A foundation on volcanic soil shows settlement after construction. Investigation reveals the soil has a high sulfate content. What concrete modification should have been specified?
Correct Answer
D) Sulfate-resistant cement with protective admixtures
Volcanic soils with high sulfate content require sulfate-resistant cement and protective admixtures to prevent chemical attack and deterioration of concrete.
Why This Is the Correct Answer
Sulfate-resistant cement (Type V Portland cement) combined with protective admixtures is the industry-standard solution when soils contain high sulfate concentrations. Sulfates in soil react chemically with standard Portland cement compounds (primarily tricalcium aluminate, C3A), causing expansion, cracking, and progressive strength loss. Sulfate-resistant cement has a low C3A content that resists this attack, and protective admixtures (such as fly ash or slag) further reduce permeability and chemical vulnerability.
Why the Other Options Are Wrong
Option A: Reduced water-cement ratio only
Reducing the water-cement ratio alone improves density and reduces permeability, which is a secondary benefit, but it does not change the cement chemistry. Standard Portland cement will still undergo sulfate attack regardless of the water-cement ratio, making this an incomplete and inadequate solution.
Option C: Higher cement content only
Higher cement content alone increases the total amount of reactive C3A present, which actually worsens sulfate attack rather than preventing it. More standard cement means more material available for the damaging chemical reaction.
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