Question: We recently supplied a house slab where the bleedwater had a greenish tint. We discovered that the water retention pond we used for water collection had green algae that had formed on the surface and was used for concrete production. The concrete met and gained strength normally, however. Is there anything we should be concerned about?

Answer:According to ASTM C 94, any water that contains substances that discolor it should not be used unless there are sufficient service records to indicate that the water is not injurious to the quality of the concrete (Ref. 1). Although such records may not exist, it seems the quality of the concrete has not suffered, yet.

The Portland Cement Association further warns that use of material that contains algae water or aggregates can disrupt the hydration process, reduce strength, or cause poor bonding to occur between the aggregates and paste (Ref. 2).

Research has shown that certain types of algae, green or brown in particular, can contribute to loss of strength or air entrainment (Ref. 3). More than likely, there was enough cementitious material to counter any strength loss, but air entrainment might be a different story and should be checked if this is an important performance parameter.

The best remedy for a situation like this is to avoid having to ask this question. It is best to make sure that you are using water stored in large retention ponds, pits, or tanks without agitation. You should check the water supply on a regular basis, not only for solids and pH content, but also for the presence of microbial bodies and algae.

  1. ASTM C 94, "Standard Specification for Ready-Mixed Concrete," ASTM, West Conshohocken, Pa., Section, 2000.
  2. S. Kosmatka and W. Panarese, Design and Control of Concrete Mixtures, 13th ed., Portland Cement Association, Skokie, Ill., page 29.
  3. A. M. Neville, Properties of Concrete, 4th ed., J. C. Wiley and Sons, page 507, 1997.