Cementos

Cement and Concrete Research 42 (2012) 333–343

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Cement and Concrete Research
journal homepage: http://ees.elsevier.com/CEMCON/default.asp

Chemical and mechanical stability of sodium sulfate activated slag after exposure to
elevated temperature
A.M. Rashad a,⁎, Y. Bai b,⁎, P.A.M. Basheer b, N.C. Collier c, N.B. Milestone c
a
b
cHousing & Building National Research Center, HBRC, 87 El-Tahrir St., Dokki, Giza 11511, P.O. Box: 1770, Cairo, Egypt
School of Planning, Architecture & Civil Engineering, Queen's University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, UK
Immobilisation Science Laboratory, Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UKarticle

info

Article history:
Received 11 May 2011
Accepted 16 October 2011
Keywords:
Activated slag (D)
Elevated temperature (A)
pH value (A)
Residual compressive strength (C)
Sodium sulfate (D)

abstract
The chemical and mechanical stability of slag activated with two different concentrations of sodium sulfate
(Na2SO4) after exposure to elevated temperatures ranging from 200 to800 °C with an increment of 200 °C
has been examined. Compressive strengths and pH of the hardened pastes before and after the exposure
were determined. The various decomposition phases formed were identified using X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. The results indicated that Na2SO4 activated slag has a
better resistance to the degradation caused byexposure to elevated temperature up to 600 °C than Portland
cement system as its relative strengths are superior. The finer slag and higher Na2SO4 concentration gave better temperature resistance. Whilst the pH of the hardened pastes decreased with an increase in temperature,
it still maintained a sufficiently high pH for the protection of reinforcing bar against corrosion.
© 2011 Elsevier Ltd. Allrights reserved.

1. Introduction
The resistance of concrete made with Portland cement (PC) to
degradation caused by exposure to elevated temperatures depends
on the type of material used in the concrete. The least stable constituent in this respect is the hardened cement paste. Both chemical and
physical deteriorations occur at elevated temperatures because
both interlayer and chemicallybound water are lost due to the decomposition of calcium hydroxide (CH) and calcium silicate hydrate
(C-S-H) [1]. The porosity and mineralogy of the aggregate also affect
the severity of deteriorations [2].
It has been shown that the critical exposure temperature at which
concrete begins to lose compressive strength is approximately 400 °C.
This is caused by the decomposition of CH and theconsequent volume increase that occurs during cooling when rehydration of calcium
oxide initiates a volume increase by about 44%, leading to cracking
[3]. Handoo et al. [4] reported that the loss of strength in PC concretes
after 400 °C could be attributed to the loss of crystalline water, resulting in the decomposition of the calcium hydroxide. Georgali and
Tsakiridis [5] attributed the crackingand softening of the concrete
surface exposed to fire to the expansion followed by the shrinkage
of PC paste due to the transformation of calcium hydroxide to calcium
oxide in the temperature range of 450–500 °C. Petzold and Rohrs [6]
explained that the loss of strength can be attributed to the rehydration of calcium oxide, which is accompanied by a 44% increase

⁎ Corresponding authors.E-mail addresses: alaarashad@yahoo.com (A.M. Rashad), y.bai@qub.ac.uk (Y. Bai).
0008-8846/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cemconres.2011.10.007

in volume. In addition, the explosive spalling that can occur [7,8] at
temperatures between 480 and 510 °C [9], will reduce the loading
capacity of concrete structures. The different effects of exposure...