Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete

Date: July 3, 2017
Source: DOE/Lawrence Berkeley National Laboratory

A new look inside 2,000-year-old concrete -- made from volcanic ash, lime (the product of baked limestone), and seawater -- has provided new clues to the evolving chemistry and mineral cements that allow ancient harbor structures to withstand the test of time. The research has also inspired a hunt for the original recipe so that modern concrete manufacturers can do as the Romans did.

A team of researchers working at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) used X-rays to study samples of Roman concrete -- from an ancient pier and breakwater sites -- at microscopic scales to learn more about the makeup of their mineral cements.

The team's earlier work at Berkeley Lab's Advanced Light Source (ALS), an X-ray research center known as a synchrotron, found that crystals of aluminous tobermorite, a layered mineral, played a key role in strengthening the concrete as they grew in relict lime particles. The new study, published today in American Mineralogist, is helping researchers to piece together how and where this mineral formed during the long history of the concrete structures.

The work ultimately could lead to a wider adoption of concrete manufacturing techniques with less environmental impact than modern Portland cement manufacturing processes, which require high-temperature kilns. These are a significant contributor to industrial carbon dioxide emissions, which add to the buildup of greenhouse gases in Earth's atmosphere.

Also, researchers suggest that a reformulated recipe for Roman concrete could be tested for applications such as seawalls and other ocean-facing structures, and may be useful for safeguarding hazardous wastes.

Compositions of phillipsite and Al-tobermorite in Roman marine mortars and geologic deposits. Electron probe microanalyses (EPMA), as molecular proportions.
Compositions of phillipsite and Al-tobermorite in Roman marine mortars and geologic deposits. Electron probe microanalyses (EPMA), as molecular proportions.

Marie D. Jackson, Sean R. Mulcahy, Heng Chen, Yao Li, Qinfei Li, Piergiulio Cappelletti, Hans-Rudolf Wenk. Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concreteAmerican Mineralogist, 2017; 102 (7): 1435

DOI: 10.2138/am-2017-5993CCBY

Pozzolanic reaction of volcanic ash with hydrated lime is thought to dominate the cementing fabric and durability of 2000-year-old Roman harbor concrete. Pliny the Elder, however, in first century CE emphasized rock-like cementitious processes involving volcanic ash (pulvis) “that as soon as it comes into contact with the waves of the sea and is submerged becomes a single stone mass (fierem unum lapidem), impregnable to the waves and every day stronger” (Naturalis Historia 35.166). Pozzolanic crystallization of Al-tobermorite, a rare, hydrothermal, calcium-silicate-hydrate mineral with cation exchange capabilities, has been previously recognized in relict lime clasts of the concrete. Synchrotron-based X-ray microdiffraction maps of cementitious microstructures in Baianus Sinus and Portus Neronis submarine breakwaters and a Portus Cosanus subaerial pier now reveal that Al-tobermorite also occurs in the leached perimeters of feldspar fragments, zeolitized pumice vesicles, and in situ phillipsite fabrics in relict pores. Production of alkaline pore fluids through dissolution-precipitation, cation-exchange and/or carbonation reactions with Campi Flegrei ash components, similar to processes in altered trachytic and basaltic tuffs, created multiple pathways to post-pozzolanic phillipsite and Al-tobermorite crystallization at ambient seawater and surface temperatures. Long-term chemical resilience of the concrete evidently relied on water-rock interactions, as Pliny the Elder inferred. Raman spectroscopic analyses of Baianus Sinus Al-tobermorite in diverse microstructural environments indicate a cross-linked structure with Al3+ substitution for Si4+ in Q3tetrahedral sites, and suggest coupled [Al3++Na+] substitution and potential for cation exchange. The mineral fabrics provide a geoarchaeological prototype for developing cementitious processes through low-temperature rock-fluid interactions, subsequent to an initial phase of reaction with lime that defines the activity of natural pozzolans. These processes have relevance to carbonation reactions in storage reservoirs for CO2 in pyroclastic rocks, production of alkali-activated mineral cements in maritime concretes, and regenerative cementitious resilience in waste encapsulations using natural volcanic pozzolans.