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Accelerator

An admixture that speeds up concrete setting and early strength development

An accelerator is an admixture that speeds concrete setting and early strength development. Accelerators allow earlier formwork removal, faster construction progress, and cold-weather placement. Calcium chloride is the most effective and economical accelerator, though non-chloride alternatives exist for applications where chlorides are prohibited.

Why It Matters

Time is money in construction. Accelerators can reduce setting time from 5-7 hours to 2-4 hours, allowing same-day finishing or form stripping. In cold weather, they enable placement when concrete would otherwise freeze before gaining strength. The productivity gains often exceed the admixture cost.

However, accelerators have tradeoffs: increased shrinkage, reduced long-term strength slightly, and corrosion risk with chloride types. Understanding when to use accelerators and selecting appropriate types ensures benefits without compromising long-term performance.

Technical Details

How accelerators work:

  • Speed hydration reactions
  • Accelerate cement compound dissolution
  • Increase rate of calcium silicate hydrate formation
  • Earlier strength gain and set

Types of accelerators:

Calcium chloride (most common):

  • Most effective and economical
  • 1-2% by weight of cement typical dosage
  • Reduces setting time 30-50%
  • Doubles 1-day strength
  • Prohibited where corrosion concern exists

Non-chloride accelerators:

  • Calcium formate, calcium nitrate, sodium thiocyanate, others
  • Less effective than calcium chloride
  • No corrosion risk
  • Required for reinforced concrete, prestressed
  • Higher cost (2-5× calcium chloride)

High-early strength cement (Type III):

  • Not an admixture but similar effect
  • Finely ground cement hydrates faster
  • Can combine with accelerators for maximum effect
  • Higher cost than Type I with accelerator

Effects on concrete properties:

Setting time:

  • Initial set: 1-3 hours earlier
  • Final set: proportionally earlier
  • Temperature and cement type affect magnitude
  • Useful for early finishing or cold weather

Early strength:

  • 1-day strength: 50-100% increase
  • 3-day strength: 25-50% increase
  • 7-day strength: 10-25% increase
  • 28-day strength: similar or slightly lower

Long-term strength:

  • 28-day and beyond: 0-10% reduction typical
  • Trade early strength for ultimate strength slightly
  • Rarely significant for typical applications

Shrinkage:

  • Increased drying shrinkage (10-50% more)
  • Increases cracking potential
  • Control joints and curing more important
  • Consider in crack-sensitive applications

Heat of hydration:

  • Accelerated hydration generates heat faster
  • Higher peak temperature
  • Beneficial in cold weather
  • Potential issue in mass concrete

Dosage guidelines:

Calcium chloride:

  • 1% by weight of cement: moderate acceleration
  • 2% by weight: maximum practical dosage
  • Higher dosages increase side effects without proportional benefits
  • ACI recommends maximum 2%

Non-chloride:

  • Manufacturer recommendations vary widely
  • Typically 0.5-4% by weight of cement
  • Less standardized than calcium chloride
  • Follow specific product guidelines

Applications:

Cold weather concreting:

  • Protects against freezing before adequate strength
  • Reduces required protection time
  • Allows construction in marginal weather
  • Combined with heated materials and enclosures

Fast-track construction:

  • Earlier form removal
  • Faster construction cycle
  • Reduced equipment rental
  • Earlier project completion

Repair work:

  • Quick return to service
  • Overnight or weekend repairs
  • Minimizes disruption
  • Fast-setting patches

Precast concrete:

  • Faster production cycle
  • Earlier stripping and handling
  • Increased throughput
  • Economic benefit in manufacturing

Restrictions and prohibitions:

Chloride accelerators prohibited for:

  • Reinforced concrete (corrosion risk)
  • Prestressed concrete (stress corrosion)
  • Concrete with embedded aluminum (aluminum corrosion)
  • Permanent contact with chloride-bearing soils or water

Where non-chloride required:

  • All reinforced applications
  • Corrosion-critical environments
  • Specifications prohibiting chlorides
  • Most structural applications

Unreinforced applications safe for calcium chloride:

  • Plain concrete sidewalks
  • Mass concrete (minimal reinforcement)
  • Non-structural slabs without reinforcement
  • Where corrosion not a concern

Compatibility and interactions:

With other admixtures:

  • Compatible with air entrainment (may need adjustment)
  • May reduce water reducer effectiveness
  • Can combine with retarders to fine-tune set time
  • Test combinations in trial batches

Temperature effects:

  • More effective in cold weather
  • Less needed in warm weather
  • Overdosing in hot weather causes flash set
  • Adjust dosage for temperature

Side effects to manage:

Increased cracking potential:

  • Higher shrinkage from accelerators
  • Adequate control joints essential
  • Proper curing critical
  • May require closer joint spacing

Finishing challenges:

  • Faster set reduces finishing window
  • Coordinate crew and schedule carefully
  • May require larger crew
  • Practice and planning essential

Color variation:

  • Can cause darkening or mottling
  • Inconsistent dosing creates variation
  • Critical for decorative concrete
  • Use integral color consistently

For residential concrete, accelerators are most useful in cold weather or time-critical repairs. For typical reinforced slabs and foundations, use non-chloride accelerators only. For plain concrete like sidewalks in freezing weather, calcium chloride is economical and effective.

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