Issues with High Alumina Cement

A Hidden Problem in Cast Stone and Cast Concrete?

Calcium aluminate cement, or high alumina cement (HAC), can be a hidden problem in your cast concrete or cast stone buildings.

JBC analyzed cast stone samples from a decorative molded architrave and a flat pier of a building constructed between 1925 and 1927. Though they appeared to be composed of the same material, the architrave samples were soft and crumbing, while the sample from the pier was sound.

Figure 1: Sample from the architrave..

Figure 2: Sample from the pier.

Physical testing of the samples showed that the architrave sample was 6 times more absorbent than the pier sample in the initial rate of absorption; 3 times more absorbent in the 48-hour soak and 5-hour boil absorption tests; and 36 times weaker in compression.

The differences in physical properties between the samples are primarily due to compositional differences. Petrographic analysis of the samples revealed that the cast stone sample from the molded architrave contained HAC, while the sample from the pier did not.

HAC was patented in France in 1908 by the Lafarge Company.  It is manufactured by fusing limestone and bauxite (aluminum ore), instead of limestone and clay/shale used for Portland cement, resulting in a mixture of hydraulic calcium aluminates. HAC develops initial strength very rapidly, which made it an ideal material for cast elements, as it allowed for the quick removal and reuse of formwork.[1]

Unfortunately, HAC goes through a natural and inevitable reaction known as “conversion”. Conversion is a mineralogical change in which the metastable hydrates of the cement undergo a change over several years or decades to their stable form (aluminum hydroxide or aluminum hydroxide gel). This change results in an increase in porosity and a reduction in strength,[2] which was evident in the sample taken from the architrave.

REFERENCES:

[1] Jeremy P. Ingham, Geomaterials under the Microscope (London: Academic Press, 2013), 117; David Odgers, ed., English Heritage Practical Building Conservation: Concrete (Surrey: Ashgate Publishing Limited, 2012), 47; Adam M. Neville, Concrete: Neville’s Insights and Issues (London: Thomas Telford Publishing, 2006), 50.

[2] Ingham, 117.