Our water source for use in ready-mixed concrete is acidic, with a pH ranging from 5 to 6. What are the effects of this low pH, and what can be done to reduce acidity?
The 5 to 6 pH may have harmful effects, but this seems to be a borderline case. The U.S. Army Corps of Engineers Handbook for Concrete and Cement states that water with a pH between 6 and 8 may be regarded as safe for use in mixing concrete, so your water is outside those limits. However, limits published in the Seventh (revised) Edition of Fulton's Concrete Technology (South Africa) range from 4 to 8.5. The book further notes that high alkalinity of the pore water should neutralize any acidity in the mix water during hydration. Concrete stability may be affected if the low pH is caused by organic acids in the mixing water, according to a Danish concrete manual. ASTM C 94, Standard Specification for Ready-Mixed Concrete, doesn't include pH limits for mixing water but does require testing questionable-quality water to assess harmful effects on compressive strength and setting time. To do this testing, make one batch of mortar with the low-pH water and a control batch using city water or distilled water. Use the same proportions in each batch, including the same volume of water. Make and test cubes from each batch in accordance with ASTM C 109, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars. Cubes made with the low-pH water should have a seven-day strength equal to at least 90% of the strength of control-batch cubes. Check the setting time of cement pastes containing the low-pH water and city or distilled water, in accordance with ASTM C 191, Standard Test Method for Time of Setting of Hydraulic Cement by Vicat Needle. Set time for the low-pH paste should be no more than one hour earlier or 1 1/2 hours later than the set time of the control paste.