The right protocol to measure yield stress of cement paste

The rheology of cementitious material paste is complex and difficult to be investigated. One value of rheology is yield stress. The best method to measure the yield stress in laboratory is using the vane method. However, there is no much literature reporting an practical and detailed testing protocol to measure the yield stress of cement paste. Here I briefly introduce the protocol I am using at our laboratory.

To obtain stable and reliable results, we should choose the same container and the same mixing procedure. We normally use a plastic beaker with a volume 500 ml, for the beaker has enough diameter and depth for the test without the influence of boundary effect, i.e. the diameter is two times larger than the diameter of the vane. For each test, 270 g of cement is mixed with relevant water at the required water to cement ratio. For the mixing, a high-shear mixer is applied with a shear rate 800 RPM for 2 minutes.

As soon as the well mixed paste is ready, let the blades of the vane of rheometer immerse in the paste. Make sure the top surface of the paste staying between the two lines labeled on the shaft.

The principle to measure the yield stress is applying a very low fixed shear rate on the paste, e.g 0.02-0.001 1/s, then the shear stress linearly increases till yield stress. As shown in the recorded data (Fig 1), the shear stress drops remarkably when the paste is yield, thus the maximum stress is the yield stress, i.e. dynamic yield stress.

Fig. 1. Measuring device: HAAKE? VT550 (ViscoTester VT550), CR (Controlled shear Rate): 0.01000 1/s; t 120.00 s; T 20.00 °C.

If the maximum stress does not occur within the testing period, longer time or higher shear rate may be tried. It is also worth to note that this method gives good result for paste with yield stress higher than 10 Pa.

How to define particle size?

Particle size is a very important property in the research field of cementitious materials, e.g. modeling, reaction kinetics.

Particles are 3-dimensional objects, and unless they are perfect spheres, e.g. emulsions or bubbles, they cannot be fully described by a single dimension such as a radius or diameter.

In order to simplify the measurement process, it is often convenient to define the particle size using the concept of equivalent spheres. In this case the particle size is defined by the diameter of an equivalent sphere having the same property as the actual particle such as volume or mass for example.

It is important to realize that different measurement techniques use different equivalent sphere models and therefore will not necessarily give exactly the same result for the particle diameter.

Below is a schematic figure illustrating the definition of particle size based on different equivalent spheres.

Definition of particle size

What is Geopolymer cement?

I have been confused by the term “Geopolymer cement (concrete)” for long time. Some researchers told me Geopolymer cement is essentially alkali activated cement/material (AAMs), such as alkali activated slag and fly ash. Since alkali activated slag is indeed a kind of geopolymer cement, it looks logical to consider Geopolymer cement as alkali activated cement.

Is it correct? Recently, I read the State of the Art report on alkali activated cement (Alkali Activated Materials State-of-the-Art Report, RILEM TC 224-AAM), finding the description:

Low-calcium alkali-activated systems, predominantly dealing with alkali aluminosilicates and including those materials which are now widely known as ‘geopolymers’.

It is clear that not all alkali activated cements are geopolymers. Just those with low calcium materials activated by alkali in cement industry are regarded as geopolymer cement, in other words, geopolymers are shown here as a subset of AAMs, with the highest Al and lowest Ca concentrations.

BTW, Davidovits is the first researcher applied the name ‘geopolymer’ to these low calcium materials activated by alkali. He was working in France, patented numerous aluminosilicate-based formulations for niche applications from the early 1980s onwards.