Concrete Technology – Mixing water impurities that significantly alter setting time and strength According to good concreting practice (IS context), which impurity present in mixing water most significantly affects the setting behaviour and ultimate strength of cement concrete?

Introduction / Context:
Water quality is a critical but sometimes overlooked variable in concrete technology. Certain dissolved salts can accelerate or retard cement hydration, change setting times, and reduce the long-term strength of concrete. Understanding which impurities are most harmful helps engineers evaluate water sources on site and avoid costly durability problems.

Given Data / Assumptions:

• Ordinary Portland cement or blended cements used for structural concrete.
• Ambient, non-extreme temperature curing conditions.
• Impurities considered are typical dissolved salts found in groundwater or recycled water.

Concept / Approach:
Different ions influence hydration differently. Chlorides (e.g., calcium chloride) act primarily as accelerators but carry corrosion risks for reinforcement. Sulphates may contribute to expansion when present with reactive aluminates. Carbonates and bicarbonates of sodium are particularly disruptive: they can cause very rapid setting (flash set) or strong retardation depending on concentration, and they often reduce compressive strength due to disturbed hydration and microstructure formation.

Step-by-Step Solution:
Identify ions with strongest adverse effect on setting and strength at typical site concentrations.Note that sodium carbonates/bicarbonates can drastically alter pH buffering and early hydration kinetics → erratic set and reduced strength.Compare with chlorides and sulphates: these are controlled by separate limits mainly for corrosion or sulphate attack, not primarily for setting disruption.Therefore, select the pair most directly harmful to setting and strength: sodium carbonates and bicarbonates.

Verification / Alternative check:
Practical field guidance often rejects water with appreciable carbonate/bicarbonate alkalinity because of abnormal setting and poor compressive strength results in cube tests.

Why Other Options Are Wrong:

• Sodium sulphates: more related to external sulphate attack scenarios; not the most acute setting disruptor in mixing water.
• Sodium or calcium chlorides: accelerators; main concern is steel corrosion, not rendering concrete unusable by setting disturbance.
• Calcium bicarbonates: common hardness salts; usually less disruptive at moderate levels.

Common Pitfalls:
Assuming any potable water is suitable; while often true, verify alkalinity and salt content. Also, do not rely on chlorides to “improve” early set because of long-term corrosion risk.

Final Answer:
Sodium carbonates and bicarbonates