Severe decay of the stone façade of the Pyramid of Quetzalcoatl, one of the more remarkable structures in the World Heritage Site archaeological zone of Teotihuacan, Mexico, is caused by the presence of humidity and salt crystallization. The floors were analyzed to determine whether water and salts were filtering up from the ground and to find a way to reduce the penetration of water. Analyses of the lime plaster and organic matter were carried out by traditional wet chemical methods in order to discover the possible recipe that is the basis of the technology used by Pre-Columbian masons. The presence of an organic mucilage and fiber was identified. The organic components of the floors was analyzed using FTIR of extracts, while mucilage from leaves of a local cactus, Opuntia sp, that is used traditionally as a cementing material in plasters, was analyzed for comparative purposes to determine whether the mucilage was also used in pre-Columbian times. X-ray spectrometry and x-ray diffraction were also used to complement the study to identify the mineralogy of the plasters. This study shows that the Teotihuacanos used a composite material that has great durability, permitting the survival of the floors for almost 2000 years, but that also is susceptible to damage in a modern, stressed environment with high tourist traffic.