Influence of Limestone Fillers on the Properties of Concrete
Can cement in concrete be reduced without having any adverse effects on the properties of concrete?
Concrete : The movement towards a phase of sustainability
Particle Packing Theory
The water content in a concrete mix is one of the governing factors for the compressive strength of the concrete. A low water to cement ratio tends to produce concrete mixes with high compressive strengths. However, if the water content is too low, the workability of the mix is compromised. Hence, to achieve the right trade-off between strength and workability, the particle packing theory may be used.
What does the particle packing theory say?
Water is added to a concrete mix to first fill the voids between the particles and the rest is used as excess water for flowability. When the packing density is high, the volume of voids is low and hence the amount of water available as excess water is increased, and a better flowability is achieved. While incorporating fillers such as limestone powder to compensate the amount of fines in a mix when the cement content is reduced, a better packing density may be achieved.
Considering a stable structure and a real mixture, it is found that the stable structure require less water to fill the voids as opposed to the real mixture. A stable structure is one in which where all the particles present are in contact with each other. In a stable structure, the packing density is higher and more excess water is made available. Consequently, the flowability or the workability of the mix is favoured.
The use of fillers finer than cement
Considering two mixes, Mix 1 and Mix 2. Mix 1 has limestone fillers (F) which has the same fineness as the cement particles (C). Mix 2, on the other hand has limestone fillers which are much finer than the cement particles. It can be observed from the diagrams on the right that Mix 2 achieved a better packing of particles as opposed to Mix 1, leading to more excess water in Mix 2.
Keeping the water content in both mixes constant, Mix 2 will have a higher flowability. If the flowability of Mix 2 is to be kept the same as Mix 1, by decreasing the amount of water in Mix 2, the cement content may be decreased while maintaining the same water to cement ratio. Hence, for the same workability, a higher strength can be achieved.
