Q: We've got a concrete paving contract in a mountainous region where the only water source is a lake with some algae growth. We've read that water containing algae is unsuitable for making concrete, because the algae reduces strength by interfering with cement hydration or by producing high air contents. Is there a limit on the permissible amount of algae in concrete mixing water? If so, how would we test for the amount of algae present? And if too much is present, could we use copper sulfate to kill the algae before using the water?

A.:Laboratory tests (Ref. 1) are the probable basis for prohibiting the use of algae-infested water as mixing water for concrete. A researcher made an algae concentrate by sieving lake water containing algae, then freezing the concentrate and thawing it immediately before adding it to the mix water. Total organic matter in the concentrate was determined from loss on ignition at 650º C, and this value was converted to organic content of the mixing water, based on water content of the concretes tested. The researcher's results are shown in the table. Except at very low levels, increasing organic concentrations increased air content and decreased strength.

Mixing Water
Organic Content,
Air Content,
28-Day Compressive
0 2.2 4830
0.03 2.6 4840
0.09 6.0 4040
0.15 7.9 3320
0.23 10.6 2470

If no other source of organic material is in the water, a testing lab could probably determine algae content by inexpensive loss-on-ignition tests (about $30 per test). However, we caution against total reliance on such a test. Instead, have the lab test concrete made with the water, which will tell you immediately if there is an air-content or time-of-set problem. And 28-day cylinder tests on the same concrete will indicate the effect of algae on strength. If you need quicker results, consider accelerated strength tests.

We also advise against using water containing copper sulfate, unless lab tests demonstrate no harmful effects. A broad survey of the effects of inorganic salts on strength of cement paste showed that copper chloride (in an amount chemically equivalent to a 2% calcium-chloride dosage) retarded set so much that no tests could be made at two and three days (Ref. 2). Depending on concentration, the sulfate ion might also lower the concrete's sulfate resistance.

You might consider buying a trailer-mounted water-filtration system similar to the one shown in the photo. This system has a flow rate up to 700 gallons per minute through a 6-inch-diameter filter that removes particles down to 0.004 inch in diameter. Semi-automatic cleaning with nylon brushes during the filtering process eliminates downtime during cleaning.

The manufacturer is Amiad Water System Technologies, 2220 Celsius Ave., Unit B, Oxnard, CA 93030 (805-988-3323).


  1. Benjamin C. Doell, "Effect of Algae Infested Water on the Strength of Concrete," Journal of the American Concrete Institute, Dec. 1954, pp. 333-342.
  2. Harold H. Steinour, "Concrete Mix Water How Impure Can it Be?" Research Department Bulletin RX119, Portland Cement Association, Skokie, Ill., 1960, p. 43.