Track 2: Materials over Time: Points of Change
Battle of the Cements: The Effect of Portland Cement Repairs on Original Natural Cement Mortars
Tuesday, September 25
8:30 AM - 10:00 AM
Location: BNCC- 101BG
After countless restorations costing several millions dollars, the Metropolitan Transit Authority asked why the repointing repairs in the Grand Central Terminal tunnels repeatedly failed. The answer lay in the incompatibility between the original and repair cements.
In 1818, Natural cement was discovered by Engineer Canvass White in Fayetteville, NY while looking for an alternative to lime-based concrete during the construction of the Erie Canal in 1818. This cement was then used to construct the 365-mile canal system. In the late 1830s as the Industrial Revolution reached America, large manufacturing structures were built, as well as dams, power plants, roads, bridges, and railroads to support them. The vast majority of these structures used natural cement either solely or in combination with lime for use in masonry mortar, concrete, and stucco. By 1896, natural cement was being produced at 71 sites in 17 states.
Although Portland cement was developed in 1825 by James Aspdin in Great Britain, the first Portland cement plant in the United States did not began production until 1875. By the end of the 19th century Portland cement usage began to catch up with natural cement, and by the turn of the 20th century Portland cement had become the dominant cement used in America. This was due in part to the high strength that was achieved using Portland cement and the shorter cure time as compared to natural cement. Natural cement producers could no longer compete with Portland cement and the last original producer of natural cement closed in 1976. The need for sympathetic repair materials for natural cement structures rekindled commercial production of natural cement in 2004.
The unavailability of natural cement, the resultant loss of knowledge of how to use natural cement, and the idea that “stronger is better” caused structures that were originally constructed with natural cement to be repaired using Portland cement. Unfortunately, the same properties that helped make Portland cement popular are the same properties that make it incompatible with natural cement. Natural cement mortars retain a low modulus of elasticity as they age, unlike Portland cement-based mortars which continue to become harder and brittle. The low modulus of elasticity allows natural cement mortars to deform when the masonry units expand and contract thereby remaining intact while not damaging the masonry
This presentation will explore the effects of using Portland cement mortar to repoint structures originally set with natural cement mortar. The lessons learned during each project will be discussed including the need for surveys and testing during design and contractor training for how to use natural cement.
- Upon completion, participants will be able to know the difference between Portland and natural cements.
- Upon completion will know the history of natural cement and Portland cement use in the United States.
- Upon completion, participants will know the effects of Portland cement mortars used in conjunction with natural cement mortars.
- Upon completions, participants will know how to investigate and analyse their structures for the presence of natural cement.