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Better Foundations, Part 1: An Introduction to Concrete

Written by Iver P. Cooper

Concrete—"Liquid Stone"—has made possible many innovations in architecture. Yet concrete is no Space Age wunderkind; it has its roots in antiquity. Concrete, albeit of a kind inferior to the modern product, was used by the Romans in the construction of the Pantheon, which has endured since the time of the Emperor Hadrian.

While the Roman concrete structures endured, concrete technology languished after the fall of Rome. The seventeenth century is still the Dark Ages so far as concrete is concerned. But the up-timers will bring about a "Concrete Renaissance" in short order.

What Is Concrete?

Concrete is a composite material, made by combining an aggregate (a hard particulate material) and a cement (a matrix forming material) with enough water to cause the cement to set and bind the aggregate together. The binding is the result of the chemical reaction of the cement with the water. The aggregate is a combination of fine aggregate (sand) and coarse aggregate (e.g., gravel).

Mortar is a paste-like mixture of sand, a binder (e.g., cement) and water. It doesn't contain coarse aggregate, but of course the mortar is used to bind together cut stones or bricks, and to fill in gaps between them. Those stones and bricks are much larger than the coarse aggregate of concrete.

Concrete and Cement in Canon

We know that when Grantville made its involuntary journey into seventeenth century Germany, some concrete construction came along for the ride. Mark Huston's "Gearhead" (Grantville Gazette, Volume9) mentions "a pair of concrete bridges." There is a concrete floor in the building which Chad Jenkins has converted to a shop for washboard manufacture, see Rittgers, "Von Grantville" (Grantville Gazette, Volume7). There is also a concrete floor at the farm where Harmon Manning suffered his ultimately fatal fall, see Ewing, "An Invisible War" (Grantville Gazette, Volume 2). Pam Miller has a concrete porch, see Vance, "Protected Species" (Grantville Gazette, Volume13). The high school has a concrete "awning" over the entrance, see Flint, 1632, Chapter 11. And there is at least a concrete slab in the Grantville city jail, see Weber, "The Company Men" (Grantville Gazette, Volume2).

Concrete was used in the displaced West Virginia mine featured in Mark Huston's "Twenty-eight Men" (Grantville Gazette, Volume10), in a wall separating the working and non-working sections of the mine. The wall was built out of concrete blocks, and thus, even if the wall was assembled after the Ring of Fire, the blocks themselves may have been cast up-time.

Since the Ring of Fire, there has been some new concrete work. Sometime before March 1632, Delia Higgins sold the remaining dolls in her collection, and used the proceeds for two projects. The first was building a warehouse. Her intent was to build a concrete warehouse, a "work of art", with "the best combination of up-time and down-time construction techniques possible." Gorg Huff, "Other People's Money" (Grantville Gazette, Volume 3)(timeframe March-October 1632). What she got was, "if not exactly a work of art," a structure which "was functional, and very large." It was built with "fairly standard down-time construction techniques, with concrete pillars added for support."

In the process of trying to persuade the high school chemistry teacher, Alexandra Selluci, to help with the warehouse project, Delia got talked into becoming the "sugar grandma" for the Grantville High Tech Center's "brand new concrete research program, complete with structural engineering courses where the teachers were half a chapter ahead of the students, or sometimes half a chapter behind." In Delia's opinion, "the kids that had gone into concrete were phenomenal. They were about four to one down-timer to up-timer, about average for the high school. They wanted to build things. Great big things, dams, skyscrapers, and roads, and were willing to work at it."

Later in OPM, Delia reveals that she saw the warehouse as a stepping stone to a grander project, the Higgins Hotel. "The concrete program at the school was developing a group of young people who could make structural concrete, and form it into structures that would support tremendous weight. Hiring Michel Kappel was done both to get a down-time builder familiar with up-time building techniques, and as favor for Karl Schmidt. Claus Maurer was a master builder with more experience than Herr Kappel, but again, part of the reason for hiring him was to get him familiar with the available up-time tech. It wasn't her fault that they had fought with each other and with the teachers at the tech center and Carl over at Kelly Construction. Besides, materials were so expensive that the cheapest halfway decent material was quarried granite from the ring wall."

By December, 1631, the high schoolers are working on "some sort of concrete project," and mortar is available. See Huff and Goodlett, "Birdie's Village" (1634: The Ram Rebellion). In Cooper, "Stretching Out, Part One: Second Starts" (Grantville Gazette, Volume11), there is a passing reference to an equally mysterious "concrete project" which is apparently looking for venture capital in July 1633.

By July, 1633, the conservatory at the new hospital has "cement paths" (people often confuse cement and concrete, cement is a component of concrete). See Ewing, "An Invisible War" (Grantville Gazette, Volume2). I can't help but wonder whether the hospital itself, a three story building completed a year earlier, is of concrete construction.

The Higgins Hotel is at least partially built as of summer 1633, see Cooper, "Stretching Out Part 1" (Grantville Gazette, Volume 11) and "The Chase" (Ring of Fire II), but the stories don't say whether it used concrete.

Somewhat inconsistently, in the Friends' "Burgers, Fries, and Beer" (Grantville Gazette, Volume7), set in January 1634, Julio Sanabria wonders where he would get the cement, fire clay, and lime he needs in order to put his masonry tools to use. You can't make concrete without cement, and concrete was already being made.

The Grid lists a concrete company, started by William Roberts and his brother Ronald Chapman. Roberts is a managerial type and Chapman worked "for a company in Fairmont as a foreman of a team that built pre-fab metal buildings." The relationship of this company to the high school concrete research lab is unclear. It is possible that the company existed only on paper.

Concrete and Cement Knowhow

Grantville is in do-it-yourselfer territory. There are also going to be a fair number of "how-to" manuals (some even read by their owners), as well as homeowners with hands-on experience making (and repairing, especially those who didn't read the manuals) concrete flatwork (floors, driveways, patios, porches, walkways), foundations, walls and even outdoor furniture. But no pink concrete flamingos, I hope.

In addition, Grantville had at least two general contractors before the RoF, Happy Acres and Home Center (Grid). I doubt that either of them has built a skyscraper, and there isn't much concrete construction in Mannington, but chances are reasonable that they have employees who have worked with concrete.

There were also construction technology courses offered at the Marion County Technical Center. I have no idea which courses were offered in 1999-2000, but the current catalog includes "Basic Masonry and Landscaping," "Foundations and Framing," "Fundamentals of Building Construction," "Masonry and Plumbing," and "Construction Systems."

The up-timers with college engineering degrees are most likely to have attended either West Virginia University or Fairmont State. WVU offers a degree in civil engineering, with undergraduate elective courses in Civil Engineering Materials (CE310), Concrete and Aggregates (CE412), Construction Methods (CE413), Construction Engineering (CE414), Advanced Concrete Materials (CE416), and Reinforced Concrete Design (CE 462). While there is no guarantee that any particular civil engineer in Grantville has taken any of these courses, it is certainly possible.

In any event, there are going to be up-timers who know how to estimate how much concrete is needed for a job, prepare the forms to receive the concrete, put down the steel reinforcements, monitor the pour, and cure the concrete. A smaller number (those who didn't just use ready-mix) will know how to proportion concrete, that is, decide the proper ratios of cement, aggregates and water.

What is less certain is that the up-timers will know, firsthand, how to make cement. Cement is usually bought ready-made and even a building contractor needn't have firsthand knowledge of cement manufacture. If any up-timers do, it is probably because they worked in a cement plant outside Grantville. An EPA ranking of Portland cement plants, by size, listed Capitol Cement, in Martinsburg, in 29th place. That was the only West Virginia listing. (EPA) However, I have found a 1976 reference to the Marquette Cement Manufacturing Co. plant near Fairmont (PSC).

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Grantville, according to canon, has pretty much every encyclopedia you can imagine. "Not just the great one, the 1911 Britannica, which they guarded so carefully, but all of them—the later Britannica editions, the World Book and Americana, Columbia, and Funk and Wagnalls, old and new, large and small." (Flint and DeMarce, 1635: The Bavarian Crisis, Chap. 5). My understanding is that the public library has the Encyclopedia Americana, and both the modern and the 1911 editions of the Encyclopedia Britannica. The high school has the World Book, and the junior high, the Collier's. There's also a nearly complete ninth edition of the Britannica, and, I suspect, several CDROM-based encyclopedias, most likely Compton's and Encarta. Besides having articles on concrete and cement, these encyclopedias have related tidbits scattered across their many volumes, which a sufficiently diligent researcher can uncover.

Grantville is modeled on Mannington, West Virginia, and I have checked the high school and public library holdings for more specific works. North Marion High School has the Time-Life Masonry (1977). It and the public library have Kicklighter's Modern Masonry: Brick, Block, Stone (1977).

There may also be other relevant books. For example, the high school has Trachtenberg's Brooklyn Bridge: Fact and Symbol (1979), and Stevens' Dam: An American Adventure, and concrete was used in their construction.

There is no easy way of determining what books might be in private (home and work) libraries, or at the Voc-Ed Center. Bear in mind that any engineer almost certainly has kept all of his or her college engineering textbooks. Even a retired engineer would hesitate to part with them.

There is also a documented relationship between the North Marion High School of Mannington, WV and LaFarge, an Ohio cement company. LaFarge gave the high school a $300,000 atomic absorption spectrophotometer in 1997 (Zeller). Surely the student research projects developed using the AAS would have included ones dealing with cement. And perhaps the school got some cement technology texts along with the AAS.

Down-timers' Cement and Concrete Technology

Previously, I said that Roman concrete was inferior to modern concrete, and I should explain why. First, it had a compressive strength of only 2800-3000 psi (RomanConcrete.com; Spratt), comparable to the "low end" of the strength range of modern concrete. Secondly, it was not reinforced.

Many sources state that concrete technology was "lost" in the Middle Ages. But I very deliberately used the term "languished" in the introduction. The Normans used concrete in the construction of parts of Reading Abbey (1130), the White Tower of London, and other structures. (Davidovits, May, Ferguson). But Ferguson comments that "concrete in the hands of the Normans was a total failure," and lists a dozen Norman concrete towers which fell down.

Mukerji asserts that the Roman formula for "hydraulic cement" (that is, one which hardens in contact with water) wasn't lost, but rather remained "tacit knowledge" among masons and military engineers, at least in limited areas, so that it was known in, e.g., seventeenth century France. Idorn (38) says that use of hydraulic cement was monopolized by the authorities; e.g., Christian IV of Denmark imported trass (from Dutch merchants) for making hydraulic mortars for his palaces and castles.

In 1568, the French architect Philibrt de L'Orme taught preparing a mortar from burnt quicklime, river sand, pebbles and water, with the pebbles being "of all sizes." (Jackson 23).

A crude form of concrete was apparently used as ballast between the frames in the galleon Nuestra Senora de Atocha (1620)(Crisman).

The Advantages of Concrete

Concrete competes as a building material with steel, wood, and brick, and as a road pavement with asphalt. Concrete has numerous advantages as a structural material.

On-site Fabricability. The 1911 Encyclopedia Britannica (1911EB) says that concrete has "the immense advantage over natural stone that it can be easily molded while wet to any desired shape or size." It has similar advantages over steel and wood. Steel can be cast only at a high temperature and wood not at all. Steel can be bent but only through persistent application of great force, and wood can be bent only gingerly and slowly, to avoid breakage.

Convenience. "Its constituents can be obtained in almost any part of the world, and its manufacture is extremely simple." (1911EB).

Compressive strength. Like natural stone, it possesses great resistance to compression; its compressive strength is usually 4,000-15,000 pounds per square inch (psi), or higher, of cross-sectional area. (Levy/Down, 279). (In 2000, concrete with a strength of 8,000 psi or higher was considered "high strength"; Nilson 50.) The compressive strength of wood, parallel to the grain, is comparable, perhaps 6,000-7,500 psi, but perpendicular to the grain, wood is much weaker, perhaps 450-1050 (Green). As for natural stones, granites and marbles are stronger (up to 30,000 psi), and soft limestone weaker (700 psi)(Cowan 105).

Strength-to-Cost. The figure-of-merit (two-thirds power of strength, divided by cost per unit volume) is 80 for concrete, 60 reinforced concrete, 80 wood, 45 brick and stone and only 21 steel (Ashby 100).

Stiffness-to-Cost. The figure of merit (half power of Young's modulus of elasticity divided by unit cost) is 40 for concrete, 20 reinforced concrete and brick, 15 wood and stone, and only 3 steel.

Stiffness-to-Weight. For columns which fail by buckling, the figure of merit is the half power of Young's modulus, divided by the density. Steel is 59, but concrete is almost as good, 49 (Gordon 321). So by making the columns just a little thicker, you can use reinforced concrete instead of steel, saving perhaps 99% in steel consumption.

Fire resistance. Concrete itself is non-combustible, and has a thermal conductivity about 5% that of steel (PCA). However, it should be noted that with reinforced concrete, the reinforcing steel becomes ductile at high temperature, and since it presumably is there to provide tensile strength, the result may, ultimately, be structural failure.

Biological and chemical resistance. Wood rots, and is attacked by termites (or, at sea, teredo worms). Steel corrodes. Concrete isn't vulnerable to these threats, but it can be attacked by acids, sulfates and chlorides (from deicing salts). Special concretes are used for construction in the vicinity of high-sulfate soil (or groundwater). And of course the reinforcement in reinforced concrete can corrode if corrosive agents can reach it.

Temperature stability. A concrete wall or floor will absorb heat during the day and re-radiate it at night. That's true of any material, but concrete has a greater "thermal mass" than wood (HousingZone). (An interesting variation is a panel with a lightweight thermal insulating material sandwiched between layers of concrete.)

Soundproofing. The airborne sound insulation of a concrete first floor is 9-22 dB higher than that of a timber floor ("Going Up").

Disadvantages of Concrete

Low Tensile strength. Unfortunately, concrete's tensile strength (resistance to being pulled apart) is only about 10% of its compressive strength (Twelvetrees 41); Gordon (44) quotes a value of 600 psi (whereas commercial mild steel is 60,000 and high tensile engineering steel is 225,000). Because plain (unreinforced) concrete is strong in compression and weak in tension, it can be used in columns, arches and domes, but not in beams (horizontal structural members).

Low Compressive Strength-to-Weight Ratio. If we divide the compressive strength by the density (~2), we get values of about 2000-7500 psi/unit weight for unreinforced concrete. For steel, despite its greater density (~7.5), we get values of 4800-8000 (Twelvetrees 31).

Flexural strength. Concrete is not good at resisting failure from bending; its flexural strength is perhaps 12-20% compressive strength. (Cadman).

Brittleness. Concrete is also brittle, that is, once cracked it is easily fractured. The "fracture toughness" of concrete is 0.2-1.4, compared to 0.7-0.8 for soda lime glass and 50 for steel (Matt Gordon).

Shrinkage and Expansion. Freshly laid concrete shrinks as a result of the chemical reactions between its ingredients, the evaporation of water from the concrete, and the rising of air voids to its surface. Once hardened, concrete expands and shrinks in response to changes in temperature and moisture levels. Of course, all these dimensional changes stress the concrete, perhaps causing cracks.

Fortunately, if concrete is reinforced with a material, like steel, which is strong in tension, and ductile, it becomes an all-purpose structural material.

CONCRETE COMPOSITION

We start by reviewing the ingredients of concrete: cement, aggregate and water. Cement itself is a complex material. Once we know what goes into both concrete and cement, we can consider how concrete is mixed, laid, cured and tested.

Cement

Cement, in essence, is a binding agent. It can be used in mortar or in concrete. Cements are traditionally classified as being either hydraulic (those which, at least after setting, are resistant to water) or non-hydraulic (those which must be kept dry). The term "hydraulic" also has come to imply that when first mixed with water to make concrete, the cement reacts chemically with the water, forming hydrates which help bind the aggregate. (These hydrates are themselves insoluble in water, thus conferring the water resistance.)

The pozzolanic cements result from the mixture of a source of calcium (usually lime) and a source of silica (possibly also containing alumina). The lime is derived by heating chalk or limestone (calcium carbonate); the process is called calcination. The lime may be either the highly reactive quicklime (calcium oxide) or the somewhat less reactive slaked (hydrated) lime (calcium hydroxide), the latter being obtained by reacting quicklime with water.

Portland cements likewise are derived from a mixture of calcium- and silica-rich materials, but this mixture is subjected to a further calcination at a high temperature.

The natural cements are prepared from a source material which naturally contains both lime and silica. Hence, no mixing step is needed. Like Portland cements, it is calcinated, but at a lower temperature than that typical in Portland cement production. (Eckel, 151).

High-alumina cements are made from limestone and low-silica bauxite; they were invented in 1908.

In the twentieth century, the commercially dominant hydraulic cement was "Portland cement." However, we will first discuss the older pozzolanic cements.

Pozzolanic Cements

A pozzolan is a source of silica (silicon dioxide) which can react with lime to form a cement. A material can contain silica but not be useful as a pozzolan. For example, most sands contain silica, yet are unreactive. Moore says that this is because they have a tightly bound structure which frustrates the reaction. (Sands are used in concrete, but as aggregate.)

The ancient Roman cement, which is pozzolanic in nature, is described in Marcus Vitruvius Pollio's De Architectura, Book II. This classic text became available to Europeans, in Latin, Italian and German printed translations, in the fifteenth and sixteenth centuries. There is also Sir Henry Wotton's The Elements of Architecture (1624), which is derived from Vitruvius.

The first known post-Roman use of a pozzolanic cement was in Italy. The Venetians used the "black lime of Abetone" in the fifteenth century, and the Roman pozzolana was used by Fra Giocondo in the mortar of the pier of the Pont de Notre Dame in Paris (1499). (Giocondo published an edition of Vitruvius in 1511.)

Vitruvius refers to "rubble work," with stones mortared together. In chapters 4-5, Vitruvius says that one may mix sand (a silica source) with lime to make mortar. He recommends use of three parts sand to one of lime when the sand is from a pit, and a two to one ratio if the sand is from sea or river. According to Moore, Vitruvius' "pit sand" is actually volcanic ash, specifically, pozzolana.

Pozzolana. The eponymous pozzolana is a volcanic ash discovered at Pozzoli, near Vesuvius, but also found elsewhere in Italy (including near Rome). The 1911 Encyclopedia Britannica article on "cements" gives compositions for both Neapolitan (27.8% soluble silica, 5.68% lime) and Roman (32.64%; 4.06%) "Pozzuolana."

Vitruvius, in chapter 6, says: "There is a species of sand which, naturally, possesses extraordinary qualities. It is found about Baiae and the territory in the neighborhood of Mount Vesuvius; if mixed with lime and rubble, it hardens as well under water as in ordinary buildings." This "sand" is obviously a pozzolanic ash. Herring points out that this pozzolan could react with lime because it was already calcined by the volcano.

Santorin earth. This is really a volcanic tuff, which blankets the Greek island of Santorini (Thera). It is about 64% silica and 3.5% lime (USBM). It was used in ancient Greek mortar (Lea 3) and, millenia later, it was still exported for use in making pozzolanic cement (1911EB "Santorin").

Pottery shards were ground up, in antiquity, to produce a pozzolan. Pottery is made by heating (calcinating) clay, and clay is rich in silicate minerals. Brick could be recycled in a similar way. Vitruvius says that if mortar is made from river or sea sand, it is improved by addition of one-third part of ground potsherds.

A modern clay-derived pozzolan is metakaolin. It is obtained by calcinating the clay mineral kaolinite, an aluminosilicate clay mineral. Metakaolin is one of the most reactive pozzolans.

Pumice is a very light, highly porous igneous rock, with a silica content of 60-75%. In 1911, pumice was chiefly obtained for commercial use from the Lipari Islands north of eastern Sicily, and especially from Monte Pelato and Monte Chirica. The Lipari Islands have exported pumice since antiquity, and Canneto is the center of the pumice trade.

Trass, a Germanic pozzolan, is a tuff (rock derived from volcanic ash) found in the Eifel, a volcanic region of Germany lying between the Rhine and Moselle rivers (1911EB). The article on "Trass" specifically mentions the Brohl and Nette valleys, and the town of Andernach. Eckel (635) says that trass occurs along the Rhine, from Koln to Coblenz, and that the towns of Brohl, Kruft, Plaidt and Andernach near Coblenz are significant players in the trass industry. 1911EB characterizes it as 19% soluble, 50% insoluble silica. It is lacking in lime so it is less reactive than pozzolana. Nonetheless, the Romans recognized its resemblance to the Vesuvian material.

(Johnson, 387). In 1837, trass sold for $5.225 per cubic meter, whereas common sand cost $0.85. (Treussart 90).

Extinct volcanoes can also be found in the Vogelsberg (west of Fulda), the Roehn (east of Fulda), the Lausitz (north of Dresden), and in the Eschwege at the Werra, east of Kassel. (MB).

Kieselguhr (diatomaceous earth, diatomite), which is derived from the silica skeletons of fossil diatoms, is over 80% silica. In 1911, it was not an economical material for cement making, because it was in demand as an absorbent for the nitroglycerin in dynamite. The 1911EB mentions deposits of diatomite in Richmond, Virginia, in Aberdeenshire (between Logie Coldstone and Dinnet), in Wales (Llyn Arenig Bach), and on Skye. It is in fact found in Germany (e.g., Obrehole) , but I don't know whether it was a known substance (e.g., for filtering beer) insofar as the down-timers are concerned.

Ground granulated blast furnace slag (GGBFS) is a product of steelmaking (1911EB). Slag cements were first used in 1774, in mortar (Prusinski).

While USE Steel in Grantville will no doubt be happy to sell its slag, King warns that the slag "requires a fair amount of processing to become a useful pozzolan." GGBFS is produced by rapidly quenching (cooling) molten iron blast furnace slag by immersing it in water or blowing air over it, in a "granulator," and then grinding it. The GGBFS is then combined with lime to make slag cement. (It should be noted that slag, processed differently, can be used to make an aggregate.)

Coal fly ash. When coal is burnt, it leaves behind both bottom ash and fly ash, the latter being the particles which are carried up into the smokestack. Fly ash was first used in a pozzolanic cement in the construction of the Hoover Dam (1929).

The silica content of the fly ash is dependent on the type of coal; 20-60% for bituminous, 40-60% for sub-bituminous, and 15-45% for lignite. The ash also contains lime; 1-12% for bituminous; 5-30% for sub-bituminous, and 15-40% for lignite. Fly ash is classified as being either Type C (calcium-rich) or Type F (calcium-poor). The type C ash is more reactive than the type F ash, and is even self-cementing. ("Fly Ash," Wikipedia).

Fly ash particles have diameters of 1-100 microns. The particles with sizes under 10 microns are the most pozzolanically active, and ASTM limits the concentration of particles larger than 45 microns to 38%. The particle size distribution varies depending on the coal deposit and also on the plant design and operating parameters.

By way of a bonus, since fly ash particles are almost perfect spheres, they act like microscopic ball bearings, improving the workability and pumpability of the concrete in which they are used (Copeland).

Grantville has a coal-burning power plant which may already be equipped with devices for filtering out fly ash to minimize air pollution. By the beginning of 1634: The Baltic War, there is a coal gas plant in Magdeburg. They are connected by rail and water, and the fly ash can therefore be shipped to any point along the line which is convenient for cement and concrete manufacture.

How much the up-timers know about this utility of fly ash? It is not mentioned in 1911EB. The Encyclopedia Americana notes that it can be removed from the smokestack gas by electrostatic precipitators ("Power, Electric") but doesn't mention its significance for cement-making.

On the other hand, the Allegheny Power Company (which presumably owns the Grantville power plant) reported to the SEC that its subsidiaries sold 131,000 tons of fly ash (and 168,000 tons of bottom ash) in 1996, and that the uses of the ash included "cement replacement."

So I am sure that at least the power plant manager, Bill Porter, knows about this possibility.

Talmy, USP 5521132 gives the composition of the fly ash from the Rivesville Power Plant, which was the model for the Grantville plant. It is 58.79% silica, 27.91% alumina, 8.41% iron oxide, and only 1.20% lime. Its LOI (loss on ignition),a measure of the unburnt carbon on the particles, is 28.3%. That's high, so it will have to be burnt off. ASTM C618 requires that the LOI be no more than 6% (King 5).

Rice husk ash. Traditionally, rice was milled just to remove the chaff (outer husk), leaving brown rice. The brown rice may be further milled to remove the bran (inner husk), leaving white rice. If the husks are burnt, about 20% of the husk weight remains as ash, and this ash is about 95% silica, and constitutes a highly reactive pozzolan (Allen, King). The difficulty in preparing the ash is burning the husk at a temperature low enough so that the silica doesn't form inactive crystals while burning it long enough to ensure that all the cellulose is consumed.

Americans don't think of rice as a European crop, but it was brought to Spain and Portugal by the Moors ("Rice," Wikipedia), and has been grown in Italy at least since the fifteenth century. Lombardy was the first major Italian producing region. By 1644, there was rice production in the Veneto, and in the nineteenth century canal construction made it possible to grow rice in the Piedmont. (Seed). Rice can also be grown in France and Greece.

Silica fume. Once the semiconductor industry is reestablished, there will be the possibility of using silica fume (0.1 micron silica particles, a byproduct of silicon production) as a high-activity pozzolan. Silica fume is expensive and difficult to work with, so it will probably be relegated to the same niche market it enjoys now (concrete with compressive strength exceeding 15,000 psi and with high chloride resistance). (King 7; SFA).

Portland Cement

"Portland cement" was patented by Joseph Aspdin in 1824, and improved by his son William in 1843. Aspdin's cement was made by heating together finely ground limestone and clay. He cooled the resulting "clinker" and pulverized it. This powder could be stored until it was ready to be activated by addition of water.

The Aspdins used too low a temperature (probably lower than 1400 deg. C) to achieve a true Portland cement. (Blezard 8). (EB11 specifies a "clinkering" temperature of 1500 deg. C (2732 deg. F) which is in accord with modern practice.)

There is a good description of the modern American cement-making process in "Cement," Encyclopedia Americana. Sources of lime (e.g., limestone, chalk, marl, marble, shells), and of silica (sand, sandstone, clay, slag, ash) and alumina (clay, shale, bauxite) are quarried and crushed, then mixed together and ground up some more. The grinding can be done wet (that is, in a water slurry) or dry. Wet grinding yields a more homogeneous blend, but the powder has to stay in the kiln longer. (Camp)

This "rawmix" flows into a continuously operated, inclined, rotating kiln. EA says that this is typically 300-400 feet long, inclined at one half inch to the foot, and rotated at 30-90 revolutions per hour. The kiln is hottest at the discharge end.

The material takes 2-4 hours to pass through the kiln, and reaches a temperature of 2600-2800 deg. F. First water is driven out, and then the carbonates decompose into oxides. Ultimately, some of the material liquefies, and the lime (calcium oxide) reacts with the silica to form calcium silicates, notably dicalcium silicate (belite) and tricalcium silicate (alite).

Shale and clay often have a high aluminum and iron content. Alumina (aluminum oxide) serves as a flux, that is, it reduces the melting point so that more of the charge is liquefied at the peak kiln processing temperature. Thanks to the flux, liquid appears at about 2400 deg. F, but even at the peak temperature, only 20-30% of the charge is in the liquid phase. When the charge is cooled, the alumina is converted into tricalcium aluminate.

The EA "Concrete" article explains that tricalcium aluminate "produces a very high heat of hydration" and "has poor durability because it reacts with sulfate alkalis found in soil and water." Overly high aluminate levels may be reduced by adding iron ore to the kiln.

Iron oxides also act as fluxes, and they react with aluminate to form tetracalcium aluminoferrate. This iron compound is responsible for the grey color of the cement; if cement is made from low-iron materials, it will be white in color. On the other hand, the iron improves the resistance of the concrete to sulfate water.

Tricalcium aluminate forms because a source (e.g., bauxite) of aluminum oxide (alumina) is added to the kiln when making Portland cement. It is provided to reduce the melting point of the composition so it is liquid at the peak kiln processing temperature, thereby favoring formation of alite and belite. Unfortunately, it has undesirable properties, Like tricalcium aluminate, the tetracalcium aluminoferrate acts as a flux.

The product of the calcination reaction in the kiln are black hard nodules with diameters of one-quarter to one inch diameter, called "clinker" because they make a clinking noise in the kiln. These are mixed with 4-5% gypsum (hydrous calcium sulfate). EA states that the purpose of the gypsum is to slow down the "setting" of cement, since otherwise a Portland cement concrete mix might set, and become unworkable, before pouring was complete.

On average, every thousand tons of cement requires roughly 1511 tons of various oxides (1315 tons calcium oxide, 71 tons silica, 108 tons alumina, 17 tons ferric oxide), and 53 tons gypsum. (Van Oss 22). To get those oxides probably means processing up to twice the weight in raw rock.

Vertical Kiln Development

Of course, the post-RoF cement industry is going to begin more humbly than with the monster rotary kilns described in EA. The first cement kilns were intermittent, vertical kilns. Such kilns are "old" technology, already used in pottery, lime and brick making, and so there will be a rapid adaptation.

The simplest kiln design is a pit kiln, in which the fire is allowed to burn downward. Unfortunately, most of the heat is wasted, because it escapes upward.

An improved design is a simple shaft kiln; this involved digging a horizontal tunnel into the side of a hill, and a vertical shaft down to meet it. An arch of limestone is built at the junction. The rawmix is piled on top of the arch, and the fuel goes below it ("separated feed"). The fuel is lit and the fire burns upward. (Lazell 24-30; Eckel 409-19).

In both pit and shaft kilns, earth acts as the insulator. The dome kiln is the free-standing equivalent, made of brick or perhaps brick-lined metal. The interior was egg- or bottle-shaped, with the top portion serving as a chimney. The arch was replaced with a grating, and the fuel (preferably coke, but sometimes firewood) and rawmix was piled above the grating in many alternating layers ("mixed feed"). A typical dome kiln was 15-20 feet high and perhaps six feet in diameter.

Normal operation was discontinuous. The kiln would be loaded with perhaps 50 tons slurry and 12 tons coke. It will take two days to fire up, two or three days to burn through, and additional time for cooling down, drawing out the clinker, and reloading the kiln. Dome kilns produced perhaps thirty tons clinker per batch, and one batch per week (EB11). According to Eckel, production is 0.5-1 ton clinker per cubic meter of burning space, and 23-30 pounds of fuel are needed per 100 pounds clinker.

Intermittent operation is wasteful of energy, since the kiln must be cooled down and then reheated for the next batch. But the new arrangement made it theoretically possible to operate the shaft or dome kiln continuously. One worker could (cautiously) collect clinker which has fallen through the grating, while another added new layers at the top. We then have a "running kiln."

In practice, the clinker tended to hang up, forcing a cool-down (Redgrave 158). Also, it was difficult to maintain a consistent burn in running lime kilns (Johnson) and I suspect that the same problem would have carried over to cement kilns. Lipowitz (32) said in 1868, "many attempts to establish a kiln on the perpetual system have been devised, but hitherto the desideratum of a perfectly unexceptionable running kiln is still unattained."

Chamber kilns were adapted from brickworks, and the basic concept was that excess heat from one chamber was transferred to another. They thus achieved a substantial fuel savings. Chamber kilns are an old technology, but they reached their pinnacle in 1858, when Hoffman invented the "continuous" chamber (ring) kiln, briefly described by EB11.

The first vertical kilns capable of sustained operation appeared in the 1880s. These were larger than dome kilns (the Coplay Cement Company's nine Schofer kilns, operated in Delaware 1893-1904, were ninety feet tall), with separate drawing, burning and loading floors for the workers, and multiple ports and chutes through which to regulate the supply of rawmix and fuel. They probably had better linings, too. However, my sources are maddeningly vague about just how they avoided the problems of the old "running" dome kilns.

The new kilns differed in terms of where exactly the fuel and rawmix were added, the fuel used (coke or small coal), and where the interior narrowed and widened. EB11 diagrams the Dietzsch type, in which the shaft is staggered to create a horizontal ledge to which the fuel was added. A pair was usually built back-to-back. The upper vertical shaft contains the unburnt rawmix and the lower shaft is the burning zone.

EB11 also mentions the Schneider kiln, which had a single vertical path. The Schofer (Aalborg) kiln was similar. Eckel says it produced 10-15 tons clinker daily, consuming 280 pounds coal per ton product.

Rotary Kiln Development

It took roughly ten years (1885-1895) to achieve a truly practical rotary kiln. There were "many practical difficulties" and "an immense amount of expensive experimenting" (Sabin 23; ER Chap. 20; Brown 39-42; Redgrave 167-176).

One problem with the early rotary kilns is that they were simply too short, Ransome's being twenty-six feet long and Navarro's, forty feet. Consequently, there was a lot of underburnt clinker, and also much heat was wasted. While our heroes will know that the modern rotary kilns are hundreds of feet long, without foreknowledge of the problems of the pioneers, the first post-RoF rotary kilns are likely to be short prototypes, of underwhelming performance.

Secondly, there were various problems with the kiln lining, both spalling of the lining and balling of clinker upon it .

And finally there were the issues of finding the right fuel and minimizing fuel consumption. The first fuel experimented with was gas. Next came a "jet of burning petroleum," because it allowed precise control of the temperature of the kiln. Indeed, a chemist asserted at the time that the "rotary kiln can be successfully operated only in localities where crude oil is abundant and cheap." (Prentice) However, in most places oil was expensive, and the rotary kiln didn't really catch on until it was adapted (1895) to use blown pulverized coal.

The great advantages of the rotary kiln, once perfected, were its low labor cost (20-30% that of continuous shaft kilns) and high production rate (over double) (ER 188). Its bugbear was fuel consumption.

For several decades, the standard dry-process kiln was sixty feet long and six feet diameter, and the wet-process cousin could be up to eighty feet. The dry-process kiln produced 160-180 barrels (each 376 pounds) of clinker daily, consuming 110-150 pounds coal per barrel. (Eckel 424; Sabin says 175-250 barrels for 95-120 pounds coal/barrel.) The wet process was even more wasteful of fuel (ER 17).

It took Edison from 1899 to 1902 (Vanderbilt) to build the first "long" (150 foot) kiln, despite his study of the "short" kiln technology. The Edison kiln tube, nine feet in diameter, was made from ten-foot sections of cast iron, bolted together. It was suspended on fifteen rollers, rotated by an electric motor, and there were ten thousand bearings, lubricated with an automatic oiling system. The kiln had a pitch of eighteen inches and powdered coal was forced in by pressurized air to create a forty foot combustion zone at the lower end. Only two men were needed per shift.

Edison shocked the industry by producing 350-375 barrels daily, while consuming only 65 pounds coal per barrel (Eckel 424). Edison ultimately increased production to 1100 barrels/day (Vanderbilt 185). The "light bulbs came on," and by 1918 there were kilns over 200 feet long.

In the standard sixty footer, the combustion zone, in which clinker was formed, was near the lower end, and about ten feet long, and the heated gases which rose from it had only the upper forty feet in which to decompose the rawmix. Much of the heat of the gases was wasted, and the processing path was so short that the rock still retained much of its carbon dioxide, releasing it when it reached the combustion zone. (Dyer)

By increasing the length, the cylinder could be fed faster, tilted more steeply and rotated more quickly, and still burn the stone properly, without the produced carbon dioxide interfering with the combustion. Eckel (429) estimates that ...