Figure 6-1: Maya engineers used native limestone from mines like this. Note the center support pillar. Author’s image.
It is unknown how Maya technicians developed the geometry of the ingenious cement kiln assembly that enabled them to convert raw limestone into hydraulic cement. The process was in use by 250 BC and most probably was developed in a heuristic process of numerous trial-and-error developmental efforts for producing higher temperatures, until the optimum prototype of the classic cement kiln was developed. This kiln was used by Maya construction until the 20th century, when modern industrially produced cement became more cost effective. This process is still in use today in remote locations and for consolidation of Maya archaeological structures.
The cement firing kiln assembly developed by Maya technicians was a basic blast furnace that applied similar thermodynamic and chemical applications to those used by Henry Bessemer when he invented his blast furnace in 1856 (Figure 6-2). To achieve the requisite high temperatures for melting limestone, the Maya developed an assembly constructed of self-consuming timber fuel into a geometric configuration that induced elevated temperatures using the same process as the Bessemer blast furnace.
The timber kiln assembly serves multiple purposes: It functions as a platform for stacking the raw limestone material on the top, provides the requisite fuel for the combustion process, and develops a geometrical configuration that enables the assembly and its timber fuel to perform as a thermodynamic reactor, producing elevated temperatures at the centroid of the kiln. The streams of super-hot air are exhausted upward via the open cylinder in the center of the kiln, introducing high-temperature heat flow for melting the raw materials at the top of the kiln. The physical process of achieving these high temperatures is accelerated by the induction of cool, oxygen-rich outside air, which elevates the temperature in the center core while reducing the ambient pressure in the center core. In turn, the low pressure at the center core induces a rapid flow of cooler, oxygen-rich air from the exterior of the kiln, which increases the ambient temperature of the core center, causing super-heated air to dynamically exhaust upward (in a chimney-like effect) into the limestone material. The cycle of increased interior temperatures and the inflow of oxygen-rich air continue until the process is optimized, and the limestone is melted and converted into cement clinkers.
Figure 6-2: Maya engineers developed a thermodynamic process in their cement kilns that is similar to the modern blast furnace. Author’s image.
The cement fabrication process was in use as part of the consolidation and restoration of major Maya archaeological sites well into the 20th century, including, but not limited to, Uxmal, Calakmul and Chichen Itza. Figure 6-3 is a photograph of Maya workers applying the final adjustments to a cement kiln at Chichen Itza during the early 20th century. The cylindrical assembly, which provided the platform for the limestone and the fuel to achieve the desired final product, is constructed of locally acquired green timber. The geometrical configuration of the assembly clearly indicates the opening in the center of the kiln that served as the vertical flue and combustion chamber for the kiln.
Figure 6-4 is a graphic of the thermodynamic flow chart of the kiln assembly and the combustion and heat transfer process used by the Maya to produce a successful cement fabrication operation. The process applied the thermodynamic reaction provided by the geometrical configuration of the kiln and the fuel materials.
The diameter of a typical kiln was approximately 6 meters across the width of the assembly. The assembly was constructed of small-diameter wood logs of less than a meter in length. The logs were stacked horizontally in a radial pattern, with large logs infilled with smaller logs and chinked solid with chipped wood. The center of the kiln was open to the atmosphere, with a vertical cylindrical shaft of 8 inches in diameter. The central shaft extended from the ground level to the top or platform level of the kiln. The height from the ground level to the top of the platform is approximately 2 meters. A passageway for the flow of outside air to the center is located at the bottom of the kiln. When the kiln has been assembled, limestone is deposited on the top level of the platform formed by the timber logs. The raw limestone material is cut into small blocks and stacked to a height of 0.75 meters. The process is started by igniting easily combustible material in the form of dry leaves, dry decayed wood, or resinous wood that is deposited into the core center. This material is the tinder that is easily ignited and burns quickly to initiate the chemical reaction for producing cement.
Figure 6-3: Nineteenth-century cement kiln at Chichen Itza used for consolidation of the site. The process was thousands of years old at this time. Courtesy of Carnegie Institution.
The process involves a logical procedure that applies sound physical and chemical procedures, combined with ritual that has become a part of the cement-making process. To assure a long-term source of heat the logs must contain sufficient moisture content to control the time of combustion. Heat generated by the process will produce requisite high temperatures while assuring a long-term period of regulated temperatures. Wood that is very dry will ignite and burn too quickly. The timber logs must be green or soaked in water to impede the combustion process to the desired time of 24 to 30 hours. The atmospheric and meteorological conditions are also important. Clear weather is required for the burn; the chance of precipitation should be zero. A rainstorm during the process would result in failure, and it would have to be restarted. The process must be carried out in level terrain configurations and weather conditions without wind. The presence of wind will result in an unequal combustion process and cause the windward side of the kiln to burn at greater rate than the leeward side. The uneven burn will result in collapse of the windward side and a failure of the cement-making process. Under optimum conditions, a narrow tongue of flame will soar skyward as the flames extend to a height of 30 meters, resulting in a unique pyrotechnic manifestation. As the fire becomes very hot, the flame color changes with the temperature, from red to orange, then yellow, and finally to blue. A blue flame indicates a temperature of 1,600 degrees Celsius, which is sufficient temperature for melting limestone.
Figure 6-4: Thermodynamic diagram of a Maya cement kiln with heat flow into the limestone raw material. Author’s image.
The kiln burn process has certain ritual or superstitious attendants to the procedure. The Maya cement makers do not permit the presence of women during the burning of the kiln. This superstition could be akin to sailors on ships that forbade women to board their vessel. They believed it was bad luck, even though the sailors always referred the ship in feminine terms. The Maya cement kilns were also considered to be feminine. They believed that the kiln would become jealous if another woman was present during the burn and the kiln may refuse to operate properly.
When the burn is complete, the clinkers and cinders are allowed to cool. The mass is then allowed to cool. The cement clinkers are allowed to be exposed to dew and rain. The mass will expand into a dome of fluffy, white powder that is five or six times its original bulk. The cement can then be collected and ground into a fine powder. This powder is hydraulic cement that could be combined with water, admixture, and aggregate to create cast-in-place concrete for structures, concrete for pavements, mortar for stonework, and stucco for plastering the interior and exterior of their magnificent buildings. The cast-in-place concrete mixture was one part cement, three parts loosely ground limestone, and water. The process required five tons of wood to produce one ton of cement.
Forensic Analysis of the Chemical and Physical Properties of Maya Cement Compared With Modern Portland Cement
Archaeologists, when describing Maya structures and architectural components, freely use the terms mortar, stucco, and concrete interchangeably. These descriptions are intended to identify applications of cementitious materials in the construction of Maya buildings. However, nowhere in published work do archaeologists investigate the sources and methodologies for the fabrication of cement or cast-in-place concrete. Several references
on the Internet relating to archaeological discussions of Maya building materials state the Maya used a material that “mimicked” concrete, but was not concrete. As the old saying goes, if it feels like concrete, looks like concrete, and holds like concrete, then it must be concrete.
Archaeology has not properly investigated sources of cement or the application of cast-in-place concrete in Maya structures. However, there have been attempts to probe the issue, and research indicates that in “The Engineering Knowledge of the Maya” in Contributions to American Archaeology (Volume II, 1934), the Carnegie Institution published the results of chemical tests on Maya cement. Analysis was carried out on three samples of concrete taken from the structure known as the Casa de Monjas at Chichen Itza. Tests showed that the samples were quite close to each other in percentage of calcium oxide and carbon dioxide. These tests did not include comparisons with Portland cement or physical tests for strength. Archaeologist Lawrence Roys, who directed the tests, did make the observation that the Casa de Monjas may have been constructed during a period of 150 years and the quality of the cement over that time appeared consistent. The testing, though ambitious, did not indicate the exact source of the cement or its strength.
It was not until 1999 that a science-based, chemical, and physical testing program was carried out on Maya concrete. A team of scientists from the Autonomous University of the State of Mexico under the auspices of the National Institute of Archaeology and History carried out a field collection of samples and laboratory testing of Maya cementitious materials including concrete, stucco, and mortar. The team was led by Dr. Horacio Ramirez, and included Ramiro Pérez and Heriberto Diaz. They successfully carried out the testing program and published their results in a paper entitled “El cement y el concreto de los Maya” (“The Cement and Concrete of the Maya”) in the Mexican scientific journal Ciencia Ergo Sum.
Samples of Maya concrete, mortar, and stucco were collected for the testing program from the ancient Maya cities of Palenque and Colmalcalco, as well as concrete from Yaxchilan. The ancient concrete samples were evaluated using standard laboratory testing procedures for cement-based materials. The results were compared with similar standardized test results derived from samples prepared from modern commercial Portland cement. The testing included chemical analysis, defraction of the matrix, and physical analysis, including unit weight and compressive strength.
The properties of the Portland cement, the international standard for hydraulic cement, were selected as the baseline for the comparative analysis versus Maya cement. The results indicated in Table 6-1 show a close similarity in the chemical composition and relative percentage of these materials between Maya cement and Portland cement. Physical testing of the mechanical properties included compressive strength of the concrete samples and unit weight of the samples made with Maya cement. It must be noted that the test samples were collected from degraded Maya structures. It is probable that the ancient concrete test samples included minute fissures in their matrix, which would lower the compressive strength testing. The results of the testing program conclude that though the chemical composition of Maya cement is slightly different from that of Portland cement, it should be considered as having the same chemical composition as the advanced quality control of Portland cement. This quality control meets strict international standards for manufactured cement.
The pozzolanic index is an important measure of the character of cement. The test analysis of oxides for Maya cement samples indicated a pozzolanic index of 0.12. This value is greater than the pozzolanic index 0.05, which is the lower limit acceptable in modern concrete, and indicates that pozzolanic activity was significant. The team concluded that this level of a pozzolanic index was produced intentionally and did not occur by chance.
Table 6-1: Comparative Chemical Analysis
In an additional experimental program, carried out at a later date and conducted by Dr. Horacio Ramirez and Heriberto Diaz, undamaged samples of ancient concrete were collected for testing. The samples were collected from an ancient slab at Palenque and from the wall of Temple No. 6 and Structure No. 5 at the city of Yaxchilan. The samples consisted of eight specimens of Maya concrete of approximately 61 mm in size. The program included testing for measurements of pulse velocity, compression strength, modulus of rupture, and modulus of elasticity, and was carried out in the civil engineering laboratory of the CFE (Comision Federal de Electricidad) in Mexico City. The testing produced the following results:
1. Average pulse velocity: 4.16 × 106 mm/s (horizontal), 3.3 × 106 mm/s (vertical).
2. Average compressive strength: 1330 psi (93 kg/cm2) with a max of 2430 psi (171 kg/cm2).
3. Average modulus of rupture for concrete: 260 psi (18 kg/cm2).
4. Modulus of elasticity: E = 1,970,000 psi (138,504 kg/cm2).
These results indicate that the strength of ancient Maya concrete is more than adequate for resisting the loads superimposed on Maya structures. Structural analysis of Maya structures by the Ramirez team indicated that concrete in these structures was acting under a maximum stress of approximately 350 psi (25 kg/cm2). The results of both tests indicate that Maya structures were constructed of cast-in-place concrete with a much higher strength concrete than was required. The conclusions of the authors indicated the following:
1. The advanced materials of construction fabricated by Maya engineers, including concrete, have a remarkable ability to resist severe environmental exposure due to these significant mechanical properties comparable to modern standards.
2. The values of compressive strength of concrete samples are higher than reported than tests on other ancient American cultures.
3. Maya structures were designed by Maya engineers with a sufficient factor of safety.
4. As a result of the testing program and studies, a procedure for formulating cement with the characteristics of the ancient Maya cement has been developed. In the future, this blend of cement will be used by INAH in the restoration of Maya archaeological sites.
The result of the testing program concludes that the chemical composition of Maya cement is only slightly different from that of Portland cement. The study concludes that Maya cement, though not a perfect chemical match with Portland cement, is a true cement and can be defined as a building material very similar to modern cement. The authors of the study also concluded that Maya cement has cementitious properties that characterize the material as true cement and chemical agglomerate with properties that bind the constituent components of the concrete mix together into a strong matrix. The report states that the Maya cement was well compacted, and the aggregates were well graded, with a maximum size of 40 mm, similar to modern concrete.
The unit weight of Maya concrete determined by the study falls within the range of unit weight for modern Portland cement. The unit weight of Maya concrete was 112 pcf (pounds per cubic foot), or 1.8 gm/cm3, whereas Portland cement concrete varies from 110 pcf to 150 pcf depending on the mix of aggregate. The compressive strength of Maya concrete averaged 1,300 psi, or 93 kg/cm2, in the testing program. This is a lower strength than contemporary concrete, but would have been an acceptable level of compressive strength in the early 20th century, and was definitely acceptable as a structural material in the type of buildings designed by Maya engineers. The use of unreinforced mass concrete structures by Maya engineers applied this durable, low-strength concrete for use in a variety of purposes. The conclusions of the study were clear: the cement fabricated by Maya technicians was true cement, and concrete made with this Maya cement possessed the chemical and physical characteristics to develop cast-in-place concrete structures, mortar for masonry construction, and stucco for surface applications. Maya cement proved its capabilities by modern scientific analysis and indicated that it was the structural building material that held Maya cities together against all odds.
Testing the Capabilities of the Maya Cement Kiln
The ingenious cement kiln, with its special geometry, enabled the forced flow of oxygen-rich
air into the interior of the kiln and fire cylinder, which elevated temperature in the kiln to 1,450 degrees Celsius in order to melt the limestone and induce the chemical change from limestone into cement. An actual test of the capabilities of elevating temperatures by the timber kiln would be a positive proof of its thermodynamic capabilities.
The opportunity to construct a scale working model of a Maya cement kiln came about during the filming of a production for the History Channel. I had the chance to carry out the test in conjunction with sculptor Philippe Klinefelter. The History Channel was filming a segment relating to the technology of the Maya. This sequence of the production covered the fabrication of cement utilizing Maya techniques. We started our test with intentions of using the full-scale Maya design for the blast furnace. We soon realized that if we used the full dimensions for a typical Maya cement kiln, it would require 22 cords of wood, or 11 truckloads of wood, each with a 10-cubic-yard capacity. Not only would the logistics and economics of the project be a daunting issue, but we also would be the object of special attention from the City of Austin Fire Department when the flames soared 20 to 30 meters vertically into the sky. Considering the alternatives, we choose the better part of valor and selected the scientific route for the test. We decided to build a working scale model.
The Lost Secrets of Maya Technology Page 13