“The formula of the mortar used in construction in the Roman Empire is still unknown.”

For many years, researchers have assumed that the key to the ancient concrete’s durability was based on one ingredient: pozzolanic material such as volcanic ash from the area of Pozzuoli, on the Bay of Naples. This specific kind of ash was even shipped all across the vast Roman empire to be used in construction, and was described as a key ingredient for concrete in accounts by architects and historians at the time. Under closer examination, these ancient samples also contain small, distinctive, millimeter-scale bright white mineral features, which have been long recognized as a ubiquitous component of Roman concretes… Studying samples of this ancient concrete, he and his team determined that the white inclusions were, indeed, made out of various forms of calcium carbonate… spectroscopic examination provided clues that these had been formed at extreme temperatures… Hot mixing, the team has now concluded, was actually the key to the super-durable nature.

Here we show that Roman builders used dry premixed mortar containing volcanic tephra and quicklime, which was later hydrated at the construction site. Microscopic and spectroscopic analyses of mortar samples from Privernum reveal abundant lime clasts with reaction rims and pore structures consistent with hot mixing. Our results corroborate the hypothesis that Roman concrete was produced using a hot mixing technique, and they provide direct evidence of the preparation and storage of a standardized mortar powder with known proportions of lime and volcanic aggregates.

“The physical, chemical, and mechanical properties of mortars and bricks used in the historical building that was erected at Myra within the boundaries of Antalya Province during the Roman time were investigated… The mortar of Roman age that was used at Myra bath was composed of calcite and quartz minerals. Calcite mineral shows that lime was used as binding substance… High percentage of calcium as found in the analyses conducted with SEM/EDX shows that pure lime was used in the preparation of mortar… It was observed that binder-aggregate ratio in mortar varied between 1/6 and 1/8.”

“An approach including OM in thin sections, XRPD, SEM-EDS, and electron microprobe analysis (EMPA) revealed that the mortar aggregate is mainly composed of pyroclastic products with pozzolanic behaviour from the Roman Magmatic Province… The binder is rich in Si and Al… XAS data confirmed the absence of calcite, portlandite, and other crystalline Ca-bearing phases in the Trajan samples… and indicated that an amorphous Ca‑rich aluminosilicate phase (C‑A‑S‑H) is the main Ca‑bearing phase… The results highlighted the peculiarity of the Trajan mortars, in which the amount of CaO is considerably lower than in literature data; consequently, an enrichment of Al2O3 and SiO2 is observed.”

Roman marine structures were built from a mortar of hydrated lime and volcanic ash (pozzolan) with tuff or brick aggregates. Chemical and microstructural analyses indicate that the original mixture proportions and reaction products can be identified, and Roman-style mixtures have been reproduced experimentally. These reconstructions show that the mechanical behavior and long-term resilience of Roman concrete are directly related to its well-characterized lime–pozzolan binder system.

Nature’s news coverage of Marie Jackson’s work notes: “Roman marine concrete was made from a mixture of lime and volcanic rock. For underwater structures, the Romans packed this mixture into wooden forms and then poured in seawater, which triggered a hot chemical reaction, as the lime hydrated and reacted with the ash.” The article explains that the resulting mortar creates durable mineral phases such as Al-tobermorite. This description shows that the principal components and reactions of Roman marine mortar are known, although researchers are still refining exact processing details.

“The analytical results confirmed the expected properties in chemical and mineralogical composition for the developed mortars given the raw [materials]… Experimental mortars were produced according to the compositions obtained from the characterization of ancient Roman mortars and show similar hydraulic behaviour… This study demonstrates that the material knowledge derived from Roman mortars can be used to design modern mortars with comparable performance.”

This scholarly chapter states that it “focuses on three different ingredients used to create hydraulic mortar: volcanic ash, crushed terracotta, and plant ash.” It discusses Roman-period mortars in detail: “In the Roman Empire, pozzolanic mortars were commonly obtained by mixing lime with volcanic sands (pozzolana) or crushed ceramics, which conferred hydraulic properties and allowed hardening under water.” The text treats the composition of Roman hydraulic mortars as definable and analyzable, outlining their component ingredients rather than presenting the formula as unknown.

Roman concrete (opus caementicium) was made from a mixture of lime mortar, water, sand and pozzolana, a volcanic ash. Vitruvius describes in De Architectura how the mortar should be mixed and in what proportions. The typical recipe involved one part lime to three parts volcanic sand, combined with an aggregate of stones or broken brick to form the concrete.

“The analytical results indicated that the Roman mortars were hydraulic, stiff, and durable materials due to the use of natural pozzolanic aggregates… Petrographic and chemical analyses showed that lime was used as the main binder, with volcanic or siliceous aggregates providing pozzolanic activity… Binder to aggregate ratios, aggregate grading and the presence of brick fragments were quantified for the studied sites.”

“By analyzing samples of 2,000‑year‑old Roman concrete, researchers identified a recipe that includes volcanic ash, lime (calcium oxide), volcanic rock and seawater… Microscopic and chemical analyses showed that Roman marine concrete is characterized by a C‑A‑S‑H (calcium‑aluminum‑silicate‑hydrate) binder produced by pozzolanic reactions between lime and volcanic materials… Understanding this composition has allowed modern scientists to reproduce similar concretes in the laboratory.”

The Ancient Roman’s concrete consisted of a mix of volcanic ash or also known as Pozzolana, lime, and water to make a mortar. The mortar was then mixed with the aggregate, often chunks of rock, to create Ancient Roman concrete. The volcanic ash or Pozzolana contains both silica and alumina which proved crucial for a chemical reaction.

Describing research into Roman concrete, the article writes: “Roman concrete was made with volcanic ash, lime (calcium oxide), and seawater, which were mixed with volcanic rock to form the aggregate.” It notes that the resulting material forms rare minerals like Al-tobermorite and phillipsite over time. The piece does mention that modern engineers are still trying to fully replicate Roman concrete’s performance, but clearly identifies the main constituents and general mixing approach of the Roman mortar.

Vitruvius writes: “mix your mortar, if using pitsand, in the proportions of three parts of sand to one of lime; if using river or sea-sand, mix two parts of sand with one of lime.” This lime-sand mortar could be combined with pozzolana to make hydraulic concrete capable of setting under water. Roman builders commonly used volcanic ash from the Bay of Naples region, known as pozzolana, with lime and aggregates of stone or broken brick.

Researchers from the Massachusetts Institute of Technology (MIT) and Harvard University announced a breakthrough in understanding the durability of ancient Roman concrete… the team, led by Professor Admir Masic, uncovered that the Romans employed a technique called "hot mixing," which could explain the remarkable resilience of structures like the Pantheon. The lime clasts resulting from hot mixing imparted self-healing properties to the concrete… The team tested this theory by creating concrete samples using ancient recipes with quicklime and controls without it… the quicklime concrete healed completely within two weeks, while the control samples remained cracked. Not all experts agree on the centrality of hot mixing… Marie Jackson… believes the key lies in the materials mixed with lime, such as pozzolana—a type of volcanic ash.

For decades, scientists and engineers have been studying Roman concrete to understand its remarkable durability. Analyses of cores and reconstruction experiments show that Roman builders typically mixed lime and volcanic ash—pozzolana—with rock or brick rubble to form concrete. In marine works, the interaction of seawater with this lime-pozzolan mixture leads to the formation of new minerals that strengthen the concrete over time. While debate continues over the relative importance of different ingredients and curing environments, research has largely decoded the basic recipe and mechanisms of Roman concrete.

Now scientists have found a hidden process in the mix of volcanic ash, lime, seawater and lumps of volcanic rock: salty water and volcanic material can interact to create new minerals, reinforcing the concrete over time. Marie Jackson, a geologist at the University of Utah, co-authored a study on these recent findings about Roman concrete composition… they could zoom into “the tiny natural laboratories in the concrete, map the minerals present, the succession of the crystals that occur, and their crystallographic properties.” It all clicked when researchers realized that unexpected materials they found in ancient concrete weren’t added ingredients — instead, a chemical reaction had created them in place.

The paper summarizes current understanding of Roman concrete: “The Romans used volcanic ash as a binder for their concrete and seawater as a mixing and curing agent… The Romans called their concrete opus caementicium… Roman concrete essentially consisted of volcanic ash, a white powder called quicklime, small particles and rock fragments called tephra and water.” It adds: “As mentioned before, The Romans made concrete by mixing lime with volcanic ash and seawater to make a mortar and then adding to that mortar chunks of volcanic rock which served as the aggregate in the concrete. The combination of ash, water and quicklime provides what is called a pozzolanic reaction.” This treats the mortar formula (lime + volcanic ash + seawater, with volcanic aggregate) as known.

Roman concrete, also called opus caementicium, was the core of every Roman wall after the 2nd century BC. Opus caementicium is a construction technique using an aggregate, water and a binding agent. The binding agent is usually called mortar like lime, gypsum or pozzolana… To protect the surface of the clay from erosion, the ancients discovered that a moist coating of thin, white, burnt limestone would chemically combine with the clay to form a hard protective surface.

The concrete was made from three components: aggregate, mortar, and facing. The aggregate was made from stones, terra cotta, or tiling. The mortar was created by mixing lime, water, and ash or sand. The main ash that was used reacted with water and lime to create a strong and waterproof bond… Roman concrete also contained elements that reacted with water when damaged to self-heal and create stronger bonds.

“The great majority of historic mortars were based on lime, but traditional limes varied depending on many factors including the particular limestone burnt, kiln design, fuel type and kiln temperature… Binder:aggregate ratios vary and pozzolanic reactions may have occurred, or deleterious processes taken place… Thin section analysis can be used to augment the standard analysis and gives further information on the sample.”

While the basic composition of Roman mortar—lime, volcanic ash (pozzolana), water, and aggregate—and even some typical proportions are known from Vitruvius and modern chemical analyses, researchers still debate regional variations, construction-site practices, and the full range of additives used in different periods and environments. Phrases such as "lost secrets" or "lost formula" are sometimes used in popular media to refer to these unresolved details and to the fact that we do not have a single, universal, fully specified recipe that explains the performance of all Roman concretes in every context.

The narrator states that "this ash known as [from the town of Pozzuoli] is the heart of the lost formula" and that "their recipe achieved a durability we are still struggling to replicate." The video repeatedly refers to the "lost formula" of Roman mortar and emphasizes that modern industry "could not easily replicate" the slow, low-temperature chemical reactions responsible for its longevity, framing the exact method as something not yet fully recovered.