Question

Passage:

For decades, architectural historians and civil engineers attributed the extraordinary longevity of ancient Roman maritime and architectural concrete—structures such as the Pantheon and seawalls at Caesarea Maritima that have survived intact for millennia—primarily to the reaction between volcanic ash (pozzolana) and slaked lime. Under this traditional model, known as the pozzolanic hypothesis, builders mixed lime with water to form a calcium hydroxide paste, which was subsequently blended with volcanic ash to yield cementing calcium silicate hydrates. Ubiquitous millimeter-scale white mineral inclusions visible throughout Roman mortar matrices, termed "lime clasts," were routinely dismissed by twentieth-century conservators as incidental artifacts of defective mixing or low-grade precursor limestone.

Recent spectroscopic and micro-computed tomography investigations have overturned this dismissal, demonstrating that lime clasts were not accidental contaminants but the deliberate consequence of an unrecorded manufacturing technique known as "hot mixing." In this process, Roman builders introduced quicklime (calcium oxide) directly into the pozzolanic aggregate prior to adding water, rather than pre-slaking the lime into a paste. The hydration of quicklime is an exceptionally exothermic reaction, generating localized temperatures in excess of eighty degrees Celsius. This intense thermal surge conferred two immediate rheological advantages: it significantly shortened overall curing periods and expedited structural stabilization, which was vital for underwater maritime constructions where tidal erosion threatened unhardened mortar.

More critically, hot mixing imparted an autonomous self-healing capability to the cured concrete. The elevated temperatures precluded the complete dissolution of the quicklime, leaving brittle, calcium-rich clasts distributed throughout the matrix. When subsequent mechanical strain or seismic shifting induced microcracks through the concrete, these fissures preferentially propagated through the lime clasts rather than the harder surrounding matrix due to the clasts' high specific surface area and structural fragility. As ambient moisture or seawater permeated the resulting fissures, it dissolved the residual calcium from the clasts, recrystallizing it as calcium carbonate or calcium aluminosilicate hydrates that sealed the cracks within weeks. However, researchers emphasize that this self-healing mechanism is strictly limited to micro-fissures narrower than half a millimeter; structural fractures exceeding this threshold overwhelm the available calcium reservoir within adjacent clasts, permitting unmitigated water infiltration and structural failure.

Based on the passage, which of the following is explicitly cited as a factor that causes microcracks to propagate preferentially through lime clasts rather than the surrounding matrix?