Question

In maritime construction, modern structures fabricated from ordinary Portland cement frequently deteriorate within decades when exposed to the corrosive action of seawater. This degradation occurs primarily because sulfate ions dissolved in seawater attack calcium hydroxide and hydrated calcium aluminate phases in the cement matrix, inducing expansive crystallization, micro-cracking, and structural disintegration. By contrast, submerged Roman maritime concrete structures—such as breakwaters and piers built along the Mediterranean coast over two millennia ago—have demonstrated remarkable long-term durability.

Geochemical analyses of core samples extracted from the ancient harbor of Portus Cosanus have illuminated the specific mechanism responsible for this resilience. Unlike modern builders who combine clinker-derived cement with inert silica aggregates, Roman engineers blended reactive volcanic ash (pozzolana), quarried primarily from the Phlegraean Fields near modern Naples, with quicklime and volcanic tuff rock fragments. When this slurry was immersed in seawater during harbor construction, the lime reacted rapidly with water in an exothermic reaction known as slaking, forming calcium hydroxide.

Crucially, however, the durability of Roman concrete was not merely a consequence of initial curing, but of post-depositional mineralogical changes driven by sustained interaction with percolating seawater. Over decades and centuries, alkaline seawater dissolved volcanic glass present in the pozzolanic ash. This dissolution released soluble silica and alumina into pore fluids, reacting with the existing calcium hydroxide matrix to precipitate platy crystals of aluminium-tobermorite—a rare layered calcium silicate hydrate mineral—alongside interconnected crystals of phillipsite, a zeolite mineral. Rather than degrading the mortar, these interlocking crystalline structures actively reinforced the matrix by filling interstitial voids and bridging macroscopic microfractures. Consequently, rather than causing expansive failure, the ingress of seawater initiated an autogenous strengthening process.

Nevertheless, early modern mineralogists often mischaracterized Roman concrete's longevity, attributing its survival solely to exceptional compaction techniques or to the hypothetical inclusion of secret organic additives such as animal fats and blood. While historical Roman treatises, including Vitruvius's De architectura, did describe the manual tamping of mortar during placement to eliminate macroscopic air voids, recent spectroscopic and X-ray diffraction assays have confirmed that compaction alone cannot account for the chemical resilience of the binding matrix. Instead, the continuous precipitation of aluminium-tobermorite remains the decisive factor distinguishing ancient pozzolanic marine concrete from its modern industrial counterparts.

According to the passage, the precipitation of aluminium-tobermorite within submerged Roman concrete was directly enabled by which of the following?