Introduction

Through the rise and fall of the Roman Empire, concrete preserved. Due to it’s strength and versatility in composition, concrete became the building block for civilizations dating over two thousand years ago. As more of ancient Rome is uncovered, scientists have experienced firsthand the durability of Roman concrete. The unearthed mixture is a testament to the ingenuity of Romans; concrete allowed for large scale engineering projects from the Pantheon, to the maritime city of Ostia. Such achievements helped to advance Rome as a society for centuries. A concrete base combined with other construction components, such as bricks, marble or travertine, formed many buildings in ancient Rome, great number of which still stand today. Many of these concrete buildings now act as the foundation for present day Rome; the durability of Roman concrete quite literally holds up the city as we know it today. Though numerous studies have been performed on concrete since its rediscovery in the 18th century, there are few sources on Roman concrete that date back to the time of it’s discovery and widespread use. It is likely that academics and researchers at the time were documenting their findings about concrete as it developed, but very few of their findings survived. One of the largest remaining texts from the time of development is titled “De Architectura” or “The Ten Books on Architecture.” Written by Roman military engineer and architect Marcus Vitruvius Pollio, this work comprises of personal experiences and general advice on topics ranging from building materials, to water and aqueducts, to miliraty tactics and equipment. This text has been studied extensively, as it gives great insight as to how architecture was processed in the time of Julius Caesar (Cartwright, 2015). With such texts as a starting point, large quantities of research have since been done concerning concrete, cement, aggregates and all other aspects from the chemical reactions occurring within to strength tests in laboratories. This paper gives a historical background of the development of concrete and examines Vitruvius’s take on concrete and its compositions compared to today’s concrete. Figures 1 and 2 show two examples of concrete structures that stand today in the heart of Rome.

Figure 1. The Roman Colosseum is an iconic sight in Rome today, largely composed of Roman concrete. Photo by author, 2017.

Figure 2. The Roman Pantheon is one of the oldest churches in Rome, and an important figure in the world of concrete domes. Photo by author, 2017.

Ancient Concrete

Though older mixtures with cementitious properties were found in India as early as 2500 B.C., Romans routinely utilized mixtures of lime, water and pozzolan in their everyday lives (Moore, 1995). Despite how long concrete has been around, the development of concrete as a building material was a long and drawn out experiment. Unique reactions were often observed when pozzolans (in many Roman cases, a loose volcanic ash) from different sites were used in different situations, leaving much up to trial and error. The process to develop concrete, though scientific, was anything but singular. It involved numerous materials that slowly developed into what we now recognize as concrete. Many of these steps are recorded in the following sections.

Predecessors

Hydraulic mortar (a key part in concrete) in Rome can be traced back as far as the third century B.C., but before concrete came a plethora of other materials (Oleson, 2014). Most notably, early builders used clay to form walls of structure. Clay could be used in a multitude of ways, as it is malleable and adhesive. The silica and alumina found in concrete were also present in clay, giving the two materials similar properties (Moore, 1995). Though not as advanced as concrete, clay offered early builders a base from which to build upon.
First, clay could be used in conjunction with brick, stacked effectively to form walls. In “The Ten Books of Architecture,” Vitruvius details many types of bricks, as well as their uses and differences. He advises bricks made from red or white clay, as they are lighter, more durable, and easier to lay. Early Romans used such bricks in their construction; they used sun-dried bricks because undried bricks posed threats. When undried bricks lost their moisture and shrunk, they receded from the cementitious lime stucco holding them together. This could then endanger the structural integrity of the wall (Vitruvius, 20 BC). Despite the numerous conditions that Vitruvius suggested for using bricks, many buildings in early Roman history used this technique. Bricking did not require skilled labor, and they could be produced in bulk. Though clay was often replaced later with concrete, the use of bricks in Roman buildings continued. Today, bricks are still widely used, and many of their characteristics align with Vitruvius’ findings. Along with small additives and shale (pulverized sedimentary rock, similar to that which was used in Roman bricks), clay is still used in the bricking industry today (Hughes, 1979). Though the type and composition may vary from that which Vitruvius advises, the basic idea aligns with what is used today giving credit to Vitruvius’ findings. Though effective, bricks quickly gave way to cementitious like materials examined next.

Key Roman Components

Lime

The bricks alone, while stable, could not withstand the abuse of weathering. In need of a new technique, Romans began mixing lime with sand and other materials to create paste. The lime used in the stucco introduced a new development in concrete. Although a defined date is unknown, Romans began baking limestone in a process to produce a hardened shell. Vitruvius details that white limestone should be used, and that solid and porous lime should be used for structural and stucco respectively (Vitruvius, 20 B.C.). These specifics indicate that Romans examined different limestone and their uses in order to determine which worked most effectively in different cases. 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… Further, in using river or sea-sand, the addition of a third part composed of burnt brick… will make your mortar of a better composition to use” (Vitruvius, 20 B.C.). Testing and observation, though different from our scientific method today, were present and prevalent in ancient Rome. As seen in many scientific journals today, such as “Masonry Repair Lime-Based Mortars: Factors Affecting the Mechanical Behavior,” scientists and engineers are still searching for the ideal compositions. In this study, published in 2003, different ratios by volume of lime to aggregate were tested to see how the different compositions affected strengths and behaviors (Lanas, 2003). This work is similar to the work that Vitruvius and other scientists at the time would have performed.

The process of creating this stabilizer was simple; limestone was one of the main components in the hardened paste used in many Roman buildings. Limestone is a sedimentary rock comprised of organic remains. Although it is likely to have been used earlier, lime was in use as early as 2600 B.C by the Egyptians (Knibbs, 1924). Limestone in Italy could be obtained directly in many areas near the Bay of Naples. The heating aspect drove off carbon dioxide in the material and Romans were left with quicklime. When placed in water, quicklime turned to slaked (or hydrated) lime and became paste-like. This was then mixed with pit sand, creating a workable paste. (Vitruvius, 20 B.C.). This mixture could be spread and when exposed to air, it gained hardened properties (Moore, 1995). This was an important component in ancient wall construction. However, it will be discussed later how lime provided much more than a hardened shell in the development of concrete.

Pozzlolan

One of the most important aspects of Roman concrete was pozzolan, an amorphous silica that bonded with the stucco. Numerous pozzolans were used, but the most common were volcanic ash and burnt brick that was abundant in regions of Italy. Pozzolan gains its name from Pozzuoli, near Naples. This was where the first pozzolan was mined (Blake, 1892). Pozzolan continued to be mined throughout the age of in order to supply cementitious material during periods of large urban development.

Due to the large number of then active volcanoes near Rome, many different types of pozzolan were used and had different effects. Quality ranged a great deal, often depending on where the pozzolan was procured. It is assumed that the strengths and weaknesses of different pozzolans were largely determined by trial and error, and Vitruvius writes of many Roman findings. Vitruvius wrote quite extensively on pozzolans. He states pozzolan can be found in the areas surrounding Mount. Vesuvius. He comments on the pozzolan and lime composition, stating that constructing in the sea further strengthens the process (Vitruvius, 20 B.C.). These findings have been mirrored today, and many scientists credit the Roman Naval power to be attributed to their concrete that is not dissolved by water. Vitruvius credits much of this power to pozzolan.
One of the easiest manners of differentiating pozzolans was by their color. Pozzolans that were red in color often created the highest quality, and the location of the pozzolan also played a large factor in pozzolan strength (Moore, 17). The pozzolan and lime combination provided Romans with an unusually hardened mixture that formed the basis of Roman concrete. Romans used this mixture combined with bricks or other earthen material for hundreds of years.

One of the earliest recorded uses of pozzolan was found in a wall in Pompeii. Said wall dates back to as early as 300 B.C., where black pozzolan was found. Though this porous type of pozzolan was considered inferior, it was generally found atop the other types of red and yellow pozzolan and therefore would have been used prior to the latter types (Blake, 1892). Many Roman emperors continued to ship pozzolan like that in Pompeii to Rome during times of large-scale construction. Figures 3 and 4 below show a visual representation of where the pozzolan was often found, and some of the routes by which it ended up in Rome. This routine usage of a lime-pozzolan mixture aided the Roman empire in a time of massive conquest and construction advancement. Rome, however, could not expand forever and material construction was generally halted.

Figure 3. A map of Naples, Italy. Just east of the city lies Mount Vesuvius, a large source of Roman pozzolan. Encyclopedia Britannica.

Figure 4. A map of Pozzuoli, Italy. First traces of pozzolan were found here. Pozzuoli lies west of Naples. Encyclopedia Britannia.

Rediscovery and Advancement

Despite the leaps and bounds that Romans made in the development in concrete, construction largely grinded to a halt after the death of Hadrian in 138 A.D. By this time, enormous amounts of concrete had been utilized to build and rebuild after the Great Fire of Rome. However, massive urban development stalled in the period after Hadrian due to the need to repair rather than expand (Moore, 1995). The repair required after difficult times fell on Rome used concrete just as expansion had, but advancement in concrete development fell as focus shifted. Monumental structures were still appearing all across the Roman empire, but the science of concrete composition slowed. Flooding, fires and earthquakes all contributed to the slow but steady halt of development. By the time of the first sacking of Rome in 410 A.D., many aspects of Roman concrete were lost as focus shifted from expansion and development to the need for stabilization (Moore, 1995). After a decline in it’s use and experimentation, methodological advances in concrete would not resurface for centuries. In “Lea’s Chemistry of Cement and Concrete,” Peter Hewlett writes “A gradual decline in the quality of the mortar used in buildings set after Roman times, and continued throughout the Middle Ages. Saxon and Norman buildings, for instance, show evidence of badly mixed mortars, often prepared from imperfectly burnt lime. The conclusion appears certain, from the examination of French buildings, that during the ninth, tenth and eleventh centuries the art of burning lime was almost completely lost” (Hewlett, 1998). The decline in Roman concrete advances occurred around the same time as the general decline in quality and stability of concrete around the world. Real concrete advancement would not be seen until the construction of the Eddystone Lighthouse, as pictured in Figure 5.

Smeaton’s Process

Figure 5. John Smeaton’s Eddystone Lighthouse in England. Smeaton was credited with the rediscovery of hydraulic lime. Encyclopedia Britannica.

It wasn’t until centuries later that civil engineer John Smeaton was credited for hydraulic lime, the process of joining limestone, clay and water to create a hardened mass similar to that of Roman concrete (Moore, 1995). The chemical reactions present in his creation were comparable to those present in Roman concrete, and many parts of modern concrete have been based off of his findings. Smeaton was studying impure limestone and selected a mixture high in clay content to build the Eddystone Lighthouse. His mixture contained silica, alumina and iron oxide, which were compounds also found in pozzolan (Teutonico, 1993). When exposed to water, the limestone mix hardened into a hydraulic lime similar to that used in ancient Rome. This discovery paved the way to more findings in the making of modern concrete.

Modernizing Quicklime

After the completion of Eddystone Lighthouse, James Parker received the first patent on natural cement in 1796. He burned lime into quicklime as many others had, but changed the delivery method of quicklime into water. Parker ground the quicklime into dust, thus increasing the surface area ratio of quicklime to water (Thurston, 2014). This crucial step helped by accelerating the reaction. The cement required less heating and the technique of grinding quicklime has been continued to this day.

Vicat’s Contributions to Understanding

One of the most important advances in understanding and advancing Roman concrete came in 1818 with C.J. Vicat’s experimentation with lime and its reactions. By this time, pozzolanic materials from Italy were available and Vicat published a paper on his laboratory findings. He was the first documented scientist to show exactly how the chemical reactions in the pozzolan-lime caused hardening when exposed to water, while reactions in sand-lime mixtures did not experience the same strength (Moore, 1995). By this time, many aspects of Roman concrete were understood and could be replicated. The following components help understand concrete as it is known today, and why it acts as it does.

Compositions

To fully understand concrete, a closer look must be taken at the reactions occurring at each step of the process. It is simplest to look at limestone first, as it was one of the first to be utilized.

Lime Mortar

Limestone is an overarching name for materials that are predominately calcium carbonate (CaCO3). Concerning the history of lime use, Knibbs writes, “Lime was one of the first chemical reagents to be used by mankind, and lime-burning is one of the oldest chemical industries. The process of calcining limestone is probably nearly as old as the use of fire, because in any limestone country the effect of lighting a fire on the stone would be to produce lime, but the intelligent application of the process must have come much later” (Knibbs, 1924). As is common today, we begin our process with lime.

Lime itself must be manufactured from existing compounds. Namely, lime can be produced when burning limestone, chalk, or other materials high in calcium carbonate. Lime undergoes distinct chemical reactions when converting to different steps in the chemical process; the chemical makeup is different for lime in unique forms. Below are some of the common types:

• Lime refers to a mix of both calcium oxide (CaO) and magnesium oxide (MgO).
• Limestone refers to lime mixture that is primarily (at least 90 percent) calcium carbonate (CaCO3).
• Quicklime refers to the material produced when limestone is cooked (CaO and MgO). Calcination point is approximately 900 degrees Celsius. Lighter than limestone due to water and carbon dioxide being driven off.
• Slaked (hydrated) lime refers to the material produced when quicklime is introduced to water (Ca(HCO3)2). The mixture turns paste and plaster-like when mixed with sand, creating a composition known as hydraulic mortar.

The slaked lime is then hardened in a process called recarbonation. Hydroxide combines with the air to give the material solid properties (Moore, 1995).
The chemical reactions listed in Table 1 show the physical changes that occur to harden lime:

Table 1: Reactions of Lime

(Massazza, 1998)
This process of hardening has preserved the mixture from air for centuries, and the airtight properties have prevented the establishment of new chemical reactions. Though Vitruvius does not comment on the chemical compositions of concrete at the time, the processes by which Romans made concrete would have yielded reactions similar to those we see today. Evidence of this lies in the concrete’s strength in water.

Pozzolan and Other Aggregates

Aggregates were a key part to Roman concrete. Aggregates include the materials in concrete that were not cementitious, notably sand, crushed brick, or other rocks of varying sizes. Of the common aggregates used in ancient Rome, pozzolan was generally the most effective. Originating from the areas surrounding Naples, pozzolan is composed primarily of silica. Franco Massazza described the origins of natural pozzolan in “Lea’s Chemistry of Cement and Concrete” when he states “Pyroclastic rocks result from explosive volcanic eruptions which project minute particles of melted magma into the atmosphere. The rapid pressure decrease occurring during the eruption causes the gases originally dissolved in the liquid magma to be released. As a consequence, each particle will contain a number of microscopic bubbles and ducts forming a microporous structure. Simultaneously, the particles are subject to a quenching process which is responsible for their glassy state. The material can be deposited either on the ground or in the water” (Massazza, 1998). Unique pozzolans with differing effects could be found all around Italy because unique volcanoes produce differing pozzolans. As the Roman had settled in an area of high volcanic activity, natural pozzolan was abundant. Pozzolan was then mixed with lime to create a mortar that became central to Roman concrete. The term pozzolan grew to encompass any compound that was predominantly silica and could react with lime; volcanic debris, residual ash from burning coal, crushed and burned clay, and burnt rice hulls (waste product of rice) all fell under the general term pozzolan.

Vitruvius goes into significant detail about why pozzolans from one region have properties similar to those a great distance away. He explains “That there is a burning heat in these regions may be proved by the further fact that… there are places excavated to serve as sweating-baths, where the intense heat that comes from far below bores its way through the earth, owing to the force of the fire, and passing up appears in these regions, thus making remarkably good sweating-baths” (Vitruvius, 20 B.C.). This provides information about how pozzolans from non-volcanic sources were possible. Figures 6 and 7 show two different types of volcanic eruptions: explosive and non-explosive. Each provided Romans with unique pozzolans with unique properties.

Figure 6. Mount Vesuvius erupts in 1944. This is an example of an explosive eruption, leading to small bubbles in pozzolans. The Telegraph, 2014.

Figure 7. Mount Etna erupts in 1983. This is an example of a non-explosive eruption, leading to large bubbles in pozzolan. The Telegraph, 1983.

On top of the numerous different types of pozzolan, pozzolans of the same type also contained large discrepancy in their chemical make up. The general makeup of pozzolan involves some mixture of silicon dioxide, aluminum oxide, titanium oxide, magnesium oxide, and iron oxide. The following table was accessed from David Moore’s “The Roman Pantheon: The Triumph of Concrete,” and originally by H. F. W. Taylor in “The Chemistry of Cements”.

Chemical Analysis of Pozzolans (Moore, 1995)

The remaining percentage of pozzolan is taken up by some differing combination of FeO, CuO, K2O and MgO.
The above table features natural pozzolan. Man made pozzolans can have slightly different composition. The following is taken from Moore, the data also credited to Taylor.

Analysis of Burned Clay and Gaize (Moore, 131)

The closest thing Vitruvius has written in regards to composition or materials is when he advises two parts pozzolan to one part lime. The chemical compositions (or specific compaction factor) of the pozzolan, however, is not noted by Vitruvius. Despite this, scientists today are still looking to Vitruvius’ work for guidance. For example, E. Gotti et al performed a study in which they “modified the Vitruvian formula to 2.7 parts pozzolana to one part lime” in an effort to “obtain information concerning the rate at which Roman concrete cured, and the efficacy of the Vitruvian formula” (Gotti, 2008).

An important factor in effectiveness of pozzolans is the amount of surface area that is exposed to lime. Most pozzolans were ground into a fine dust, much like the equivalent material used in concrete today. This increased surface area enabled Romans to create an extremely potent mortar, as the greater surface area led to a stronger reaction and a more potent paste. The silica that is formed when pozzolan is mixed with lime expands into voids in the mixture and bonds the two together (Moore, 1995).

Along with a large ratio of large surface to whole, compaction of the concrete was essential for durability. A compact mix will hold far more reliably than one that is more free-flowing. This has to do with how well the pozzolan and lime can mix, and how many bonds can be established in voids. It was common practice for Romans to use wooden tools to pound the mixture into the ground, thereby increasing the durability. Chemically, compaction is important because the less distance between the silica and calcium hydroxide, the stronger the paste will be (Blake, 1947). If the mix is not compact enough, sufficient bonds cannot establish and the mix will be lacking in strength.

Perhaps one of the most important aspects of Roman concrete was that of hydraulic mortar. Hydraulic mortar is any mortar that undergoes strengthening through hydration (as mentioned above). Non-hydraulic mortars include simple pastes such as mud and simple clay, though Romans did not prefer these materials. They did not hold well and were rarely used. Despite this, Marion Elizabeth Blake wrote, “Still, it was common enough for certain properties to become apparent, since clay was retained for number of specialized uses after the introduction of a mortar of lime and sand or pozzolana. It is found particularly in construction exposed to fire, such as kilns, ovens and hypocauses, where it was hardened by the heat” (Blake, 1947). Alternative to the bonding in water, heat was commonly used but many Romans discredited the process. Among them was Vitruvius who considered the non-hydraulic mud process dissatisfactory when used alone. Romans shifted many project away from this procedure. Hydraulic mortar, on the other hand, was widely used in ancient Roman concrete (Vitruvius, 20 B.C.). The process of hydrating quicklime gave Rome concrete of considerable strength by their standards. It was therefore used far more commonly and when added to pozzolan, water, and aggregate, created a mixture similar to that used today. Because many ancient Roman buildings are still standing today, scientists have been able to extrapolate information about their construction methods, ways of life, and much more. These aspects are present in the following applications.

Roman Applications

Roman concrete enabled Romans to create lasting structures on a scale previously unseen. Though not all visible above ground, many churches, housing complexes and other buildings still exist today. They offer researchers an insight as to how Romans might have lived over two thousand years ago. The three following concrete structures once played major roles in Rome as a city and could be linked directly to the importance of Roman concrete.

Figure 8 features the Pantheon, an important religious site and iconic figure dating back to the age of Marcus Vipsanius Agrippa. As well as being a longstanding site of religion importance, the Pantheon is one of the world’s greatest engineering feats as it features the world’s largest unreinforced concrete dome. By using the emerging concrete technology and design, builders created a structure with light aggregate to minimize the structural load (Moore, 1995). That Romans were able to complete such a structure with their concrete mix is astounding; it is difficult to imagine engineers today attempting a dome on this scale without some type of reinforcing. The completion and Pantheon marked a historic point in time for both concrete development and structural analysis.

Figure 8. The Pantheon in Rome. The Pantheon has the largest unreinforced concrete dome in the world. Photo by author, 2017.

Figure 9 features the interior Aqua Claudia, one of the eleven aqueducts that provided water to Rome. Rome has been known for its aqueducts for centuries, and for good reason; ranging from ten to fifty seven miles in length, these structures were a testament to Roman water and structural engineering. Incorporating both over and underground transportation of water, many of the Roman aqueducts utilized concrete in multiple ways (Kosonen, 2017). At a different section than that seen in figure two, the Aqua Claudia was at least partly constructed and held up with concrete in the supports and arches. Arches were a large part of structural design in ancient Rome, and significant portions of them were completed with concrete. Along with an important role in helping hold up the Aqua Claudia, concrete was used on the interior of the aqueducts. Due to its hydraulic nature, exposure to water hardened and strengthened the concrete and made it a natural choice for the interior. Both finished and unfinished concrete was used to line the aqueducts, which helped to reduce the failure in the interior of the structure. The concrete insides were noteworthy because they helped keep the structure together as the concrete was strong in nature. On top of this, fewer failures in the interior meant fewer repairs needed and less time that Romans were down one of their essential aqueducts (Morabito, 2017). Today, aqueducts in Rome are a distinct feature.

Figure 9. The interior of the Aqua Claudia. Finished and unfinished concrete were used inside Roman aqueducts, as concrete retained strength in the presence of water. Photo by author, 2017.

Figure 10 captures a small housing unit at Ostia Antica, what was once a large port city that fed into Rome. A majority of the buildings at Ostia were concrete in nature. Aside from the structural importance of using concrete for the strength, ancient Romans utilized concrete to create extensive seaports. Roman concrete hardened when exposed to water, which made it a logical choice of material for building in water. This hardening process allowed Romans to effectively trade by sea at greater distances, an important aspect for growth in any civilization (Rankin, 2017). This was just another manner in which Roman concrete helped expand the ancient Roman Empire, both physically and culturally.

Figure 10. A housing complex at Ostia Antica. Roman concrete enabled Romans to expand to trading cities with ports, as their concrete could withstand exposure to oceans and rivers. Photo by author, 2017.

Closing

Roman structures were largely based on concrete, much as ours are today. Through years of careful observations and experimentation, Romans were able to produce a concrete mixture that has held up for over two thousand years. Many concrete structures from ancient Rome still stood to see the rediscovery of concrete in the seventeen hundreds, which is a testament to the durability and reliability of the Roman mixture. Concrete allowed the Romans to build massive monuments, and has allowed engineers today to do the same. By continuing to experiment and develop our concrete mix today, stronger and more efficient concrete may become apparent in the years to come.

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