Amazing stuff! Pompeii keeps on giving!
"Construction material uncovered by archaeologists in Pompeii has revealed that the Romans used ‘hot mixing’ to make concrete, new chemical analyses suggest.1 The building materials and tools were discovered abandoned in rooms that were under construction when Mount Vesuvius erupted in 79AD, destroying the city. ...
now reporting that concrete production at Pompei involved transporting quicklime and volcanic ash separately to a construction site, mixing the two dry powders and then adding water last to trigger an exothermic reaction that heated the mortar. ...
The MIT group previously [in 2023] analysed clumps of lime in ancient Roman mortars and proposed that they were the result of such hot mixing, which created hot spots in the forming concrete that could exceed 200°C. The researchers concluded that these clumps wouldn’t form if slaked lime was used and that this pointed to hot mixing as the method used to make these mortars. They came to this conclusion after chemically mapping the lime clasts and surrounding matrix and observing the microstructure of Roman mortars. ..."
From the abstract:
"Recent excavations at Pompeii’s Regio IX have uncovered an intact ancient construction site, offering insights into Roman building techniques at the time of the eruption of Mount Vesuvius in 79 CE.
Microstructural and chemical analysis of materials collected from previously constructed walls, walls under construction, and adjacent dry, raw material piles show unequivocally how quicklime was pre-mixed with dry pozzolan before adding water in the creation of Roman concrete.
This construction method, also known as hot mixing, results in an exothermic reaction within the mortar and the formation of lime clasts, key contributors to the self-healing and post-pozzolanic reactivity of hydraulic mortars. The analysis of reaction rims around volcanic aggregates demonstrate aggregate/matrix interfacial remodeling, where calcium ions originating from the dissolution of lime clasts diffuse and remineralize, producing amorphous phases and various polymorphs of calcium carbonate (including calcite and aragonite).
Furthermore, the parallel discovery of masonry materials and tools permits elucidation of the entire construction workflow, including the steps required to process binding mortars and larger aggregates (caementa).
These findings advance our understanding of ancient Roman construction and long-term material evolution, providing a scientific basis for developing more durable and sustainable concretes and restoration materials inspired by ancient practices."
Fig. 1: Pompeii Archeological Park site map, with Regio IX denoted in light blue (upper, middle) and Domus IX 10, 1 shown in additional detail, with color-coded piles of raw construction materials (right): purple: debris; green: piles of dry pre-mixed materials; blue: piles of tuff blocks.
Fig. 7: Schematic summary of the ancient production of Roman concrete.
Each step in the schematic captures a specific part of the preparation and building process, highlighting both the materials and the tools used.
A Preparing the dry mortar mix: This process begins with a dry, pre-mixed pile of pozzolan and quicklime.
B Adding cocciopesto: To enhance the mortar’s water resistance and durability, Romans often added cocciopesto, a mix containing ground terracotta or pottery fragments.
C Hydrating the mortar: The dry mortar mixture is then hydrated, where workers would carefully add water to initiate a chemical reaction with the quicklime.
D Slaking lime: slaking, or hydration of quicklime to produce slaked lime before being added to the mortar mix, often took place in smaller vessels. This step was not included in the preparation of Roman concrete and was generally used in the preparation of finishing and repairing mortars and fresco decorations.
E Transporting mortar in broken amphorae: Broken amphorae, or pottery vessels, served as practical containers for moving mortar. These vessels were readily available on Roman construction sites and were a sustainable choice, repurposing broken pottery that would otherwise be discarded.
F Applying the mortar with a trowel: The mortar was then moved to the wall and applied with a trowel, carefully layering it between caementa and stones or bricks.
G Ensuring geometric and structural integrity with a plumb line: As construction progressed, workers used a plumb line to verify vertical alignment. This simple yet effective tool helped maintain precision during the construction process, which was essential for stability, especially in tall or expansive structures.
H Shaping aggregate with an ax: To create aggregate, an essential component of Roman concrete, workers used axs to break larger stones into smaller pieces.
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