What are the properties or phenomena that contribute to the compressive strength of the following mixtures: cement paste, standardized mortar, and concrete?
Most of the concrete technologist believe that strength is because of the hydration product C-S-H gel i.e. Calcium Silicate Hydrate which is having binding property to bind coarse and fine aggregate into a solid mass what we call Concrete. The strength is due to how strong this inter-facial or transition zone is?
Compressive behaviors of cement-based materials are sensitive to strain rate.
Article Experimental and modeling study of dynamic mechanical proper...
Due to the transition zone in Concrete, it is most helpful to view this complex mass as a three-phase composite structure:
A coherent mortar phase
Aggregate phase
The transition zone (TZ) which represents the interfacial region between the particles of coarse aggregate and the hydrated cement paste.
Characteristics of the TZ:
Large crystals of Ettringite and CH with the preferred orientation.
Porous Structure
A transition zone exists on a thin shell, typically
10-15 um thick around large aggregates.
It is generally weaker than either of the two
main components of concrete, and it therefore
imposes a far greater influence on the
mechanical behavior of concrete than is
reflected by its size. In freshly compacted concrete, water film form around large aggregate particles. This account for high w/c ratio that exists closer to
large aggregates than in the bulk mortar.
So, Concrete is brittle in tension but relatively tough in compression. (tensile strength) is almost 1/10th of (compressive strength). At a given w/c
ratio, mortar is stronger than the corresponding concrete. Cement paste and aggregate are elastic, concrete is not. The coefficient of permeability of mortar is much lower (1/100) than typical concrete of the same w/c.
Hydration and the formation of C-S-H product as noted above controls the strength of cements. However, the environmental temperature, workmanship to keep off air bubbles from the fresh paste, and curing are important factors that can greatly affect the strength of cement paste.
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