Hydration process of fly ash blended cement pastes by impedance measurement

Highlights

• Hydration of high-dosage fly ash blended cement pastes is evaluated.

• Four hydration stages are identified.

• Gismondine is found in the zeolite formation stage.

Abstract

This study focuses on the early-age hydration development in fly ash blended cement pastes. The characteristic features in different hydration stages and the evolution of zeolites phase, viz., gismondine, are identified in high-dosage fly ash blended cement-based materials using the innovative non-contact impedance measurement (NCIM), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), heat evolution test and pore solution analysis. It is found that the zeolite formation stage is comparatively distinct at the late hydration period. Besides, the influence of fly ash dosages in cement pastes on setting time and compressive strength are further examined.

Introduction

The current global production of fly ash is reported as around 800 million metric tons per year [1]. The amount of fly ash is expected to dramatically increase due to the continuous demand of coal for power production in China and India since 2004. The re-utilization rates of fly ash in United States in 2010 and European Union in 2008 are only 38% and 45% for the concrete production [1]. A good deal of the remaining ash is taken as waste products and dumped into landfill sites or even the ocean [2]. It is also known that fly ash always includes large amount of leachable toxic trace elements. The improper deposition of waste ash may lead to the serious contamination of soil, underground water and ocean, and thus, is a potential threat to the ecological environment and human health [3], [4].

It seems that incorporating fly ash in cementitious materials is an effective method to eliminate the replacement burden of fly ash to a great extent [5]. From the view point of chemistry, in fly ash blended cement-based materials, the highly glassy silica and alumina phases in ash can react with the portlandite and/or other hydrated products in the cement matrix in alkaline environment to form additional calcium-silicate-hydrate (C-S-H) and/or calcium-aluminatesilicate-hydrate (C-A-S-H) phases [1], [2], [6], [7]. Enhanced durability properties of concrete with fly ash by so-called pozzolanic reactions above have been extensively investigated [8], [9], [10], [11], [12], [13], [14]. Besides, it was reported that calcium-aluminatesilicate-hydrate (C-A-S-H) phases (also known as zeolites phases) produced in fly ash blended cement-based materials could be employed to remove the ammonium from wastewater [2].

To achieve the goal of sustainable development as possible, the high-dosage fly ash cement-based materials in which more than 50% of cement by mass has been replaced by fly ash is favor for low carbon footprint and cost efficiency [6]. However, studies on the hydration mechanism of fly ash blended cement-based materials, especially high-dosage fly ash ones, based on non-destructive, continuous and in-situ techniques are limited [1]. Meanwhile, the formation process of C-A-S-H in fly ash blended cement-based materials has not yet been fully investigated until now, which may restrict the application of fly ash blended cement-based materials.

In this work, the hydration evolution of fly ash blended cement pastes with different fly ash dosages is investigated using innovative non-contact impedance measurement (NCIM) in addition to X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis test (TGA), heat evolution test, ion chromatograph and inductively coupled plasma tests. In particular, characteristic features in each hydration stage of high-dosage fly ash blended cement pastes are discussed. Moreover, the relations among the dosage of fly ash, setting time and compressive strength of blended cement pastes are analyzed.