Effect of microfines associated with aggregates on concrete performance and microstructure

Microfine materials (particles diameter < 75 mum) are normally found as a part of aggregates used in concrete production. Our research has confirmed that their effect on concrete performance not only depends on their concentration but also on their mineralogy. The water absorption capacity is the principle property responsible for their high impact on the performance of concrete. Accordingly, microfine minerals having a low water absorption capacity such as dust, carbonates, and certain clay minerals are consider innocuous, while other clay minerals, in particular 2:1 phyllosilicates having a high water absorption capacity, can be considered as deleterious. This investigation contributes to a broadening of our current understanding concerning the mechanisms through which microfines impact concrete performance.The introduction of 2:1 phyllosilicates microfines into concrete as coarse aggregate coatings rendered poor workability, low strengths, and high drying shrinkages. The scope of these deleterious mechanisms is highly dependent not only on the swelling characteristics of the clay minerals as discussed above, but also on how they are introduced into concrete, such as coating or disperse in the cement paste (additive).In this thesis, we have studied concrete containing microfines from dolomitic limestone and igneous aggregates, respectively, under ASTM C33 thresholds, cured under both wet and dry conditions. The results showed that under dry curing conditions, microfines associated with dolomitic limestone aggregates become deleterious. These microfines aggravate problems in concrete due to insufficient moist curing. However, the presence of moderate quantities of phyllosilicates in the microfines contributed to ameliorating these damaging effects.The chemical reactivity and the pozzolanic properties of microfines along with their impact on the microstructure of concrete was also a subject of study. The dolomitic limestone particles transformed into a protodolomite (dolomite poor in magnesium) or dissolved under the pore water conditions present in concrete. Magnesium ions leached from the dolomite particles forming calcium magnesium silicates or brucite based on the concentration of dissolved species of silica. In the siliceous microfines one could notice the disappearance of feldspar and the dissolution of phyllosilicates under basic conditions. Studies on the properties of cement paste containing these siliceous microfines revealed a certain degree of pozzolanic character.Lastly, the particles of both types of microfines were still visible in the concrete after 28 days of aging and concentrated in a strip, 30 mum wide, adjacent to the surface of coarse aggregates. In addition, the concentration of these microfines is not uniform across this strip, but reaches a maximum at the fringe of this strip located between 10 and 30 mum from the surface of the aggregate.