Frost resistance of fiber-reinforced blended slag and Class F fly ash-based geopolymer concrete under the coupling effect of freeze-thaw cycling and axial compressive loading

Highlights

• Three types of fiber were added to enhance the compressive strength and frost resistance.

• The coupling effect of freeze-thaw cycling and axial compressive loading was studied.

• S fiber has a complicated impact on the structural compactness and frost resistance of FGPC under pressure.

Abstract

The poor frost resistance of low-calcium geopolymer concrete is an urgent problem that needs to be solved before its application in practice. In this study, polypropylene (PP) fiber, polyvinyl alcohol (PVA) fiber and steel (S) fiber were added to enhance the frost resistance of blended slag and Class F fly ash-based geopolymer concrete (SFGPC). More importantly, to evaluate the frost resistance of fiber-reinforced SFGPC under axial compressive loading, a coupling experiment with 20 MPa of compressive stress and 125 freeze-cycles was conducted. It was found that the addition of fibers did not inhibit the initiation of microcracks but could suppress their propagation. The water penetration depth and mercury intrusion porosimetry (MIP) test results verified that the applied compressive loading further compacts the concrete, which significantly improves the frost resistance. In comparison, the S fiber-reinforced SFGPC exhibited an inconsistent frost resistance with and without compressive stress due to the extremely high elastic modulus of the S fiber. Overall, 0.3% vol PVA fiber had the best effect on improving both the mechanical properties and the frost resistance of SFGPC.

Introduction

The process of producing cement consumes a substantial amount of energy, produces high carbon emissions and causes environmental pollution [1]. Geopolymers are a kind of cementitious material obtained by mixing an activator and binder materials, which are generally industrial wastes such as fly ash (FA) or slag and are used as a replacement for cement, and the carbon emissions in the geopolymer production process are very low [2], [3]. In addition, geopolymer concrete exhibits favorable mechanical properties and durability characteristics, such as high-temperature resistance [4], chloride attack resistance [5] and sulfate attack resistance [6]. Therefore, geopolymer concrete is receiving increasing attention from researchers [7], [8].

Frost resistance is a crucial aspect of concrete durability [8]. At present, research on the frost resistance of geopolymer concrete is limited. Fu et al. [9] studied the frost resistance of slag-based geopolymer concrete and found that it can withstand more than 300 rapid freeze-thaw cycles. Our previous work found that the frost resistance of Class F FA-based geopolymer concrete was weak but can be improved to withstand 225 rapid freeze-thaw cycles by adding 50% slag [10]. For ordinary Portland cement concrete (OPCC), an air-entraining agent can be used to enhance frost resistance [11]. However, Sun et al. [12] and Brooks et al. [13] found that using an air-entraining agent is not effective for geopolymer concrete and even has a weakening effect. On the other hand, fiber additives have been considered to improve the mechanical properties and durability of cement-based materials [14], [15], [16]. Some investigations have shown that fiber additives can improve fatigue resistance and thus have a strong potential for use in pavement applications [17], [18]. Park P et al. [17] found that proper use of steel fiber can improve the low-temperature cracking resistance of asphalt concrete. Moreover, adding fibers with appropriate lengths and diameters is critical for this performance improvement. Erhan Güneyisi et al. [19] investigated the effect of the steel (S) fiber aspect ratio on concrete fracture properties, and the highest fracture energy values were achieved at an aspect ratio of 80. Nam J et al. [20] studied the frost resistance of polyvinyl alcohol (PVA) fiber and polypropylene (PP) fiber-reinforced cementitious composites and found that the interfacial transition zone (ITZ) between the substrate and the blended fiber is the weak link. Recently, it has been found that the addition of fiber can improve the workability and mechanical properties of geopolymer mortar [21], [22]. Overall, there are few reports on the use of fiber to improve the frost resistance of geopolymer concrete, and this topic deserves further research.

Structures suffer from environmental erosion and bear loads at the same time. For example, a submerged pier in a cold zone bears the coupling effect of freeze-thaw cycling and axial compressive loading, which has an adverse impact on the safety and durability of the structure. At present, many papers have studied the durability of concrete under the influence of the single factor of freeze-thaw cycling [23], [24], [25]. Only a limited number of papers have studied the coupling effect of freeze-thaw cycling and loading. Jingzhou Lu et al. [26] studied the coupling effect of freeze-thaw cycling and cyclic loading through the following methods: (a) Mode F-FT: first, a fatigue load was applied to the specimens; then, the specimens were exposed to freeze-thaw cycling. (b) Mode FT-F: first, specimens were exposed to freeze-thaw cycling; then, a fatigue load was applied to the specimens. The results showed that the compressive strength reduction in concrete due to the F-FT damage history is greater than that due to the FT-F damage history. Bin Lei et al. [27] studied the coupling effect by alternating freeze-thaw cycles and load cycles, as shown in Fig. 1. With the increase in the number of alternations and the level of stress, the number of cracks that formed under freeze-thaw cycling clearly increased. However, none of these methods can truly reflect the real failure process in which loading and freeze-thaw cycling are applied simultaneously.

Some researchers have worked to address this coupling problem [28]. Kong et al. [29] designed a novel loading method to study the combination of sustained compressive stress and freeze-thaw cycles on carbon fiber-reinforced polymer (CFRP)-wrapped concrete cylinders. After 300 freeze-thaw cycles, the strength of the samples under the compressive load decreased by 12%, while the unloaded samples were completely destroyed. However, measuring only the compressive strength provides only an approximation of the frost resistance. Song et al. [30] conducted a similar experiment on concrete-filled fiber-reinforced polymer tube (CFFT) columns. The test results showed that the degradation of the ultimate strength is negligible compared with that of the ultimate strain. Wang et al. [31] studied the water absorption of concrete under the coupling effect of freeze-thaw cycling and axial compressive loading. It was found that for OPCC, 0.3fc could relieve the damage caused by freeze-thaw cycling, while 0.5fc could aggravate the damage. In their study, 100 mm × 100 mm × 100 mm cubic specimens were used to test the relative dynamic elasticity modulus (RDM), but this is not a standard specimen size (100 mm × 100 mm × 400 mm), as required in ASTM C666 [32] and GB/T 50082-2009 [33], which may lead to deviations in the results. Considering the frost resistance evaluation method and the size effect, the existing research is still insufficient.

The purpose of this paper is to study the effect of fiber type and fiber content on improving the frost resistance of SFGPC and to study the effect of axial compressive loading on the frost resistance of fiber-reinforced SFGPC.