Archive for category: Materials

Out of the 4 billion tons of cement produced every year, the overwhelming majority is Portland cement.  Invented over 200 years ago in France by Louis Vicat — then patented by Englishman Joseph Aspdin —Portland is a star in the world of building materials. Its almost unparalleled durability has allowed it to outperform its competitors, so much so that the synthesis methods and other cement formulations used in the 19^th and early 20^th centuries have since been forgotten. Yet, buildings constructed with these cements still stand today, and cannot be restored using Portland, which has become a monopolistic cement. In a quest to retrieve this lost technical expertise, Vincent Thiéry, a researcher at IMT Lille Douai, has launched the Cassis^[1] project. In the following interview, he presents his research at the border between history and materials.

How can we explain that the cement industry is now dominated by a single product: Portland cement?

Vincent Thiery: Cement as we know it today was invented in 1817 by a young bridge engineer:  Louis Vicat. He needed a material that had high mechanical strength, which would set and have strong durability under water, in order to build the Souillac bridge over the Dordogne river. He therefore developed a cement based on limestone and clay fired at 1,500°C, which would later be patented by an Englishman, Joseph Aspdin, under the name Portland Cement in 1824. The performance of Portland cement gradually made it the leading cement. In 1856, the first French industrial cement plant to produce Portland cement opened in Boulogne-sur-Mer. By the early 20^th century, the global market was already dominated by this cement.

What has become of the other cements that coexisted with Portland cement between its invention and its becoming the sole standard cement?

VT: Some of these other cements still exist today. One such example is Prompt cement, which is also called Roman cement — its ochre color reminded its inventor of Roman buildings. It’s an aesthetic restoration cement invented in 1796 by Englishman James Parker. It’s starting to gain popularity again today since it emits less CO₂ into the atmosphere and can be mixed with plant fibers to make more environmentally-friendly cements. But it’s one of the few cements that still exist along with Portland cement. The majority of the other cements stopped being produced altogether as of the late 19^th or early 20^th centuries.

Have these cements always had the same formulation as they do today?

VT: No, they evolved over the course of the second half of the 19^th century and were gradually modified. For example, the earliest Portland cements, known as “meso-Portland cements,”  were rich in aluminum. There was a wider range of components at that time than there is today.  These cements can still be found across France, in railway structures or old abandoned bridges.  Although they are there, right before our eyes, these cements are little-known. We don’t know their exact formulation or the processes used to produce them. This is the aim of the CASSIS project – to recover this knowledge of old cements. The Boulogne-sur-Mer region in which we’ll be working should provide us with many examples of buildings made with these old cements, since it was where cement started its industrial rise in France.  In the Marseille region, for example, research similar to that which we plan to conduct was carried out by the concrete division of the French Historic Monument Research Laboratory (LRMH), one of our partners for the CASSIS project. This research helped trace the history of many “local” cements.

How do you go about finding these cements?

VT: The LRMH laboratory is part of the Ministry of Culture. It directly contacts town halls and private individuals who own buildings known to be made with old concretes. This work combines history and archeology since we also look for archival documents to provide information about the old buildings, then we visit the site to make observations. Certain advertising documents from the period, which boast about the structures, can be very helpful. In the 1920s, for example, cement manufacturer Lafarge (which has since become LafargeHolcim) published a catalogue describing the uses of some of its cements, supported by photos and recommendations.

Once a structure has been identified as incorporating forgotten cements, how do you go back in time to deduce the composition and processes used?

VT: It ‘s a matter of studying the microstructure of the material.  We set up an array of analyses using rather conventional techniques in the field of mineralogy: optical or scanning electron microscope, Raman spectroscopy, X-ray diffraction etc. This allows us to detect mineralogical changes that appear during the firing of clay and limestone. This study provides us with a great deal of information: how the material was fired, at what temperature and for how long, as well as whether the clay and limestone were ground finely before firing. Certain characteristics of the microstructure can only be observed if the temperature has exceeded a certain level, or if the cement was fired very quickly. As part of the CASSIS project, we’ll also be using nuclear magnetic resonance since the hydrates— which form when the cement sets — are poorly crystallized.

It is this, along with other types of microstructural evidence within a cement paste, whether mortar or concrete, that makes it possible to gain valuable insight into the nature of the cement used. It is a relic of a speck of non-hydrated cement in a mortar from the early 1880s. To make these observations, samples are prepared in thin sections (30 micrometers thick) in order to be studied with an optical microscope. The compilation of observations and analyses of these samples provides information about the nature of the raw mix (the mix before firing) used to make the cement, its fuel and firing conditions:  the same approach will be used for the Cassis project.

Do you have a way of verifying if your deductions are correct?

VT: Once we’ve deduced the possible scenarios for the mix and process used to obtain a cement, we plan to carry out tests to verify our hypotheses.  For the project, we will try to resynthesize the forgotten cements using the information we have identified. We even hope to equip ourselves with a vertical cast-iron kiln to reproduce firing from the period, which is marked by the irregularities of the firing conditions in the kiln. By comparing the cement obtained through these experiments with the cement in the structures we’ve identified, we can verify our hypotheses.

Why are you trying to recover the composition of these old cements? What is the point of this work since Portland cement is considered to be the best?  

VT: First of all, there’s the historical value: this research allows us to recover a forgotten technical culture. We don’t know much about the shift from Roman cement to Portland cement in the industry of the period. By studying the other cements that existed at the time of this shift, we may be able to better understand how builders gradually transitioned from one to the other. Furthermore, this research may be of interest to current industry players. Restoration work on structures built with forgotten cements is no easy matter: the new cement to be applied must first be found to be compatible with the old cement to ensure strong durability. So from a cultural heritage perspective, it’s important to be able to produce small quantities of cement adapted to specific restoration work.

^[1] The CASSIS project is funded by the I-SITE Foundation (Initiatives-Science – Innovation –Territories– Economy) at the University of Lille Nord Europe. It brings together IMT Lille Douai, the French Historic Monument Research Laboratory (LRMH) of the Ministry of Culture, Centrale Lille, the Polytechnic University of Hauts-de-France, and the technical association of the hydraulic binders industry.

Article written in partnership with The Conversation France.
By Rodolphe Sonnier, IMT Mines Alès.
This article was co-authored by Clément Lacoste (IMT Mines Alès), Laurent Ferry (IMT Mines Alès) and Henri Vahabi (Université de Lorraine).

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[dropcap]T[/dropcap]he discovery of fire is often cited as the most important discovery in the history of mankind, given its major impact on the development of the Homo genus. By reducing the amount of energy required to digest food, cooking led to an increase in brain size. Fire seems to have been mastered approximately 400,000 years ago, although evidence of its use much earlier has been found. However, with urbanization, fire has also become a serious problem when it spreads uncontrollably. Examples include the great fire of Rome in the year 64 AD or the recent fire at the Notre Dame de Paris Cathedral.

What is fire?

A fire requires a combination of three elements: a fuel source, an oxidizer and a heat source. This combination of elements is called the fire triangle. These elements interact through a complex process involving physical phenomena, such as heat transfer, and chemical phenomena, such as pyrolysis of the fuel source and combustion of the pyrolysis products.

Technically, a distinction is made between reaction to fire and fire resistance. Reaction to fire involves the combustible materials, which are likely to release heat when they decompose as a result of the temperature and in the presence of an oxidizer (most often the oxygen present in the air). Fire resistance considers an element’s ability to maintain its load-bearing capacity, thermal insulation and smoke and gas tightness properties during a fire. Since wood is a combustible material used as a structural element in buildings, it is considered in light of both of these aspects, which rely on specific standards and a variety tests.

When it comes to fighting fire, there are two strategies which are not mutually exclusive. The first calls for using what are referred to as active systems in the event of a fire: extinguishers, smoke detectors or automatic sprinkler systems. The second consists in using materials that will contribute as little as possible to the propagation of the fire.

Fireproofing

Since many materials, including most plastics and wood, are naturally highly flammable, additives called flame retardants must be incorporated within or on the surface of the flammable material. These flame retardants make it possible to modify the material’s behavior by disrupting the fire triangle.

Their effects are mainly to delay the appearance of flames, slow down flame propagation speed, reduce the heat released and the power of the fire, and limit the opacity and toxicity of the smoke.  All these effects are assessed through standardized reaction to fire tests. They result in classifications that determine the potential use of a material for a given application according to regulations. There is no universal flame retardant. A fireproofing system must be tailored to the material it is intended to protect, in particular by taking into consideration its decomposition process. Furthermore, the choice of a flame retardant is also informed by the process used to manufacture the material and must not have a significant effect on its intended functional properties.

Archeologists place the beginnings of fireproofing in antiquity. Around 400 BC, the Egyptians used minerals to make certain fabrics like cotton or linen fire-resistant. Later,  during the siege of Piraeus (23 BC), alum solutions were used to make the wooden ramparts fire resistant. Yet it was not until 18 June 1735 that Englishman Obadiah Wyld would file the first patent, patent number 551, for a cotton treatment. In the 19^th century, the king of France, Louis XVIII, requested that a solution be found to prevent fires in Parisian theaters which were lit with candles.  Joseph Louis Gay-Lussac filed a patent for the use of a mixture of ammonium phosphate, ammonium chloride and borax to fireproof the curtains in theaters.

Flame retardants

There are several families of flame retardants, which are based on different chemical elements and work in various ways. Historically, halogenated molecules containing chlorine or bromine have been widely used since they are effective even in small quantities. These molecules act by disrupting the combustion reactions that take place within flames, which makes it easier to extinguish them and limits the amount of energy released. This is referred to as flame inhibition. However, the toxic nature of certain halogen compounds has led to a ban on their use. Since it is impossible during recycling to easily distinguish between authorized and prohibited brominated molecules, it is no longer possible to recycle plastics treated with these flame retardants. Moreover, these molecules lead to the formation of opaque, corrosive smoke in the event of a fire. For all these reasons, this family of fire retardants has increasingly come under scrutiny.

It has been replaced largely by phosphorous flame retardants. Given the wide variety of such retardants , they are able to act in a number of ways. But the main mode of action remains facilitating the formation of a residual layer on the surface of the combustible material, protecting the healthy part of the material. The strategy consists in disrupting pyrolysis reactions (decomposition of the material due to the heat) and facilitating the formation of a thermally stable residue rich in carbon, called “char.” Particularly effective systems are called intumescent because the char forms an expanded, insulating, very protective layer. This type of intumescent system is used in protective coatings for metallic components or wood.

We can also mention metallic hydroxides, which are inexpensive but proportionally less effective, meaning they must be incorporated at higher levels (up to 65% in mass in outer sheathes for wires) to produce a significant effect. As the result of the temperature, these particles release water in the form of vapor through endothermic decomposition, thereby contributing to cooling off the material and diluting the fuel in the flame.

There are also other chemistries, based on nitrogen (melamine), boron (zinc borate) or tin (hydroxystannate) for example. Nanotechnologies have also been used in the field of fireproofing for the past fifteen years. Lamelar clay or carbon nanotube type nanoparticles improve the insulating properties of the char formed, even at low levels.  But on their own, they are insufficient for providing overall protection of the material.

And wood?

In general, materials of organic origin (derived from biological organisms) such as oil, wood and coal   all have a composition rich in carbon and hydrogen atoms, which are likely to be oxidized. They are therefore combustible. Wood is a material with a complex structure and an elemental chemical composition made up of carbon (50%), oxygen (44%), and a small amount of hydrogen (6%).

Wood is a low-density material and possesses a natural ability to char, meaning a protective layer of char is formed between healthy wood and flames. When wood is burned, it first loses water, and becomes completely dry at 120 °C. Then, its structure gradually decomposes as the temperature rises. Its components remain relatively stable up to 250 °C, the temperature at which wood starts giving off smoke. At 320 °C, there is enough gas to ignite wood. Pyrolysis takes place mainly up to 500 °C,  after which point only charcoal (char) remains, which can slowly decompose through oxidation. While the char layer slows down the pyrolysis of the underlying healthy wood, its mechanical strength is negligible. As pyrolysis continues, the useful section of a wooden structural element is therefore reduced along with its load-bearing capacity.

The flame retardants used to fireproof wood belong to the families listed above (phosphorous, boron, nitrogen, metallic hydroxides). However, unlike with plastics, it is not possible to incorporate these additives when wood is manufactured.  Fireproofing therefore takes one of two forms: applying a surface coating (paint, varnish) or impregnating the core of the wood, meaning the hollow part – called the lumen – of wood cells through an autoclave process. The process involves filling all the lumens by degassing under vacuum then forcing the flame retardant to penetrate the wood by subjecting it to high pressure. This more complex solution makes it possible to prevent a diminution of the material’s fire resistant property in the event of surface defects.  When a coating deteriorates, it can no longer play its role as a flame retardant, leaving the wood without protection in the event of a fire.

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This article was co-authored by Clément Lacoste (IMT Mines Alès), Laurent Ferry (IMT Mines Alès) and Henri Vahabi (Université de Lorraine).

Rodolphe Sonnier, Mines Alès – Institut Mines-Télécom

The original version of this article (in French) was published on The Conversation and republished under a Creative Commons license. Read the original article.

The colors of cave paintings are of great interest because they provide information about the techniques and materials used. Studying them also allows fewer sample to be taken from ancient paleolithic works. Research in colorimetry by Dominique Lafon-Pham at IMT Mines Alès provides a better definition of the colors used in paintings by our ancestors.

Mammoths, steppe lions and woolly rhinoceroses have been extinct for thousands of years, but they have by no means disappeared from paleolithic caves. Paintings of these animals still remain on the walls of the caves that our ancestors once lived in or travelled to. For archeologists, cave art specialists and paleo-anthropologists, these paintings are a valuable source of information. Cave art, found at various sites in different regions and dating from a long period that covers several tens of thousands of years, reflects the distribution and evolution of prehistoric wildlife. Analysis of the complex scenes sometimes depicted — such as hunting — and study of the artistic techniques used also bear valuable witness to paleolithic social practices. They are an expression of the symbolic world of our ancestors.

Scientists examine and handle these works with minute care. “Permission to take samples of the painted works is only granted after a strict application process and remains exceptional. Decorated caves can contain a wealth of information but are also be extremely restrictive due to the fragility of the information itself,” explains Dominique Lafon-Pham. The researcher at IMT Mines Alès is developing measurement methods that do not require contact with the color and which help characterize rock paintings. She has been carrying out her work for several years in close collaboration with the French National Center for Prehistory (CNP). She alternates field work and lab experiments in partnership with Stéphane Konik, geoarcheologist at the CNP attached to the PACEA^[1] laboratory.

“Colorimetric analysis isn’t a replacement for chemical and mineralogical methods of analysis”, Dominique Lafon-Pham stresses. In certain cases it does, however, provide initial information on the nature of the colorant material. The color alone is not enough to accurately trace the constituents of the mixes, but it does provide a clue. Comparing the colors in different works is a way to avoid taking samples of the pictorial layer from the walls of prehistoric caves. The researcher’s work helps contribute to a “detective investigation” led by archaeologists at scenes dating from several tens of thousands of years ago, where even the smallest piece of evidence merits examination.

The color and, more generally, the appearance of the drawings observed by teams of scientists in caves such as Chauvet and Cussac tell us some of the history of the chosen colorant material that was prepared and applied and has been exposed to the passing of time. It is a way of entering into the work through analysis of the ancient material used. Data produced from this analysis may allow parietal archaeologists to approach the work from the perspective of its creation and even its purpose, whereas conservation specialists are more interested in its evolution over time.

Our visual ability does not allow us to compare subtle differences in color that do not fall within our visual range. We do not have perfect color memory. In addition, the impressions created by an area of color are influenced by the surrounding chromatic environment. “When we can measure the color of a mark without the problem of deterioration due to aging, we will be able to establish similarities between works of the same color, whether they are on the same rock wall or not,” indicates the researcher at IMT Mines Alès.

Objectifying the perception of colors

This comparative method may seem a simple one, but it is important not to underestimate the complexity of the site. Lighting — very often artificial — alters the perception of the human eye. A colored surface will not appear the same when lit in two different ways. The aging of the rock also has an impact. The calcite that forms in the caves sometimes covers the paintings and alters the optical performance of the material, dulling and modifying the color of the depictions. In addition, moisture conditions vary with the seasons and between different sectors at a single site, leading to reversible variation in the colors perceived and measured. All these different impacts require set procedures to be put in place to identify, in the most objective way possible, the color produced by the interaction between light and material.

Measuring the color of cave paintings is not an easy task. Researchers use spectroradiometry and a whole set of associated procedures to keep the lighting constant for each measurement, as seen here in the cave of Chauvet.

Researchers use a spectroradiometer, an instrument that measures the spectral power distribution of a luminous radiance in the range of visible light, which is a physical scale that has no correlation with the color perceived by the eye. “The advantage of working at an underground site is that we can control the lighting of rock paintings,” explains Dominique Lafon-Pham. “We always try and light the work in the same way.” The situation becomes more complex when the scientists need to work outside. “We are currently taking measurements at the site of the Cro-magnon rock shelter,” explains the researcher. This site, listed as UNESCO World Heritage, is located in Dordogne in France and was a shelter for Cro-magnon men approximately 30,000 years ago. “The analysis of potentially decorated rock walls which are exposed to the open air is much more complex due to changes in the natural light. It is a real challenge in this situation to try and distinguish between very similar colors using measurements.”

Towards virtual caves?

The use of mixed reality (part-way between augmented reality and virtual reality) at cultural sites is increasingly common. This technology opens up new possibilities for transmitting knowledge such as through the creation of remote guided tours in an entirely virtual environment. The quality of the cultural mediation depends on the realism and exactitude of the features and objects in the virtual world. Taking objectified measurements allows standardization of data collection on the optical characteristics of the parietal art at prehistoric sites. Data collected in this way can be processed using modelling and realistic simulation tools. It provides some of the information required for the construction of virtual facsimile.

The scientific community is also keeping a close eye on such devices which capitalize on new media technology. Highly accurate virtual replicas of prehistoric sites could offer considerable research opportunities by enabling researchers to access sites regardless of how easily accessible they are or not. For conservation and safety reasons — such as the presence of high levels of CO2 in the air at certain times of the year — it is only possible to access caves for very short periods of time and under strict control of movement. Although Dominique Lafon-Pham agrees that these are particularly promising prospects, she nevertheless tempers expectations: “For the moment, the image generators we have tested are a long way off being able to render the subtlety of light and color variations that we see in reality.”

It will be a little longer before it is possible to create identical virtual replicas of paleolithic caves and their art with sufficient realism to allow quality cultural and scientific mediation. Nevertheless, this doesn’t stop the researchers at Mines Alès continuing to study the colors of rock paintings and, in particular, the way they looked at the time of our ancestors. 30,000 years ago, our predecessors painted and viewed their art by firelight, which has been replaced in caves today by very different electric lighting. “The light cast by fire flickers: what does that mean for the way in which the painted or engraved work is seen and perceived?” wonders Dominique Lafon-Pham. Another question: if researchers today are able to detect multiple shades of red in a single drawing using these systems of measurement, were these different shades seen by our Homo sapiens ancestors? If so, were they accidental or deliberate and did they serve a purpose for the artist?

^[1] “From Prehistory to Today: Culture, Environment and Anthropology” (PACEA) laboratory. A mixed research unit attached to the CNRS, the University of Bordeaux and the French Ministry of Culture and Communication.