A critical review of geopolymer properties for structural fire-resistance applications

Highlights

• Geopolymers are chemically stable.

• Thermal deformations occurring in geopolymers.

• Strength of geopolymers is affected by microstructural and phase composition changes.

• Geopolymers show inherently superior fire resistance.

• Geopolymers require careful mix design to achieve superior fire resistance.

Abstract

Protection of structures from fire is of extreme importance. Geopolymer is a novel material that has wide-ranging applications, and this review article focuses on assessing the potential of geopolymers towards enhancing the structural fire resistance by critically reviewing its properties subjected to elevated temperature exposure. The properties of geopolymers are categorized into three scales, namely, micro-scale, meso-scale and macro-scale, and are discussed at length. It is noted that geopolymers are chemically stable and do not undergo breakdown of chemical structure in contrary to OPC hydration products. Thermal deformations occurring in geopolymers, which cause macro-cracking, are discussed. Compressive strength of geopolymers is observed to be affected by microstructural changes (including crack formation, pore structure changes, densification, sintering, and melting) and phase composition changes (such as growth or destruction of crystals and transformations in geopolymer paste). Geopolymer-based binders show inherently superior fire resistance as compared to Portland cement-based binders. However, it requires careful mix design, to achieve substantial chemical stability, low volume changes, strength endurance, and spalling resistance. Factors such as choice of precursor, use of aggregates, total alkali content in geopolymer, water content, etc. are critical and should be controlled. The influence of these factors is discussed at length in this article. The current applications of geopolymers for heat and fire resistance have also been briefly presented.

Introduction

Fire damage can be critical in structures and in all cases, the protection of people and property is essential. Despite all the technological advancements and measures for fire prevention, there always remains a threat of fire breakout. The fire arising from 9/11 attack on World Trade Centre, the Windsor tower fire, the fire in Mont Blanc and Channel tunnels, have caused heavy losses of life and human assets. Protection of structures from fire is vital since it would surely contribute to a reduction of losses in all respects. Therefore, research in a matter of fire safety is of utmost importance.

Concrete and steel are the most widely used construction materials [1]. The reasons are that concrete has inherent fire resistance, is non-combustible, and has a relatively low thermal conductivity. Cracking, spalling, and loss in mechanical strength of concrete after exposure to fire, however, are key concerns, especially with high strength concrete [2], [3], [4], [5], [6], [7], [8], [9], [10]. All concrete structures, especially structures such as concrete liner of tunnels, must offer sufficient fire resistance. There has been an experience of severe dramatic tunnel fires in the past few decades. Table 1 [11] summarizes the life and property losses due to these fires. In case of a tunnel fire, there is a rapid increase in air temperature within the first few minutes of fire. This results in considerable spalling of concrete and consequently, the tunnel lining suffers damage. It leads to safety issues both for the tunnel users and the fire rescuers. As can be seen from the table, there are direct economic losses from repair activities and indirect losses related to tunnel closure. Fire resistance tests on concrete elements are required, in most tunneling projects, to address the tunnel fire issues. Temperature-versus-time curves such as the ISO standard fire curve, RWS curve, hydrocarbon fire curve have been adopted in various tunnel fire testing regulations. All these highlight that fire safety of any concrete structure, not just tunnels, is a serious concern.

The use of polypropylene (PP) fibers in concrete is generally undertaken to reduce the danger of spalling of concrete in the event of a fire [12], [13], [14], [15]. Two chief causes for spalling are – a rise in pore pressure and development of thermal stresses. Although PP fibers have been shown to reduce the spalling caused by pore pressure rise, they have not been very effective in reducing the thermal stresses. This is one of the drawbacks with using PP fibers as a measure to prevent concrete from fire damage. Use of thermal barriers is another solution for concrete fire protection. However, this solution is available at a cost. Moreover, when OPC concrete is subject to high temperature, ordinary Portland cement (OPC) hydration products, namely, calcium silicate hydrate, calcium hydroxide and calcium carbonate degrade (Fig. 1). It is challenging to avoid this deterioration of concrete micro-structure [16], [17].

Moreover, steel, both as reinforcement in concrete and for structural applications, is much more vulnerable to high temperature as compared to concrete. Thus, steel structures need fire protection. For steel members, in many applications, it is imperative to provide insulation to prevent softening of steel at high temperatures. Four common types of passive fire protection materials are listed below:

• Traditional materials (concrete, plaster, bricks): These are readily available, and their implementation does not require special skills. These materials are weather resistant but are slow to apply on site. The thick casing takes up the floor space, and they are heavy.

• Board materials: These are pre-fabricated non-combustible boards made by gypsum and reinforced with inert fibers. The pre-fabricated insulation boards can be applied to existing structures as a method for fire refurbishment. However, this is an expensive method and requires a long installation time.

• Spray-applied fire-resistive material (SFRM): This is the most commonly used passive fire protection material for steel structures. It is also used in reinforced concrete structures to prevent explosive spalling. SFRM usually consists of cement matrix or gypsum plaster containing vermiculite, shredded polystyrene or mineral fibers. They can be sprayed either in the form of dry-mix (sprayed mineral fiber SFRM) or wet-mix (sprayed cementitious SFRM). They have relatively low cost and can be applied to existing structures as well. However, the application of SFRM makes the appearance of the structural element less aesthetically appealing. The adhesion or bonding to the surface of structural element is important. Missing fireproofing due to poor adhesion to steel and brittleness of SFRM has been observed. One primary reason of the collapse of the World Trade Center (WTC) towers in 2001, for example, was the dislodging of SFRM from structural members due to aircraft impact, which resulted in rapid heating of unprotected structural steel. In reinforced concrete structures, heat-induced spalling can result in severe loss of integrity and collapse of structures.

• Intumescent coating: Specially formulated paint-like coating can be applied onto steel surface. The coating swells upon heating. This approach is aesthetically pleasing and most used in architecturally exposed structural steel. However, the material is much more expensive than SFRM. Intumescent coating commonly provides a fire rating of 1–2 h. Generally, the intumescent coating is applied on site. Application of intumescent coating is fast, and it does not take up space or adds to the weight. The concern with it is the high cost.

Overall, it has been reported that for many applications including fire protection of tunnels, underground structures, high rise buildings, strategic objects, metallic structures such as I-beams, thermal insulation of furnace, chimneys, boilers or as a composite material for ships, automotives and aircrafts, it is necessity to develop new materials which have superior fire and mechanical performance to conventional concrete and other construction materials [19].

Geopolymer is a novel construction material which can be an alternative binder to OPC. It has also been referred to in the literature as ‘inorganic polymer glass’, ‘alkali-activated cement’, ‘mineral polymer’, and ‘alkali-bonded ceramic’ [20]. The term ‘Geopolymer’ was coined by Joseph Davidovits in 1978 [21]. It refers to a solid material formed by the reaction of aluminosilicate source with an alkaline solution [22]. Aluminosilicate sources that are commonly used are metakaolin, fly ash and blast furnace slag. Geopolymer is greener than OPC and can provide a route to reduce carbon footprint caused by excessive usage of OPC [23], [24], [25], [26], [27], [28]. Producing geopolymer concrete based on coal combustion by-product, fly ash, is an useful technology to utilize fly ash, instead of amassing or simply dumping this material to cause ecological concern [27], [29], [30]. The mechanism by which geopolymer hardens and gains strength is markedly different from OPC. Geopolymer possesses excellent mechanical properties, and its compressive strength can be greater than 100 MPa [31]. It has excellent potential for fire, alkali-silica reaction (ASR), and acid resistance. It has an inorganic framework and does not combust like organic polymers. It is non-smoking and non-toxic and has low processing temperature as compared to ceramic composites [32]. Geopolymer can serve as a green construction material with immense potential for sustainable development and fire resistance.

Based on the above-presented descriptions, it is realized that there is need of a green construction material for structural applications with admirable fire and mechanical performance. It is also anticipated that geopolymer can fit the bill. As a consequence, there is a need to conduct an in-depth literature review on the current state of the art related to the performance of geopolymer subjected to elevated temperature. The discussion to follow will be focused on assessing the potential of geopolymers towards enhancing the structural fire resistance by reviewing the phase stability, microstructural changes, thermal deformations, strength endurance, and fire-induced spalling resistance of this material on exposure to high temperature. It is an effort to critically review the latest developments on this matter and present it concisely. The review of industrial applications and existing literature in an article such as this is by necessity less than exhaustive. However, the understandings provided by the current article will be valuable for researchers and engineers aspiring to develop high temperature-resistant or fire-resistant materials for structural applications. Fire-resistant geopolymer concrete/composites may be a potential solution for the construction of critical structures such as tunnels, underground caverns and high-rise buildings which necessitate fire safety.