Aggregation and cementation

In addition to chemical and mineralogical alterations, weathering and soil formation also induce physical changes in the rock. A more or less dense, non-porous, massive rock turns into a porous material containing air and water and thereby becomes suit- 

In contrast, in cementation which is usually much more stable than aggregation, solid-solid contact through chemical bonding may be involved this is not responsive to classical dispersion treatments and requires destruction of the cement. It is accepted that there is no clear separation between the two mechanisms and combinations will certainly exist. 

Microscope observations and chemical analyses help locate the Fe oxides in the fabric of matrix soil particles. Single Fe oxide-containing aggregates which appear uniform to the naked eye, may vary appreciably in Fe content and mineralogy (Ford-ham Norrish, 1979). SEM and TEM photos show goethite and hematite crystals in strongly developed soils to be associated in a more or less systematic fashion with flakes of kaolinite, usually the main matrix mineral in such soils (Fig. 16.19 a-c) 

Selective removal of the Fe oxides usually, but not always (Kretzschmar et al., 1993), leads to destruction of the aggregates. Other chemicals which removed only little Fe have also led to substantial dispersion of such aggregates (Cambier Picot, 

50 nm (courtesy R.C. Jones) Jones et al., 1982 with permission) e) Small hematite crystals associated with large, tabular gibbsite from a bauxitic saprolite on basalt (SEM), Jos Plateau, Nigeria (see Zeese et al., 1994 courtesy G.F. Tetz). 

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Here, V and 1/ are the volume fractions of aggregate and cement paste, and E and E are their moduli. As Fig. 20.6 shows, experimental data for typical concretes fit this equation well. 

Aligizaki, K. du Rooij, M.R. Macdonald, D.D. (2000) Analysis of iron oxides accumulating at the interface between aggregates and cement paste. Cement Concrete Res. 30 1941-1948... 

Have ability to form continuous films in mortar or concrete, due to a lower minimum fihn-forming temperature than the application temperature, which adhere well to the aggregates and cement hydrates and possess good alkali and water resistance. 

Hsu, T.T.C. and Slate, F.O. (1963) Tensile Bond Strength between Aggregate and Cement Paste or Mortar. ACI Journal, Proceedings 60,4, 465 -85. 

The space occupied by the concrete should, as much as possible, be filled with solid aggregate and cement gel free from honeycombing. Compactness may be the primary criteria for those types of concrete which intercept nuclear radiation. 

The composition of a concrete specimen can be essentially defined by the water/cement ratio, aggregate and cement types, and aggregate and cement contents. Therefore, like shrinkage, an increase in the water/cement ratio and in the cement content increase creep. Also as in shrinkage, the aggregate induces a restraining effect such that an increase in aggregate content reduces creep. 

Fresh concrete must be mixed, transported, placed in the mould and compacted without compromising the homogeneity of the mix. Voids that remain in the mix after compaction, due both to entrapped air and to separation of coarse and fine aggregate and cement paste (segregation), are deleterious for strength and durabiHty. 

A difference in adhesion between the aggregates and cement paste of unmodified and modified mortars is difficult to detect. However, the cohesion of the bulk cement paste is improved by polymer modification. Irrespective of the crack initiation mechanism, a much higher amount of microcracks is detected in the bulk paste of unmodified mortar. Fig. 7. 

Concrete is a composite typically composed of aggregate and cement paste. It is the connection between these phases - the interfacial transition zone (ITZ) that is most important.. The ITZ is the weakest link in the composite system with lowest mechanical properties of the three [1-4], The ITZ can affect the overall elastic module and the stress distributions in a concrete material. The ITZ is comparatively more porous than that of bulk cement paste, and often less well bonded to the aggregate [3]. This region can have a low formation of calcium-silicate-hydrate (C-S-H gel), a product of Portland cement hydration responsible for the good mechanical properties and durability [5]. 

Density tests. Referring to Table 2 below, the densities of all the trial mixes ranged from just above 1000 kg/m to 1073 kg/m. The addition of polymers into the trial mixes had a negligible effect on the densities of the concrete. The control mix, CL had a density of 1034 kg/m which is very close to the densities of the other trial mixes thus the density of the concrete is affected more by the lightweight aggregate and cement binder content rather than by the quantity or type of polymer addition. 

Concrete is a mixture of gravel (called aggregate) and cement. The spectacular 142-foot internal diameter dome of the Pantheon in Rome is made of concrete. What material did the Romans use for aggregate in the construction of the Pantheon Conld the material they used be classified as a ceramic ... 

Real joints do not of course consist of simple, separate, elastic materials with a clear mathematical geometry. Metal adherend surfaces are micro-rough, possessing oxide layers, while concrete surfaces are macro-rough comprising aggregates and cement paste, and both surfaces readily adsorb air-bome contamination. The thickness and modulus of primer layers, if employed, is often unknown, and the thickness and properties of the adhesive layer are difficult to regulate and to determine. 

Aquino, M. ]., Li, Z., Shah, S. P. (1995) Mechanical properties of the aggregate and cement interface . Advanced Cement Based Materials, 2 211-23. 

Lyubimova, T. Yu., Pinus, E. R. (1962) Crystallization and structuration in the contact zone between aggregate and cement in concrete (in Russian), Colloidnyi Zhurnal, 24(5) 578-87. 

Wu Xueqan, Li Dongxu, Wu Xiun, Tang Minshu (1988) Modification of the interfacial zone between aggregate and cement paste , in Proc. Symp. Bonding in Cementitious Composites, Boston, 2-4 December 1987, Pittsburgh, Pennsylvania Materials Research Society, pp. 35 0. 

Taylor, M.A. and Broms, B.B. Shear bond strength between coarse aggregate and cement paste or mortar. Journal of American Concrete Institute, 1964, 61, No. 8 (Aug.) 939-956. 

Now, there are three common methods used for improving the interfacial zone between aggregate and cement paste using silica fume as additive, decreasing W/C ratio of bulk paste and the Sand Enveloped with cement (S.E.C) technology. The addition of silica fume into concrete will decrease the pH value of the solution in pores of hydrated cement paste (hep), so as to increase the... 

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