine- to heat (as inorganic materials) to a high temperature but without fusing in order to drive off volatile matter or to effect changes (as oxidation or pulverization) What are the products? What are the products? Two-dimensional processed SEM/X-ray image for cement 133 issued by the CCRL (NIST) in June of Color assignments are: red- C 3 S aqua- C 2 S green- C 3 A yellow- C 4 AF pale green- gypsum white- free lime (CaO) dark blue (purple)- K 2 SO 4 light magenta- periclase (magnesium containing phase). Image is 256 µm x 200 µm. When water is added to cement, what happens? Dissolution of cement grains Growing ionic concentration in water (now a solution) Formation of compounds in solution After reaching a saturation concentration, compounds precipitate out as solids ( hydration products ) In later stages, products form on or very near the surface of the anhydrous cement Ca +2 SiO - OH - Al +3 Ca +2 OH - Al +3 Al +3 Ca +2 SiO - OH -

3 concrete.cee.hiroshima-u.ac.jp/ research_e.html Is the chemical combination of cement and water to form hydration products Takes time May not proceed to 100% completion Formation of hydration products over time leads to: Stiffening (loss of workability) Setting (solidification) Hardening (strength gain) Let s look at the hydration reactions in more detail Hydration of the Calcium Silicates 2C 3 S + 7H C 3 S 2 H 8 + 3CH 2C 2 S + 7H C 3 S 2 H 8 + CH H=-500J/g H=-250J/g Both produce C-S-H and CH as reaction products C 2 S produces less CH (important for durability in sulfate rich environments) More heat is evolved during C 3 S hydration C 3 S hydration is more rapid, contributing to early age strength (2-3h to 14 days) C 2 S hydration occurs more slowly to contributing to strength after ~14 days. C-S-H Calcium silicate hydrate C/S varies between H is even more variable Structure ranges from poorly crystalline to amorphous - highly variable and poorly understood Occupies 50-60% of the solid volume of the hydrated cement paste (hcp) Huge surface area ( m 2 /g) Strength due to covalent/ionic bonding (~65%) and Van der Waals bonding (~35%) within the complex structure Primary strength-giving phase in portland cement concrete

4 C-S-H CH Calcium hydroxide or Ca(OH) 2 Definite stoichiometry Large, hexagonal prism morphology Size of the crystals depends on the amount of space available Occupies 20-25% of the solid volume in the hcp Much lower surface area than C-S-H Does not contribute much to strength Keeps the pore solution alkaline (ph ) Hydration of the Calcium Aluminates Reaction of C 3 A with water occurs very quickly and liberates much heat - Flash Set Gypsum (CŠH 2 ) is added to the cement to control the hydration of C 3 A C 3 A + 3CŠH H C 6 AŠ 3 H 32 H=-1350J/g When more C 3 A remains, C 6 AŠ 3 H C 3 A + 4H 3C 4 AŠH 12 Reaction of C 4 AF occurs more slowly C 4 AF + 2CH + 14H C 4 (A,F) H 13 + (A,F)H 3 Hydration of the Calcium Aluminates C 6 AŠ 3 H 32 (Ettringite, A ft ) C 4 AŠH 12 (monosulfate, A fm ) Needle-like morphology Needles interlock, take up much water contributes to stiffening of mixture some early strength hexagonal plate morphology arranged in rosettes vulnerable to sulfate attack Calcium Aluminates and Calcium Sulfoaluminates Includes ettringite, monosulfate hydration, calcium aluminate hydrates, and ferric-aluminum hydroxide gels Comprise 15-20% of solid volume of hcp Do not contribute much to strength Formation of ettringite, in particular, does influence setting time High heat of hydration for C 3 A can be favorable or unfavorable, depending upon application

5 Heat evolution can be used to map the progress of hydration: (1) Dissolution of ions (2) Induction period (3) Acceleration (4) Deceleration (5) Steady state Formation of hydration products over time leads to: Stiffening (loss of workability) Setting (solidification) Hardening (strength gain), sulfate attack Cement Types Type Use C 3 S C 2 S C 3 A C 4 AF Comments I general no limits on 4 principal components II general/ mod sulfate resistance/ mod heat of hydration III high early strength IV low heat of hydration V high sulfate resistance

6 Cement Types: Historical Changes Other Cement Types Influence of Ambient Temperature Type IA, Type IIA, Type IIA Blended cements (e.g., Types IP, IS, ISM, IIP, IIS, etc) White portland cement Masonry cement Expansive cements Calcium aluminate cements Rapid setting cements Oil well cements Microstructure Solids C-S-H CH Ettringite Monosulfate hydrate Residual unhydrated cement Voids Entrapped air (>1mm) Entrained air (75-500um Capillary pores (macro meso) Interlayer space (micropores) Microstructure Water Capillary water Adsorbed water Interlayer water Chemically combined water

7 Solids: Review Voids are intrinsically part of the hydrated cement paste, due to the excess water used for economy and workability in the vast majority of mixtures. Voids Classification of Voids in the hcp C-S-H interlayer space nm in size; too small to adversely affect strength or permeability Capillary pores - space not take up by the cement and hydration products; nm in size in well-hydrated concrete - irregular in shape - size and amount is related to w/c and degree of hydration Entrained air - spherical voids um in size; added for freeze/thaw resistance Entrapped air - irregular in shape; can be large Pores > 2.5nm may be filled with air, water (pore solution), or a mixture The presence of voids affects Strength Stress distribution (concentrations) permeability freeze/thaw resistance Voids Voids Inverse relationship between strength (f c ) and porosity (p) f c =k(1-p) 3 k=strength of voidless mortar ~ 34,000 psi

8 Water in the hcp Ratio of mass of water to mass of cement in a mixture is the water-to-cement ratio or w/c When SCM s are used, this is the water-to-cementitious materials ratio or w/cm w/c or w/cm may range , but is typical Water in hcp Capillary water - water present in voids larger than 2.5nm - In capillaries >50nm, water exists as free water because its removal does not cause volume change - In capillaries nm, removal of water results in shrinkage because new bonds can form between C-S surfaces Water is Introduced to the concrete during mixing Permeates the concrete during service Because the water in concrete contains ions, it is usually called pore solution and has a high ph Water in hcp Adsorbed water - water physically adsorbed to the solid surfaces in C-S-H - can be removed on drying to RH ~ 30%, resulting in shrinkage Interlayer water- water associated with the C-S-H structure - can be removed only on strong drying to RH ~ 11%, resulting in shrinkage Chemically combined water - water that is an integral part of various hydration products - lost only on decomposition during heating Interfacial Transition Zone The ITZ is the region 10-50um wide around coarse aggregate; characterized by: Higher local porosity Greater density of preexisting microcracks Larger CH crystals that tend to be oriented and more prone to cleavage. WEAK link in concrete! Interfacial Transition Zone By tailoring the concrete mixture to reduce the influence of the ITZ, strength, E, and impermeability are increased. lower w/c higher cement content use of SCMs smaller MSA reactive dolomitic aggregate lightweight aggregate extended moist curing

Chapter Two Types of Cement The properties of cement during hydration vary according to:

Chapter Two Types of Cement The properties of cement during hydration vary according to: Chemical composition Degree of fineness It is possible to manufacture different types of cement by changing the