WATER, CEMENT, AND ADMIXTURES

• Cement paste


  Cement + water, serves as glue (or binder) that bonds aggregates together to make concrete
• Mortar


  Cement paste + sand
• Concrete


  Cement + water + sand + aggregates
• Uses of Portland cement concrete

  • Buildings
  • Bridges
  • Pavements
  • Concrete blocks
  • Piles
  • Earth/water retaining structures
• Portland cement


  Named after the limestone & chalk cliffs on the Isle of Portland, where it was first made in the early 1800s
• Cement production

  
• Fineness


  Smaller cement particles have more surface area to react with water, controls the rate of hydration (heat & strength gain)
• Blaine test


  • Measures air permeability against known standard material to indirectly measure cement surface area
• Wagner Turbidimeter


  • Measures sedimentation rate of cement suspended in kerosene to indirectly measure cement surface area
• Specific gravity


  Cement is much denser than aggregates, commonly has 3.15 specific gravity measured without air voids
• Hydration


  Chemical reaction of cement when mixed with water, causes hardening through crystallization
• Hardening is not a drying process, concrete can harden even in water
• Heat speeds up the setting and hardening of cement, and cold slows it down and can even completely stop the processes
• Hardening does not start as soon as cement and water are mixed, but after a certain length of time known as the "initial set time"

  
• Voids in hydrated cement

  • Interlayer hydration space
  • Capillary voids
  • Trapped voids
  • Entrained air
• Setting time


  Starts when the crystals form due to hydration, concrete should never be disturbed after final set
• Stiffening


  Change from plastic state to solid state, measured from initial setting time to final setting time
• Vicat Apparatusand Gillmore Apparatus


  • Used to measure initial and final set of cement
• Soundness


  Ability of cement paste to retain its volume after setting, measured by expansion in autoclave test
• Water-cement ratio


  Most important property of hydrating cement, water needed for hydration is about 22% to 25% of cement
• Adding more water to concrete decreases strength, durability, bond, increases permeability and volume change
• Types of Portland cement

  • Type I Normal
  • Type II Moderate Sulfate Resistance
  • Type III High Early Strength
  • Type IV Low Heat of Hydration
  • Type V High Sulfate Resistance
• Water


  Any potable (drinkable) water can be used
• Air entrainer


  Produces tiny, dispersed air bubbles into the concrete to improve durability, workability, and resistance to chemicals
• Water reducer

  Increases mobility of cement particles, improves workability and can be measured with slump test
• Plasticizer (Superplasticizer)


  • High-range water reducer that can increase slump from 3" to 9" and reduce water by 12-30% at same slump
• Retarder


  Delays or retards initial set (increases set time) for hot weather, long haul time, or special finishes
• Hydration controller


  Ability to stop and reactivate the hydration process of concrete, consists of a stabilizer and an activator
• Accelerator


  Speeds up or accelerates initial & final set (decreases set time) to reduce finishing time, curing time, and increase strength gain
• Calcium chloride (CaCl2) is the most common used accelerator
• Interlayer hydration space - space between atomic layers and if humidity is less than 11%
• Capillary voids – happens when the water-cement ratio is too high and it can decrease strength and increase permeability of concrete.
• Trapped voids – large pockets of air caused by mishandling of concrete. It can also decrease strength and increase permeability of concrete.
• Entrained air – these are microscopic bubbles caused by admixtures and can increase durability of concrete