The main cause behind this is that dry particles of cement, during mixing, cannot be reached to comparatively large aggregate particles i.e. cannot be packed closely. Thus less cement particles are available to be hydrated and filled the actual voids present due to improper packing which yields a zone of higher porosity than that of bulk cement paste hydrated at relatively greater distance; thus high porosity arises in interface zone.
The higher the porosity, the lower will be the strength. Let take a look at microstructure of interface zone:
• Immediate around aggregate surface, there has an about 0.5 µm thick layer layer comprising of oriented crystalized calcium hydroxide.
• Behind that there has identical thickness of C-S-H layer which is known as duplex film.
• Next layer is 50 µm thick, this layer is the actual interface zone which contains hydration products of cement having lager crystals of calcium hydroxide; but no unhydrated cement particles are found there.
The significance of furthermost layer is two folds; these are
- Absence of unhydrated cement means complete hydration of the same which indicates that water/cement ratio at the interface zone is higher than other portion.
- Larger crystals of calcium hydroxides will produce definitely higher porosity in this zone; thus wall effect assumption is confirmed here.
The fine aggregate also surrounded by interface zone as that of coarse aggregate. The interface zone around coarse aggregate is our primary interest, as thickness of such zone around fine aggregate is comparatively smaller.But obviously this surface effect generated by fine particles together with that of coarse aggregate enhance the overall effect of interface zone.
Now we will discuss about some special cases; like inclusion of pozzolana or silica fume, chemically reactive fine and coarse aggregate and lightweight aggregate.
Silica fume and pozzolana can be included in concrete to strengthen interface zone. When pozzolana is added to concrete, the strength of interface zone is increased due to secondary reaction of existing calcium hydroxide with pozzolana. But it takes time; thus gradually porosity is reduced. Silica fume is very much fine and especially effective as it is much finer relative to cement particles.
Microstructure of interface zone or transition zone is affected by mineralogical characteristics of fine aggregate too, especially when they are chemically reactive. Consider limestone is used in concrete, chemical reaction between cement paste and limestone yields a denser interface zone.
Now take a look at lightweight aggregate concrete; lightweight aggregate may be of two types: one type has denser outer layer and other have outer layer of more porosity or identical porosity as that of bulk. When outer layer of aggregate is denser, the condition is similar to the interface zone of normal-weight aggregate.
If outer layer of lightweight aggregate is more porous, mobile ions are attached to porous surface which leads to formation of denser interface zone. Thus mechanical interlocking between hydrated cement paste and aggregate particles is improved.
The study of interface zone is not as simple as discussed above. Therefore, experiments about interface between cement paste and individual rock particles should be resort to. But in concrete mass there have many neighbor aggregates surrounding a particular aggregate of our interest. Thus tests results may mislead as they neither encounter interference of neither neighbor coarse aggregate particles nor even fine aggregate, whose effects are usually ignored.
Again we should consider the samples we treated; laboratory-produced artifact having single particles embedded by cement paste don’t subjected to mixing process where shearing action provided during mixing influences formation or orientation of microstructure of cement paste during settling.
In practical case, moreover, water accumulated below coarse aggregate particles, due to bleeding process and voids filled with water are appeared. In this defective interface usually massive crystals of calcium hydroxides are found. In general, stress concentrations at interface between coarse aggregate and cement paste are observed as it is the junction of two elements of different poisson’s ratios and modulus of elasticities.