Thermal behavior and mechanical properties of geopolymer mortar after exposure to elevated temperatures

Highlights

• Mechanical properties of geopolymer mortar at elevated temperatures are quantified.

• Thermal behavior of geopolymer mortar under fire conditions is evaluated.

• Temperature induced mass loss and shrinkage in geopolymer mortar is evaluated.

Abstract

This paper presents results from experimental studies on mechanical properties and thermal behavior of geopolymer mortar, prepared by alkaline solution activating metakaolin and fly ash blend. Bending, compressive, tensile and bond strength tests were conducted on large sets of geopolymer mortar, Portland cement mortar, and commercially used repair mortar specimens at ambient temperature and after exposure to elevated temperatures. Thermogravimetry and differential scanning calorimetry analysis, and dilatometric tests were also carried out on geopolymer paste and mortar. Results from these tests show that geopolymer mortar exhibits higher temperature-induced degradation in bending and tensile strength, but lower degradation in compressive and bond strength than ordinary Portland cement mortar and commercially used repair mortar. Specifically, the bond strength of geopolymer mortar on cement mortar or concrete substrate is close to or even higher than that of commercially used repair mortar throughout 25–700 °C range. The microstructural damage due to temperature-induced dehydration and dehydroxylation, and thermal incompatibility between geopolymer paste and aggregates is the main reason for the strength degradation of geopolymer mortar at high temperatures.

Introduction

Geopolymer, a new environment friendly inorganic binder, derived by alkaline solution activating aluminosilicate source material (such as metakaolin, fly ash and slag), has attracted significant attention in recent years as a practical alternative to Portland cement [1], [2], [3], [4]. Geopolymer exhibits comparable mechanical properties and durability characteristics as that of Portland cement, but has lower energy requirements and lower greenhouse gas emissions during its production [5], [6]. Therefore, there is a growing research interest in developing viable processes for application of geopolymers and its resulting products in construction industry [2], [7], [8], [9], [10].

Cement mortar is a commonly used binder and repair material. Feasibility of geopolymer mortar, as a promising replacement to cement mortar, has been extensively discussed in literature [11], [12], [13], [14], [15]. Temuujin [16] reported that geopolymer binder exhibited strong bonding to sand aggregate. Chi and Huang [17] investigated the compressive strength, flexural strength, drying shrinkage and water absorption of alkali-activated fly ash/slag (AAFS) mortars, and their test results showed that with the exception of drying shrinkage, better properties have been obtained in AAFS mortars than that in Portland cement mortar. Pacheco-Torgal et al. [18] studied the effect of sodium hydroxide concentration, superplasticizer content and percentage substitution of metakaolin by calcium hydroxide on the workability, compressive and flexural strength of alkali-activated metakaolin based mortars. Ueng et al. [19] conducted a series of laboratory tests to determine the deformational moduli and strength parameters of adhesion interface between geopolymer mortar and cement mortar. Vasconcelos et al. [7] found that metakaolin geopolymer mortar exhibits a slighter lower adhesion to the concrete substrate than that of commercially available pre-pack repair mortars, but geopolymer mortar is more cost-effective.

The above review from experimental studies clearly show that geopolymer mortar exhibits great promise as a repair material, due to high compressive and flexural strength, high adhesion to ordinary Portland cement mortar and concrete substrate. However, these experimental results were obtained only at ambient temperature. The mechanical characteristics of geopolymer mortar at high temperature must be well understood if geopolymer mortar is to be used as a repair material in buildings where fire resistance is one of the primary requirements. Recently, several researchers discussed the mechanical properties of geopolymer mortar at high temperatures [20], [21], [22], [23]. However, these studies mainly focused only on the degradation of compressive strength of geopolymer mortar after exposure to elevated temperatures. There is a lack of experimental data on other mechanical properties of geopolymer mortar at high temperatures.

A series of mechanical property tests were carried out in this study on geopolymer mortar specimens, to evaluate the residual compressive, bending, tensile and bond strength of geopolymer mortar after exposure to elevated temperatures. Comparative benchmark tests were also conducted on ordinary Portland cement (OPC) mortar and commercially used repair mortar specimens. The thermal behavior of geopolymer mortar, including temperature-induced mass loss, thermal flow and expansion, were also investigated through thermogravimetry and differential scanning calorimetry (TG–DSC) analysis, and dilatometric tests.