Admixtures explained...

Chemical admixtures are the ingredients in concrete other than Portland cement, water, and aggregate that are added to the mix immediately before or during mixing. Producers use admixtures primarily to reduce the cost of concrete construction; to modify the properties of hardened concrete; to ensure the quality of concrete during mixing, transporting, placing, and curing; and to overcome certain emergencies during concrete operations.

The effectiveness of an admixture depends on several factors including: type and amount of cement, water content, mixing time, slump, and temperatures of the concrete and air. Sometimes, effects similar to those achieved through the addition of admixtures can be achieved by altering the concrete mixture, reducing the water-cement ratio, adding additional cement, using a different type of cement, or changing the aggregate and aggregate gradation.

Admixture Types:

Admixtures are classed according to function. Although our focus is predominately on water reduction technologies KemCare can offer 5 main classes of chemical admixtures:

• Superplasticising
  Enables the water content of a given concrete mix to be greatly reduced without affecting the consistency, or greatly increases the workability without changing the water content, or achieves both affects
  
  
• Air-entraining
  Introduces a specific quanity of small, evenly distributed air voids during the mixing process which remain in the concrete after it hardens
  
  
• Grout Admixtures
  Retards the time to initial set and prolongs the consistency 
  
  
• Underwater Stabilising
  Prevents the 'washout' of cements and binders in water bearing ground conditions.
  
  
• Waterproofing
  Accelerates the initial strength with or without an effect on the setting time
  

According to EN 206 -1 (a European standard), the permitted dosage of admixtures is less than or equal to 5% by weight of the cement (the effect of a higher dosage on the performance and durability of the concrete must be verified). For 'Low dosages' admixture quanitities of <0.2% of the cement are only allowed if they are dissolved in part of the mixing water. If more than one admixture is added their compatibility must be verified by specific testing.