Concrete products are a complex mixture of chemicals, fines, and heavy particles. It's always challenging to control the stability, flow and strength of it. Many admixtures have been created to overcome these challenges, often containing synthetically derived performance additives.
Microfibrillated cellulose (MFC) and nanocellulose has generated interest within this field, but why can it be a useful solution to admixture and concrete producers?
Stabilizers in concrete
It is important to control the rheology of fluid concrete. Bleeding and particle segregation is a significant problem with fluid concrete. Mix design is crucial in obtaining self-consolidating concrete (SCC) that is fluid enough to completely fill the formwork. The concrete has to be stable and robust against segregation of aggregate and bleeding. This can be obtained in at least two ways: By adding fine particles and/or filler, and/or by adding stabilizers. Viscosity modifying agents (VMA) like cellulose ethers, natural gums (xanthan, wellan) and starch are often used in concrete as stabilizers. They mainly act by increasing the paste viscosity to reduce risks of bleeding and segregation. The mechanism of action is different for MFC.
Increasing yield stress with minimal effect on plastic viscosity
MFC is an efficient tool to enhance the yield stress of concrete mixtures. Yield stress is the minimum stress for flow to occur and is important for many products and applications around us. Roussel (2005) describes this as the shear stress meeting the yield stress, thus initiating the ability of the cementitious material to start flowing.
Through our own tests at Borregaard, we have studied the effect MFC has on slump value and yield value (in self-consolidating concrete, SCC). We found that with quite a low dosage of MFC the slump decreased significantly. The result and conclusion from these tests were that by changing the dosage of the MFC, in relation to the weight of the cement, one can fairly effectively adjust the yield value of the concrete.
Figure: Yield value as a function of MFC (Exilva, Borregaard) dosage by weight of cement (sbwc = solid by weight concrete).
This is beneficial to many applications in the concrete field. For instance, increased yield stress
- results in a lower pressure exerted on the formwork by the concrete when placed (highly fluid mixes / SCC)
- is beneficial for underwater concreting (to prevent wash out)
- is an advantage for sprayed concrete/mortar/renders (to reduce rebound, improve “adhesion” to substrate, improve stability…)
- helps concrete placing by extrusion (smooth surface finish and good concrete cohesion)
MFC also provides a very interesting non-soluble network, that may help to avoid settling in concrete. Its ability to provide a physical support to the concrete matrix may, in theory, increase the opportunity to enhance the stability of the concrete at rest.
The following rheograph shows a typical effect of different materials on the rheology of concrete [1, 2]. It visualizes how different the various families of stabilizing agents are in terms of mode of action (VMA refers to viscosity modifying agent).
The following figure shows that a very small addition of MFC allows to stabilize a concrete mix. An addition of 0.015% sbwc of MFC indeed prevents segregation and bleeding without impacting the “time to flow”. “Time to flow” can, to some extent, be related to viscosity.
Self-compacting concrete mixes have to be stable and easily flowing: dense and complex architectures of steel reinforcement are usually used in concrete structures, hence it is essential that the concrete is able to flow through it, in every place. Low viscosity is therefore key in these respects.
It is also worth to underline that MFC doesn’t impact set times nor strength development at these low dosage levels. It has no detrimental effect on workability retention, contrary to silica fumes for instance, which are also frequently used to stabilize highly fluid mixes.
MFC, a 100% natural product
MFC is 100% natural, renewable and sustainable. It reduces the CO2 footprint by limiting the use of cementitious or non-cementitious additions in concrete mix designs whilst keeping good rheology and stability.
It is for sure a sustainable and green solution for producers of concrete admixtures, as well as for precast and ready mix concrete producers.
1. O. Wallevik, Course in Rheology of particle suspension, University of Reykjavik, Iceland, November 2011.
2. O. Wallevik, J. E. Wallevik, Rheology as a tool in concrete science: The use of rheographs and workability boxes, Cement and Concrete Research 41 (2011), p. 1279-1288.