Effect of Heat Curing Method on the Mechanical Strength of Alkali-Activated Slag Mortar after High-Temperature Exposure

Tai Thanh Tran,Hyuk Kang,Hyug-Moon Kwon

doi:10.3390/ma12111789

Abstract

The aim of this work was to study the mechanical strength and microstructure changes of alkali-activated slag mortar (AAS mortar) after being heat treated in the temperature range of 200–1000 °C. The AAS mortar was cured in the ambient condition (20 ± 5 °C, 60 ± 5% RH) (Relative humidity: RH) and high temperature condition (80 °C) for 27 days with three different heating regimes: curing in a dry oven, curing in sealed plastic bags, and in a steam environment. The activator for the AAS synthesis was a mixture of sodium silicate solution (water glass) and sodium hydroxide (NaOH) with a SiO2/Na2O weight ratio of 1, and a dosage of 4% Na2O by slag weight. Thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) incorporated with energy-dispersive X-ray spectroscopy (EDX) were used to assess the mortar microstructure change. The results revealed that the curing method significantly affected the mechanical strength of AAS at temperatures lower than 800 °C. The heat treatment at late age of 28 days was more beneficial for compressive strength enhancement in specimens without using heat curing methods.

Highlights

Alkali-activated ground granulated blast furnace slag has recently attracted strong attention in the literature as a potential alternative to Portland cement [1,2,3]
When used as the main binder, slag is commonly activated by an alkaline agent, such as sodium hydroxide, sodium carbonates, sodium silicates, or other alkalis [4]
Several studies have concluded that the main product of this activation process is generally a low crystalline calcium silicate hydrate (C–S–H) with a low CaO/SiO2 ratio [5,6]

Summary

Introduction

Alkali-activated ground granulated blast furnace slag has recently attracted strong attention in the literature as a potential alternative to Portland cement [1,2,3]. Several studies have concluded that the main product of this activation process is generally a low crystalline calcium silicate hydrate (C–S–H) with a low CaO/SiO2 ratio [5,6]. The properties of alkali-activated slag (AAS) were concluded to be highly dependent on the type and content of the alkaline activator, and on the curing condition [3,4,7,8,9,10,11,12,13,14]. Elevated-temperature curing greatly accelerates the slag activation process, resulting in the fast development of mechanical strength. An autoclave curing condition was reported to be appropriate for AAS mortar using an alkaline solution with low Na2 O concentrations and low weight ratios SiO2 /Na2 O

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Results

Conclusion