Tech Briefs

Geopolymer concrete is used in applications requiring high degrees of fire resistance.

Geopolymer concrete has been proposed as an alternative to Portland cement concrete in applications requiring high degrees of fire resistance, because the intrinsic chemistry of the geopolymer binder does not require the retention of water or hydration within gel phases to maintain structural integrity of the binder. Portland cement concrete contains a high level of chemically bound water, which is essential to the gel binder structure, and which is lost upon heating to several hundred degrees Celsius, whereas the water present within a geopolymer concrete is overwhelmingly present in pores and is not an essential part of the strength-generating phases. However, predictions of geopolymer concrete fire performance have up to this time been based on small-scale laboratory testing (usually on paste or mortar specimens), rather than the study of large concrete sections, which provides significance to this work.

Maximum temperatures of the Concrete Samples recorded at different points in the sample at the end of a four-hour test (furnace temperature at four hours is approximately 1150 °C) as a function of distance from the exposed (heated) face. The legend refers to the number of days after pouring that the sample was kept sealed before exposure to ambient conditions.
Fire testing of geopolymer concrete specimens and associated laboratory testing were conducted. The focus of this work is the outcomes of the series of pilot-scale (4' × 4' × 6") tests on geopolymer concrete panels, which were conducted on a single geopolymer concrete formulation (“E-Crete 40” supplied by Zeobond Pty Ltd.). Geopolymer concrete is derived from coal fly ash and metallurgical slag, which are reacted together with an alkaline “activating solution” (sodium silicate in this case), blended with fine and coarse aggregate (quartz sand and crushed granite in this case) to generate a product that is similar in mechanical properties and general appearance to Portland cement concrete.

E-Crete samples were poured, finished, and sealed in plastic sheeting for a specified number of days (1, 3, 7, 14, or 28), and then unwrapped and allowed to continue aging under ambient conditions to reach 56 days of age. Testing was then carried out following the Standard Time-Temperature Curve for a test duration of four hours.

Each data set shows an apparent flattening of the rate of temperature increase at a temperature between 100- 160 °C, holding at this temperature for a period of time, and then an acceleration following this as the temperature starts to increase again. This can be associated with the passage of a boiling front through the material; the concrete initially contains a significant quantity of liquid water in its pores. As this begins to be heated (close to the inside face of the concrete), the water expands upon heating, and fairly soon localized boiling commences. This generates pressure within the pore network of the concrete, forcing the water to flow away from the heated face. This water carries thermal energy as it travels through the sample, causing additional heating of the parts of the sample close to the unexposed face on top of the heat transport due to conduction through the sample.

The presence of more water in the samples cured for longer durations provides two competing effects relating to heat transport. The specific heat capacity and the latent heat of vaporization of water are high, which means that the requirement to heat and then boil more water provides a heat sink effect, which reduces the rate of temperature increase of the sample. The change in specific heat upon removal of liquid water from the pore structure of the geopolymer can be clearly observed, as there is an obvious change in the gradient of the heating curves before and after the boiling regime.

Spalling is a result of water entering the concrete, and forcing the surface to peel, pop out, or flake off. A tendency towards explosive spalling has been noted in many concretes, and has been the subject of much investigation internationally over the past 20 years.

The first fire test trials of geopolymer concretes, using samples that had been cured under sealed conditions but not exposed to ambient conditions for a significant period after unsealing, showed a high degree of spalling as the high water content and very refined pore structure led to the buildup of very high pore pressures. However, the more realistic curing regimes utilized in this work (a period of ambient, unsealed curing replicating the concrete being placed in service for a period of time before being exposed to fire load) did not induce spalling in any of the samples tested.

This work was done by John Provis of the University of New South Wales, Australia, for the Asian Office of Aerospace Research and Development. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Materials category. AFRL-0166

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