محاضرة ٣

• Hydration of cement


  Reaction between cement particles and water, including chemical and physical processes
• Hydration of cement compounds

  Two ways cement compounds can react with water: direct addition of water molecules (hydration) and hydrolysis
• Structure of cement paste

  Main hydrates formed: calcium silicate hydrate, tricalcium aluminate hydrate, calcium ferrite hydrate
• Hydration of C3S


  1. C3S + water → lime and silica ions in solution

  2. Formation of Ca(OH)2 crystals

  3. Formation of calcium silicate hydrate gel (tobermorite)

  4. Hydrolysis of initial gel to form CSH I and then stable CSH II
• Hydration of C2S


  1. Slower reaction than C3S

  2. Less Ca(OH)2 formed

  3. Initial gel transforms to CSH I and then stable CSH II
• Hydration of C3A


  1. Rapid reaction with water forming calcium aluminates hydrate

  2. Gypsum added to delay reaction by forming ettringite around C3A particles

  3. Eventually tricalcium aluminate hydrate (C3AH6) forms
• The rates of chemical reactions of the main cement compounds are different
• Aluminates affect the early stage hydration reactions, while silicates affect the later stage reactions
• Calcium silicates (C3S and C2S) are the main cement compounds responsible for the final strength of hardened cement paste
• The progress of cement hydration can be determined by measuring the amount of Ca(OH)2, heat evolved, specific gravity, chemically combined water, unhydrated cement, and strength
• The rate of hydration decreases continuously over time due to accumulation of hydration products, reduction of water, and reduction of unhydrated cement
• After 28 days, cement grains have hydrated to a depth of only 4 μm, and 8 μm after a year
• If C3A is exhausted before gypsum, the surplus gypsum can cause expansion and deterioration of the cement paste
• If gypsum is exhausted before C3A, the remaining C3A begins hydrating to form the stable C3AH6
• Calcium aluminate hydrates go through various metastable hexagonal forms before transforming to the stable cubic C3AH6
• Hexagonal calcium aluminate hydrates can react with sulfates to form calcium sulfoaluminate (ettringite)
• Hydration of cement


  1. Calcium aluminate hydrate is formed

  2. Preceded by metastable 3CaO.Al2O3.CaSO.12H2O

  3. Reaction of gypsum with C3A continues until one is exhausted

  4. If C3A exhausted before gypsum, surplus gypsum expands and disrupts cement paste

  5. If gypsum exhausted before C3A, remaining C3A hydrates to form C3AH6
• Calcium aluminate hydrate


  Forms many metastable hexagonal crystals before transforming to stable cubical C3AH6 crystals
• Hexagonal calcium aluminate hydrate crystals exposed to sulfates


  React to form calcium sulfoaluminate, causing expansion and deterioration of concrete
• Transformation of calcium aluminate hydrate from metastable hexagonal to stable cubical form decreases late age strength of cement paste
• C3A in cement


  Contributes little to strength except at early ages, and can cause expansion and disruption when attacked by sulfates
• C3A in cement


  Useful as flux material to reduce clinker formation temperature and facilitate lime-silica combination
• C4AF in cement


  Works as flux material and accelerates silicate hydration
• Gypsum content regulation


  1. Regulates speed of early chemical reactions

  2. Prevents local concentration of hydration products
• Necessary gypsum content increases with increases in C3A content, alkali content, and cement fineness
• Iraqi specification limits maximum gypsum content (as SO3) to 2.5% when C3A≤7% and 3% when C3A>7%
• Hydrated cement paste structure

  Consists of poorly crystallized hydrates (calcium silicate hydrate, tricalcium aluminate hydrate, calcium ferrite), calcium hydroxide, unhydrated cement, and water-filled spaces
• Capillary pores


  Part of paste volume not filled by hydration products, larger than gel pores, interconnected, responsible for permeability and freeze-thaw vulnerability
• Water/cement ratio


  Influences capillary porosity - higher w/c leads to insufficient gel volume to fill voids
• Degree of hydration


  Influences capillary porosity - more hydration reduces capillary pores
• Absence of continuous capillaries requires suitable w/c ratio and sufficient moist curing
• Gel pores


  Interconnected interstitial spaces between gel particles, much smaller than capillary pores (less than 2-3 nm), occupy about 28% of gel volume
• Gel pore properties are largely independent of w/c ratio and hydration progress