Engineers have proposed that adding crushed volcanic ash to cement reduces cracking because the ash keeps reacting for years, slowly filling fine gaps. The idea is old: Roman builders mixed ash from the Bay of Naples into mortar that still stands today. The ash supplies silica that reacts with the lime cement releases. Skeptics counter that any improvement simply reflects the lower water content of ash mixtures. Materials researcher Lucía Terán set out to separate the two, casting ninety prisms, curing them in water baths, and measuring the pore space in each one.
Which finding, if true, would most strongly support the continuing-reaction explanation over the water-content explanation?
- A) Ash mixtures were stronger than ordinary mixtures after one day.
- B) Ash mixtures required less water to reach a workable consistency.
- C) Ash mixtures and ordinary mixtures made with identical water content performed alike at one month, but the ash mixtures had far fewer fine gaps at two years. ✓
- D) Ash mixtures cracked less than ordinary mixtures at every age tested.
Why: The question asks for evidence favoring the continuing reaction over lower water content, so water has to be matched and the benefit has to arrive late, which mixtures alike at one month and far fewer fine gaps at two years give. One finding is about the first day: Ash mixtures were stronger than ordinary mixtures after one day. An advantage within a day is far too early for a reaction said to run for years, and lower water content would produce it. Another states the skeptics' premise: Ash mixtures required less water to reach a workable consistency. Needing less water to reach workability is the rival explanation, not evidence against it. The last leaves water content uncontrolled: Ash mixtures cracked less than ordinary mixtures at every age tested. Less cracking at every age, including the earliest, is what the water explanation predicts too.