Written by Howard Stone
We have now seen many 3D architectural printing projects and one thing that many of them have in common is the cement based mix they print with.
There are many materials that can be used in 3D printing but Portland cement is an easily engineered common product available world wide.
So, just for a quick rewind, what is Cement and where did it come from and what is its future in the world of 3D printing.
Joseph Aspdin of Leeds England developed the first Cement mix in the early eighteen hundreds by burning powdered limestone and clay in his kitchen stove. He called it Portland Cement because it resembled the building stone from the nearby Isle of Portland.
Joseph found that limestone, which contained alumina and silica, when mixed with clay, baked and crushed, formed a powder so fine it could pass through a sieve that could hold water.
Today’s Portland Cement dust is so fine that 1 pound contains around 150 billion grains, and that according to the US Geological Survey, in 2020, globally, over 4 trillion metric tons of cement were produced.
That is 600,000,000,000,000,000,000,000 grains, more or less…
Although the terms cement and concrete are often used interchangeably, cement is actually an ingredient of concrete. Concrete is usually a mixture of cement, aggregates and water.
Aggregates are very important to the mix and although they are most commonly inert fillers such as sand and rock, the different properties of aggregates have a large impact on the strength, durability, workability, and economy of concrete; and it’s those different properties of that allow mix designers the flexibility to meet a wide range of construction requirements.
There are many reasons to use aggregates but perhaps the biggest reason is cost. The cost of cement is usually seven or eight times more than sand or stone and while cement is always necessary, the strength is determined by the use of aggregates which make up to 70-85% of the mass of concrete.
‘Aggregate’ refers to any particulate material. This includes gravel, crushed stone, sand, slag, recycled concrete and geosynthetic products and many others. Aggregates also help control thermal and elastic properties, dimensional stability and volume stability, manage shrinkage levels and help to prevent cracking.
Aggregates can be natural, manufactured or recycled materials.
General construction aggregates generally fall into two categories.
Fine aggregates consist of particles that can pass through a 3/8-inch, or about a 10mm sieve. Coarse aggregates are particles larger than 0.19 inch, but generally range between 3/8 and 1.5 inches or 40mm. in size.
Silica fume, for instance, also known as microsilica, is an ultrafine aggregate and consists of spherical particles with an average particle diameter of 150 nm. Zeolite, pumice, crushed quartz powder and fly ash can be made into such very small particles. By comparison, a sheet of paper is about 100,000 nanometers thick and red blood cells are about 10,000 nanometers across.
The Hard Facts…
What does M -20 M -15 M -20 mean with respect to concrete?
The ‘M’ describes the mix design while the strength digits represent compressive strength in N/mm2. ( 1 N/mm2 = 1 MPA = 1000 kPa )
The ratio of cement / sand / aggregate = strength.
M10 Grade of Concrete with a ratio of 1:3:6 = 10 MPa or 1450 psi
M15 Grade of Concrete with a ratio of 1:2:4 = 15 MPa or 2175 psi
M20 Grade of Concrete with a ratio of 1:1.5:3 = 20 MPa or 2900 psi
M5, M10, and M15 grades of concrete are generally used as bedding concrete below columns and as footings.
M20 through M35 grade concrete is generally used in the construction of slabs, beams, columns, footings, etc.
M40, and M45, are used for the construction of commercial structures, pre-stressed concrete members, runways, concrete roads, it is also used for pressured concrete girders, columns, and beams.
Superplasticisers are commonly used to improve the workability and to enhance the compaction of concrete for increased density and will improve the surface finish of most any concrete product. Also known as high range water reducers, plastersizers enable the production of concrete with approximately 15% – 30% less water content.
On a Lighter note…
Aerated concrete is made by introducing air into a cement slurry so that a cellular structure is formed. While aerated or foam concrete do not have the strength of regular concrete, they do have the beneficial properties of heat preservation, heat insulation, and sound absorption. Though they are called concrete, they are not a true concrete by definition.
Foam concrete as opposed to aerated concrete has a uniform closed-cell structure which allows for more predictable engineering outcomes.
Usually it is prepared by introducing foaming agents such as hydrogen peroxide and plant surfactants similar to those found in many common dishwashing detergents. Foamed concrete is free-flowing and can be placed without vibration or compaction. It is lightweight, has low thermal conductivity and good sound insulation properties which are not available in ordinary concrete. Depending on its density, up to 80% of the volume of a typical aerated/foamed block is air. The low density also makes for lower structural compression strength which is on average about half of the compressive strength of regular concrete or, 8N/mm2 / 8000 kpa / 1,200 psi.
Foam stabilizers help maintain density, thermal conductivity, flexural and compressive strength compared to a foamed mix without stabilizer due to an improvement of pore structure.
Foam stabilizer is generally made with calcium stearate and alum. Preparation is simple, performance is excellent and it is friendly to the environment.
Looking to the Future:
Contemporary architecture is moving on from the box and has embraced the curve which gives many modern structures their sleek and futuristic appearance. This is largely thanks to the integration of innovative 3D design and cutting-edge 3D production methods.
With more advanced technologies, it is possible to create more complex geometries without compromising quality and performance. 3D printing in construction shows great promise, being able to create practically any curved shape through computer controlled processes.
Curved forms are fluidic, which allow them to blend into natural landscapes. Some of the most striking designs in contemporary architecture that utilize 3D technologies to bring these modern designs to life. But even before there was computer-aided design, CNC manufacturing and 3D printing, the curved form could be found throughout architectural history. From the dawn of time to the information age, many cultures have developed curved architecture, because of the strength and flexibility it offered. In random fact, testing by SPS found loads as high as +300 lbs can be pressed upon standard, large eggs because their form evenly dissipates pressure.
Arches, Domes and Innovation
Early examples of the curved form include structures of historical significance like the Marcello theater built in 13 BC, the Pantheon and the Roman Colosseum. Developing the arch into a dome was perhaps the greatest achievement for the renaissance builders and the first major structure was designed by Filippo Brunelleschi for the Florence Cathedral. Using modern mathematics and physics, and with no scaffolding, he constructed a beautiful dome with four million bricks that still stand to this day.
3D printing has a large part to play in our future with the integration of unibody type construction in which walls and roof are manufactured as one. While roof on frame used to be the go-to construction of choice, unibody constructs are gaining ground and will no doubt be more prevalent in the future.
Alexey Ostanin СЕО of the Company Printed Dome https://printeddome.com which is one of the early producers of 3D printed structures has recently teamed up with Howard Stone of the 3DOHM to produce affordable cyclone resistant homes throughout the pacific. Foamed concrete will play a significant role in this mass 3D printing project because of the economy, existent engineering and the basic materials that make up foamed concrete which are commonly available worldwide. This largely humanitarian project promises to be significant for Pasifika Peoples of those island nations most strongly affected by climate change.
Open Source Printing Recipe – Cement 20% ~ 35%; Mineral admixture 18% ~ 35%; Fiber 0.08% ~ 2.8%; Fine aggregate 15% ~ 40%; Foaming agent 0.2% ~ 1.0%; Foam stabilizer 0.15% ~ 2.0%; Superplasticizer 0.05% ~ 0.15%; Temperature control shrinkage reducing agent 1.5% ~ 3.5%