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Chapter 6. Concrete and mortar microstructure

This chapter covers the microstructure of mortar and concrete. It first concentrates on the interfacial zone microstructure around a single aggregate, at the cement paste level. The reasons for interfacial zone microstructure are explored in 2-D and 3-D, without and with mineral admixtures like fly ash and silica fume. Both modelling and experimental results are considered. Then the microstructure of many interfacial zones in a mortar or concrete is studied, with particular reference to the percolation properties of this phase. This basic picture of concrete is also used to study the protected paste concept and the use of light-weight aggregate.

This section studies a simple 2-D model of the interfacial zone around a single square aggregate, and introduces the concepts of the wall effect and the one-sided growth effect.

(1) Digital simulation of the aggregate-cement paste interfacial zone in concrete (13 pages of text, 49K of figures)

This section discusses similar kinds of modelling to the first section but now in 3-D, and incorporating mineral admixtures like fly ash and silica fume. The size and the chemical reactivity with calcium hydroxide are both studied.

(2) Simulation studies of the effects of mineral admixtures on the cement paste-aggregate interfacial zone (18 pages of text, 3.3M of figures)

This section discusses different ways of modifying the interfacial transition zone in concrete, and uses microstructure modelling to investigate the efficacy of each method. Methods include: mineral admixtures, absorptive lightweight aggregate, and cement clinker aggregate, both absorptive and non-absorptive.

(3) Computer modelling of the interfacial zone in concrete (10 pages of text, 161K of figures

This section describes direct comparisons, using the scanning electron microscope, between model predictions and reality for the interfacial transition zone microstructure.

(4) Experimental and simulation studies of the interfacial zone in concrete (11 pages of text, 290K of figures).

This section describes how the many interfacial transition zones in a real concrete or mortar can be described using a hard core/soft shell percolation model. Mercury intrusion porosimetry is described and explained using this model.

This image is a sample mortar microstructure, created using the model described in this section.


(5) Percolation and porosity in mortars and concrete (11 pages of text, 30K of figures)

This section discusses the effect of aggregate shape, considered to be ellipsoidal, on the interfacial transition zone threshold.

(6) Interfacial zone percolation in concrete: Effects of interfacial transition zone thickness and aggregate shape (6 pages of text, 34K of figures)

This section studies several current air-void spacing equations, and quantitatively compares them using simulated air-void systems. This is the first time this has been done, and shows clearly which spacing equations have some validity and usefulness, and which do not. Before, only qualitiative comparisons were possible.

(7) A numerical test of air-void spacing equations (31 pages of text, 46K of figures)

This section is consists of a manual for the hard core/soft shell models with which concrete has been studied. The model is very general, and has been adapted to study many other material systems (see references in manual). Programs to implement the model, in C, can be downloaded and freely used.

(8) A hard core/soft shell microstructural model for studying percolation and transport in three-dimensional composite media (51 pages of text, 46K of figures)

Go to 7. Transport, elastic, and thermal properties of mortar and concrete

Go back to 5. Cement paste percolation, transport, and elastic properties

References

(1) E.J. Garboczi and D.P. Bentz, Journal of Materials Research 6, 196 (1991).
(2) D.P. Bentz and E.J. Garboczi, ACI Materials Journal 88, 518-529 (1991).
(3) D.P. Bentz, E.J. Garboczi, and P.A. Stutzman, in Interfaces in Cementitious Composites, edited by J.C. Maso (E. and F.N. Spon, London, 1993), pp. 259-268.
(4) D.P. Bentz, P.A. Stutzman, and E.J. Garboczi, Cement and Concrete Research 22, 891-902 (1992).
(5) D.N. Winslow, M.D. Cohen, D.P. Bentz, K.A. Snyder, and E.J. Garboczi, Cement and Concrete Research 24, 25-37 (1994).
(6) D.P. Bentz, J.T.G. Hwang, C. Hagwood, E.J. Garboczi, K.A. Snyder, N. Buenfeld, and K.L. Scrivener, in Microstructure of Cement-Based Systems/Bonding and Interfaces in Cementitious Materials, edited by S. Diamond et al. (Materials Research Society Vol. 370, Pittsburgh, 1995), pp. 437-442.
(7) K.A. Snyder, J. Adv. Cem.-Based Mater. 8, 28-44 (1998).
(8) D.P. Bentz, E.J. Garboczi, and K.A. Snyder, NIST Internal Report 6265 (1999).