Tag Archives: Cementitious materials

The Importance of Cement and its Future

Concrete is the most used material in the world because of its cheap price and large quantity. It is needless to say concrete is made of cement. However, cement is also a criticized material by the public, for its intensive CO2 emission from production process. The production of cement accounts for about 5-8% of the non-natural CO2 worldwide.

Importance of Cement

Importance of Cement

Importance of Cement

People may ask, can we find an alternative construction material instead of cement? such as wood, steel. The annual usage of wood is much above the replanting level, and it is not possible to get wood in some places such as Africa and China. As for steel, it is costly and again the production of steel also gives CO2 emission. Furthermore, the production of cement is not that energy intensive compared with other materials.

Importance of Cement

Actually, it is not possible to find an alternative construction material. 98% of the earth’s crust is made of eight elements: O, Si, Al, Fe, Ca, Na, K, Mg, which are the main elements of cement as well. From the long term run, the world will still need a large amount of cement. The demand of cement mainly comes from developing countries, such as India and China because of the urbanization and its population growth.

Importance of Cement

Importance of Cement

Importance of Cement

Since we have to use cement in the future, how can we do to reduce the CO2 emission and what is the future of cement? Efforts have been doing ever since decades ago in the cement industry and research community.

On the one hand, cement plants nowadays burn (hazardous) waste, say, car tyres, as fuel to make cement. The wood floating on the water of Three Gorges Dam is even collected to make fuel for cement plants. On the other hand, the production process of cement has always been optimizing. Some cement plants can even reach 80% efficiency, which is a great achievement even from a theoretical standpoint.

Importance of Cement

Importance of Cement

Researchers of cement try to use less cement clinkers. One viable method is Supplementary Cementitious Materials (SCMs). SCMs are commonly industry by-products or raw materials, such as Slag, limestone, Fly ash, silica fume, natural pozzolan. Whether binary blended or even ternary blended, SCMs can replace part of cement without sacrificing equivalent engineering properties.

Importance of Cement

Considering the limited quantity and access of SCMs, SCMs are not a permanent sustainable way from a long term run. Some researchers are trying to utilize the unlimited amount of calcined clay, for the kaolinite content of clay has similar cementitious property. Work done replacing 30% of cement by calcined clay in Cuba shows the potential future of clay if well used.

Importance of Cement

Generally speaking, cement is a significantly important construction material, and it is not necessary to worry that much as for the CO2 emission in the future.

Buoyancy Effect of TGA Experiment

What is Buoyancy Effect of TGA experiment?

TGA (thermogravimetric analysis) is an important method to analyze the hydration products of cementitious materials. When a TGA test for cementitious materials sample is performed, the initial TGA curve may appears as such:

A typical TGA test result. Portland cement past, w/c=0.4, cured 1 day.
A typical TGA test result shows the buoyancy effect. Portland cement past, w/c=0.4, cured 1 day.

As can be seen from the curve, the mass exceeded the original mass, which is not reasonable, since the decomposition of the sample always decrease the mass of testing sample. So why does this happen? This is because of the buoyancy effect in the TGA equipment.

Every substance found in a gas atmosphere is subjected to a buoyant force. This results in “apparent” mass changes. The degree of buoyancy, and thus the degree of mass change as well, are basically dependent on the volume of the substance and the density of the prevailing gas.

However, for thermogravimetric investigations, it is not the absolute value of the buoyancy at a certain temperature that is decisive, but rather its change as a function of temperature. A typical buoyancy curve is shown as follows.

TGA test
A blank TGA test.

It was recorded using a NETZSCH STA 409 with a DSC sample carrier in a static N2 atmosphere. There is no sample in the crucible, but the recorded result shows the mass is always over ZERO.

Due to the volume-dependence of the buoyancy, it is important to specify the sample carrier type in addition to the gas atmosphere, since different sample carrier/crucible combinations give rise to different “apparent” mass changes.

How to remove the Buoyancy Effect?

Knowing this, we can remove the buoyancy effect by running a blank test. Repeat the test with the same crucible but without anything in it or with inert material, then the buoyancy effect is recorded. Subtracting the buoyancy result from the first measurement result, the true mass change of the test is obtained.

A typical TGA test result after the subtracted the blank test, removing the buoyancy effect. Portland cement past, w/c=0.4, cured 1 day.
A typical TGA test result after the subtracted the blank test, removing the buoyancy effect. Portland cement past, w/c=0.4, cured 1 day.

Why does the buoyancy curve initially show such a drastic rise?

You may find the buoyancy curve initially shows a drastic rise, why is that? This phenomenon arises from the design of TGA device.

At low temperatures, heat transfer from the furnace to the sample thermocouple is exclusively through convection. Since a certain amount of time is required before the crucible material is evenly heated and the warm air preferably flows along the inside of the protective tube, and thus does not reach the crucible immediately, the actual temperature of the gas atmosphere is far higher, particularly at the beginning of a measurement, than the temperature sensor indicates. Therefore, the gas density changes dramatically in the beginning of the test, thus subjects dramatic “apparent mass” change.

A schematic figure showing the temperature difference
A schematic figure showing the temperature difference.

What is density? how to distinguish different density definitions?

As material researchers, we know density is the physical property of materials. Even since grade school, we are taught that density is simply the mass of an object divided by its volume.

However, this is pretty complicated in the case of cementitious materials. The mass of a certain amount of harden cement (or concrete) paste is a finite value, but how about the volume? since the harden cement paste is porous, how do we consider the open and close pores inside the paste as we want to determine the volume?

I met the problem when I performed Mercury Intrusion Porosimetry (MIP) test, especially when I read the output result from the MIP machine, there are bulk density, skeletal density, envelope density and apparent density.

It is not difficult to find these definitions from either textbooks or internet. First I list them below:

Apparent particle density: The mass of a particle divided by its apparent (particle) volume (BSI).

Bulk density: (also called Bulk powder density): The apparent powder density under defined conditions.

The mass of the particles divided by the volume they occupy that includes the space between the particles (ASTM D5004).

The ratio of the mass of a collection of discrete pieces of solid material to the sum of the volumes of: the solids in each piece, the voids within the pieces, and the voids among the pieces of the particular collection (ASTM D3766).

Envelope density: The ratio of the mass of a particle to the sum of the volumes of: the solid in each piece and the voids within each piece, that is, within close-fitting imaginary envelopes completely surrounding each piece (ASTM D3766). The ratio of the mass of a particle to the envelope volume of the particle (implied by BSI).

Skeletal density: The ratio of the mass of discrete pieces of solid material to the sum of the volumes of: the solid material in the pieces and closed (or blind) pores within the pieces (ASTM D3766).

True density (also called True particle density): The mass of a particle divided by its volume, excluding open pores and closed pores (BSI).

It is a headache to me to well understand them and distinguish these different density definitions. I guess it is still not clear for people to understand these densities either. Fortunately, I found a schematic picture (below) well illustrate the physical meaning of these density definitions, which you may no longer misunderstand them any more.

density

Is it now clear?