Question

Passage:
For decades, modern materials scientists struggled to explain the extraordinary longevity of ancient Roman maritime structures. While contemporary marine concrete typically deteriorates within several decades due to the corrosive penetration of saltwater and internal chemical leaching, Roman breakwaters, piers, and harbor walls constructed over two thousand years ago remain remarkably intact. Recent mineralogical analyses of core samples extracted from the ancient harbor of Portus Cosanus have finally elucidated the geochemical mechanisms responsible for this enduring resilience.

The fundamental distinction lies in the ancient mortar formulation described by the Roman architect Vitruvius, who documented a precise mixture of slaked lime (calcium hydroxide) and reactive volcanic ash known as pozzolana, harvested predominantly from the Gulf of Naples. When Roman engineers submerged this dry mortar mixture together with volcanic rock aggregate into wooden formworks, the interaction between the alkaline components and seawater initiated a complex, multi-stage reaction. Rather than degrading the cementitious matrix, incoming seawater dissolved the volcanic glass within the ash, releasing silica and aluminum ions into the pore solution.

Over centuries of continuous marine exposure, this chemical flux fostered the precipitation of rare interlocking minerals, most notably aluminous tobermorite and phillipsite. In modern Portland cement, the formation of secondary crystalline phases often generates internal expansion stresses that cause micro-cracking and eventual structural failure. In contrast, the interlocking platy crystals of aluminous tobermorite in Roman concrete grow selectively within interstitial voids and preexisting micro-fractures. By occupying these microscopic fissures, the newly crystallized minerals effectively arrest crack propagation and reinforce the structural cohesion of the concrete over time.

Furthermore, geochemical modeling indicates that the ongoing dissolution of relict volcanic grains provides a continuous reservoir of reactive elements, allowing the concrete to self-heal when subjected to seismic tremors or relentless wave action. Thus, rather than functioning as an inert binder that slowly erodes under environmental exposure, the Roman material represents a dynamic, open chemical system that actively utilizes surrounding seawater to enhance its mechanical integrity across millennia.

According to the passage, the growth of aluminous tobermorite crystals enhances the durability of Roman maritime concrete primarily by doing which of the following?