Effects of Incorporating High-Volume Fly Ash into Tricalcium Silicate on the Degree of Silicate Polymerization and Aluminum Substitution for Silicon in Calcium Silicate Hydrate.

Sungchul Bae,Paulo Monteiro,Juhyuk Moon,Rae Taylor,David Kilcoyne

doi:10.3390/ma10020131

Abstract

This study assesses the quantitative effects of incorporating high-volume fly ash (HVFA) into tricalcium silicate (C3S) paste on the hydration, degree of silicate polymerization, and Al substitution for Si in calcium silicate hydrate (C–S–H). Thermogravimetric analysis and isothermal conduction calorimetry showed that, although the induction period of C3S hydration was significantly extended, the degree of hydration of C3S after the deceleration period increased due to HVFA incorporation. Synchrotron-sourced soft X-ray spectromicroscopy further showed that most of the C3S in the C3S-HVFA paste was fully hydrated after 28 days of hydration, while that in the pure C3S paste was not. The chemical shifts of the Si K edge peaks in the near-edge X-ray fine structure of C–S–H in the C3S-HVFA paste directly indicate that Al substitutes for Si in C–S–H and that the additional silicate provided by the HVFA induces an enhanced degree of silicate polymerization. This new spectromicroscopic approach, supplemented with 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy and transmission electron microscopy, turned out to be a powerful characterization tool for studying a local atomic binding structure of C–S–H in C3S-HVFA system and presented results consistent with previous literature.

Highlights

High-volume fly ash (HVFA) concrete, in which more than 50 wt % of Portland cement (PC) is replaced by fly ash, has been successfully employed in building structures to mitigate CO2 emissions during PC clinker production [1]
The calcium silicate hydrate (C–S–H) characteristics are affected by several factors, such as the PC chemical composition, the water-to-cement (W/C) ratio and the introduction of supplementary cementitious materials (SCMs), including fly ash [8,9,10]
The purpose of the present paper is to demonstrate the effects of HVFA on C3 S hydration by examining the hydration kinetics, morphology of the hydration products, and atomic binding structure of C–S–H

Summary

Introduction

High-volume fly ash (HVFA) concrete, in which more than 50 wt % of Portland cement (PC) is replaced by fly ash, has been successfully employed in building structures to mitigate CO2 emissions during PC clinker production [1]. HVFA enhances properties of concrete, such as its workability, durability, and long-term compressive strength, and performance losses during the early ages can be overcome by optimizing the particle size distribution of the fly ash and the concrete mix proportions [3,4,5]. The enhanced properties of concrete due to HVFA incorporation result from an increase in the number of nucleation sites for PC hydrates [4,5] and from the pozzolanic reaction between Ca(OH) and the amorphous aluminosilicate-rich ash [6,7]. The C–S–H characteristics are affected by several factors, such as the PC chemical composition, the water-to-cement (W/C) ratio and the introduction of supplementary cementitious materials (SCMs), including fly ash [8,9,10].

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