The correct answer is: C. Not less than 6.
The pH scale is a scale used to measure the acidity or alkalinity of a solution. A pH of 7 is neutral, a pH below 7 is acidic, and a pH above 7 is alkaline.
The IS 456 – 1978 code specifies the requirements for concrete mix design and construction. It states that the pH value of water used for concrete shall not be less than 6. This is to ensure that the concrete is not too acidic, which can cause problems such as corrosion of the reinforcement and deterioration of the concrete.
Option A is incorrect because the pH value of water used for concrete shall not be less than 6.
Option B is incorrect because the pH value of water used for concrete shall not be equal to 6.
Option D is incorrect because the pH value of water used for concrete shall not be equal to 7.