The microstructure of the interfacial transition zone between
steel and cement paste
*, Hu Shuguang
State Key Laboratory of Concrete Materials Research, Tongji University, Shanghai 200092, People's Republic of China
Wuhan University of Technology, Wuhan 430070, People's Republic of China
Received 3 March 2000; accepted 11 December 2000
The microstructures of steel/ordinary paste and steel/expansive paste interfaces were investigated. The Ca(OH)
orientation effect, the
crystal sizes of Ca(OH)
and AFt, the morphologies and the relative percentages of the Si, Al, S and Ca elements of interfaces were specially
studied. The results show that, under confined conditions, the microstructure of the steel/expansive paste interface is better than that of the
steel/ordinary paste interface. D 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Interfacial transition zone; Microstructure; Confined condition; Expansion; Cement paste
Experimental research [1±3] indicates that the micro-
structure of the transition zone between aggregate and
cement paste is important for the strength and durability
of concrete. Likewise, the microstructure of the transition
zone between steel and cement paste also obviously influ-
ences the properties of steel/concrete composites (such as
concrete-filled steel tube). At present, studies of the steel/
paste interface mostly focus on ordinary cement paste. The
results generally show that the transition zone between steel
and paste exists, and its microstructure is weak, and
crystals tend to be present in the transition zone
[4±7]. However, up to now, few studies on steel/expansive
paste have been done.
2.1. Raw materials
Number 525 ordinary Portland cement and expansive
agent UEA made of raw alum earth and gypsum were used.
Their chemical compositions are shown in Table 1.
As shown in Fig. 1, the pure cement paste and the
expansive paste (12 wt.% UEA was substituted for cement)
were cast in small steel tubes Y 26 Â 3 Â 20 mm (diame-
ter Â wall thickness Â height). The water to binder ratios of
both kinds of paste are 0.28. A stainless-steel disc was
inserted in the paste. The specimens were cured within the
small steel tube until the testing ages in standard condition
(20°C, RH 95 5%). During the curing period, a steel block
was put on top of the steel tube to ensure curing under
confined conditions. The following method of preparing the
steel/paste interface was employed: The hardening paste
was jacked out of the steel tube and broke off at the position
of the steel disc, thus, the interface was obtained. The matrix
surface was obtained by polishing the sample interface. The
samples to be tested were kept in ethyl alcohol to avoid
carbonization and rehydration.
* Corresponding author. Tel.: +86-21-6598-3463; fax: +86-21-6598-
E-mail address: firstname.lastname@example.org (L. Yue).
Chemical compositions of raw materials (wt.%)
CaO MgO SO
Cement 21.47 5.80 4.04 59.64 3.24 2.08 2.44
UEA 29.50 7.76 2.82 17.93 1.83 24.79 20.67
Cement and Concrete Research 31 (2001) 385 ± 388
0008-8846/01/$ ± see front matter D 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0008-8846(01)00452-5