The effect of pozzolans and slag on the expansion of mortars cured at elevated temperature: Part I: Expansive behaviour

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Abstract

The expansive behaviour of heat-cured mortars containing pozzolans and slag was investigated. In most cases, the addition of any amount of these materials to the mixture typically reduced the long-term expansion, slowed the rate of expansion, and delayed the onset of expansion. However, the efficacy of a particular pozzolan or slag in controlling expansion may depend on its Al2O3 content. Metakaolin, which contains a high amount of reactive Al2O3, was the most effective at controlling expansion at relatively low cement replacement levels. Slag and fly ash, which are also sources of Al2O3, were also effective at suppressing expansion at higher replacement levels. Silica fume was less effective at controlling expansion at conventional replacement levels, and even at higher replacement levels expansion may only be delayed.

Introduction

It has been observed that some cement products, when cured at elevated temperatures (above ∼70 °C), may exhibit expansive behaviour if subsequently exposed to sufficient moisture at ambient temperature. The resulting expansion can lead to cracking of concrete resulting in loss of strength, a decreased service life, or other serviceability and durability problems. This manner of deterioration has been referred to as delayed ettringite formation (DEF) due to the prevalence of ettringite deposits found in the voids, cracks, and gaps around aggregate particles of the damaged concrete. DEF in field concrete, though uncommon, can be potentially costly, and precautions should be taken to avoid the problems associated it.

Although the DEF expansion mechanism has not been fully explained, it is generally understood that three things are essential to promote expansion: (1) exposure to elevated temperatures (above ∼70 °C) during curing, whether deliberate or incidental, for a sufficient period, (2) subsequent exposure to moisture, and (3) a cement that has a potential for expansive behaviour when heat cured at excessive temperatures. Controlling or eliminating any one of these three elements may be adequate to prevent expansion.

While there have been some claims of DEF occurring in concrete cured at ordinary temperatures [1], [2], [3], in practice, the most effective method of controlling expansion is to avoid exposure to elevated temperatures during curing for a prolonged period of time. Moreover, the apparent role of elevated temperatures on the occurrence of DEF has led a number of countries to impose restrictions on the heat-curing process used in the manufacture of precast concrete [4], [5]. These include limits placed on preset times, rates of heating and cooling, and the maximum allowable concrete temperature. However, the risk of damage from DEF is not necessarily limited to heat-cured precast concrete elements alone; internal concrete temperatures may rise above 70 °C, to levels sufficient to promote DEF, in massive cast-in-place concrete, due to the heat evolved from hydration, or in concrete that is placed under high external temperatures (i.e., hot weather). In such instances controlling the internal concrete temperature below 70 °C may not be possible, and other mitigative measures may be necessary.

There have been a number of attempts to relate expansion from DEF to cement composition [6], [7], [8], [9], but no reliable correlation exists that can be applied to all cements. It is, however, apparent from laboratory studies that high-early strength cements have the greatest potential to exhibit expansive behaviour when heat cured at excessive temperatures. Consequently, it has been suggested that the risk of damage may be lessened to some extent by using cements that are low in SO3, tricalcium aluminate, alkalis, MgO, and alite, and which do not have a high fineness. However, such cements are likely to have relatively slow early strength development and may not be particularly suited for precast concrete. Furthermore, controlling the cement chemistry does not necessarily eliminate the risk of expansion; Hobbs [10] concluded that, based on available data, a suitable choice of Portland cement composition does not preclude expansion in materials that have experienced temperatures above 70 °C.

It has been demonstrated that additions of slag, trass (a natural pozzolan), or fly ash can reduce the expansion of heat-cured mortars and concrete if used in sufficient quantity [11], [12]. This could, in part, be due to the addition of Al2O3 present in these materials, though there are other effects of incorporating these materials that may also be beneficial. Despite these positive results, little work has been done since to better understand the mechanisms by which pozzolans and slag may suppress expansion.

This paper examines the potential mitigating effect that pozzolans and slag may have on the expansion of mortars cured at elevated temperature. The work presented here is part of an extensive study aimed at developing a fuller understanding of the role of supplementary cementing materials in controlling DEF. Data are presented for mortar prisms made from two ASTM Type III high-early strength cements, incorporating various amounts of silica fume, fly ash, high-reactivity metakaolin, and/or slag as a partial cement replacement, which were cured at elevated temperatures (up to 95 °C) and then stored in lime-saturated water at ambient temperature for as long as 4 years.

A subsequent paper will investigate the chemical and microstructural aspects of heat-cured mortars containing pozzolans and slag. In particular, pore solution extraction, X-ray diffraction and electron microscopy with energy-dispersive X-ray analysis was performed to help elucidate the role that these materials may have in controlling expansion.

Section snippets

Experimental

The chemical compositions of the cements, pozzolans, and slag used in this investigation are given in Table 1, Table 2, Table 3. The Type III cements used in this study can be characterized as being relatively high in sulfate, alkali, C3A content (as determined by Bogue calculations), and fineness. The silica fume that was used was in the form of an undensified powder. The high-reactivity metakaolin used was a commercially available grade made from Georgia (USA) kaolin conforming to ASTM C 618

Expansion of Portland cement mortars

Fig. 1, Fig. 2 show the expansive behaviour of mortars made with the Type III cements A and B when heat cured at maximum temperatures of 60, 70, 80, and 95 °C, as well as when continuously cured at 23 °C. There was a systematic increase of the ultimate expansion of mortars with increasing curing temperature. The mortars made with cement A expanded more than those made with cement B when cured at 95 °C, even though cement B had a higher sulfate and alkali content. At 80 °C, the expansion was

Discussion

It has been reported that if Portland cements are hydrated at sufficiently high temperatures, the normal formation of ettringite does not occur [17], [18]. This results in a prolonged high concentration of sulfate in the pore solution throughout the heat treatment. It has also been reported that a significant amount of sulfate enters the inner C–S–H product that forms during heat treatment, the extent of which increases with temperature [19], [20], [21]. This uptake of sulfate has been

Conclusions

  • 1.

    At the conventional replacement levels used in concrete (i.e., ∼8%), silica fume was not effective at controlling long-term expansion related to DEF when heat cured at high temperatures. The onset of expansion may only be delayed as a result of the lower permeability of the silica fume mortars.

  • 2.

    Metakaolin is effective at suppressing or perhaps even eliminating long-term expansion at relatively low levels (8% or more), when used as a partial replacement for a cement that exhibits expansive

Acknowledgments

The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada and Lafarge North America for their support of this project.

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