Revisiting the Effect of Slag in Reducing Heat of Hydration in Concrete in Comparison to Other Supplementary Cementitious Materials.

Hoon Moon,Sivakumar Ramanathan,Chang-Joon Lee,Prannoy Suraneni,Chang-Seon Shon,Chul-Woo Chung

doi:10.3390/ma11101847

Abstract

Blast furnace slag (SL) is an amorphous calcium aluminosilicate material that exhibits both pozzolanic and latent hydraulic activities. It has been successfully used to reduce the heat of hydration in mass concrete. However, SL currently available in the market generally experiences pre-treatment to increase its reactivity to be closer to that of portland cement. Therefore, using such pre-treated SL may not be applicable for reducing the heat of hydration in mass concrete. In this work, the adiabatic and semi-adiabatic temperature rise of concretes with 20% and 40% SL (mass replacement of cement) containing calcium sulfate were investigated. Isothermal calorimetry and thermal analysis (TGA) were used to study the hydration kinetics of cement paste at 23 and 50 °C. Results were compared with those with control cement and 20% replacements of silica fume, fly ash, and metakaolin. Results obtained from adiabatic calorimetry and isothermal calorimetry testing showed that the concrete with SL had somewhat higher maximum temperature rise and heat release compared to other materials, regardless of SL replacement levels. However, there was a delay in time to reach maximum temperature with increasing SL replacement level. At 50 °C, a significant acceleration was observed for SL, which is more likely related to the pozzolanic reaction than the hydraulic reaction. Semi-adiabatic calorimetry did not show a greater temperature rise for the SL compared to other materials; the differences in results between semi-adiabatic and adiabatic calorimetry are important and should be noted. Based on these results, it is concluded that the use of blast furnace slag should be carefully considered if used for mass concrete applications.

Highlights

Controlling the temperature rise in concrete structures employing large volumes of concrete is critical to prevent thermal cracking caused by large temperature differentials between the concrete surface and its interior even without external structural restraint
It is not certain if this is a result of using a different measurement system, or it is associated with pre-treatment of SL for activation, or for other reasons
Manufacturers typically apply various pre-treatment methods to increase the reactivity of SL, it is clear that such SL should be used with caution in mass concrete applications, since the core regions of the mass concrete may be similar to true adiabatic conditions

Summary

Introduction

Controlling the temperature rise in concrete structures employing large volumes of concrete (mass concrete) is critical to prevent thermal cracking caused by large temperature differentials between the concrete surface and its interior even without external structural restraint. There are several ways to control the heat of hydration of concrete These include cooling the mixing water, cooling the aggregate, using ASTM Type II moderate heat cement or Type IV low-heat cement, keeping cement contents to a minimum level, using supplementary cementitious materials (SCMs), and choosing proper curing methods [5]. SCMs that have been used for such purposes include blast furnace slag (SL), fly ash (FA), silica fume (SF), and metakaolin (MK) [8,9,10,11,12] Materials such as SF and MK reduce the overall temperature rise of the concrete depending on temperature conditions, but the effect is lesser due to their higher reactivity and lower replacement levels [13]

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