What type of problem would you expect from using chert as an aggregate in concrete?

• A. Formation of a gel which expands in moisture
• B. Rapid freezing expansion
• C. High thermal conductivity
• D. Increased compressive strength

Answer: (A)

When chert is used as aggregate, its silica can react with the alkalis from cement to produce an amorphous gel that swells upon absorbing moisture. This alkali-silica reaction causes internal pressures as the gel expands, leading to cracking and deterioration of the concrete over time. The other scenarios describe frost-related expansion, heat transfer effects, or a strength increase, none of which explain the moisture‑driven gel swelling caused by reactive silica in chert, making the gel expansion the best explanation. Mitigation involves using non-reactive aggregates, reducing cement alkali content, or adding pozzolanic materials to bind alkalis.