BDCS Notes - Portland Cement

Portland Cement

Portland Cement Concrete: most widely used manufactured construction material in the world. Most important material in modern life. Consists of portland cement, aggregates, water, air voids, and admixtures.

There are many other different types of concrete, based on different cements. Portland cement concrete is so prevalent, that unless identified, it is assumed to be used primarily.

Portland cement is an instant glue (just add water) that bonds aggregates together to make portland cement concrete.

Production

Starts with two basic raw ingredients: a calcareous material and an argillaceous material.

Calcareous material: A calcium oxide, such as limestone, chalk, or oyster shells.

Argillaceous material: Combination of silica and alumina that can be obtained from clay, shale, and blast furnace slag.

These materials are crushed, and then stored in silos. The raw materials are passed through a grinding mill, using either a wet or dry process. The ground material is stored until it can be sent to the kiln.

Modern dry process cement plants use a heat recovery cycle to preheat the ground material, or feed stock, with the exhaust gas from the kiln.

Clinker: Raw materials melted at temperatures of 1400 C to 1650 C changing the raw materials into clinker. The clinker is cooled, stored, then ground into a fine powder. A small amount of gypsum is added to the clinker to regulate the setting time of the cement in the concrete.

Standard Sack: 94 lb, equal to 1 cubic ft. of loose cement when freshly packed.

Chemical Composition of Portland Cement:

1. Lime
2. Silica
3. Alumina
4. Iron Oxide

These raw materials interact in the kiln, forming complex chemical compounds. Calcination in the kiln restructures the molecular composition, producing four main compounds of:

1. Tricalcium Silicate
2. Dicalcium Silicate
3. Tricalcium Aluminate
4. Tetracalcium Aluminoferrite

Calcination: the conversion of metals into their oxides as a result of heating to a high temperature.

Fineness of Portland Cement

Fineness: Important property that must be carefully controlled. Because hydration starts at the surface of cement particles, the finer the cement particles, the larger the surface area and the faster the hydration.

Increases fineness beyond the requirements for a type of cement increases cost.

Maximum size of cement particles is 0.09 mm.

85%-95% of particles are smaller than 0.045 mm, and the average diameter is 0.01 mm.

1 kilogram of portland cement has approximately 7 trillion particles with a total surface area of about 300 to 400 sq. meters.

Fineness of cement is usually measured indirectly by measuring the surface area with the Blaine air permeability apparatus or the Wagner Turbidimeter apparatus.

Blaine Test: Surface area of the cement particles [in (sq. cm.)/g] is determined by measuring the air permeability of a cement sample and relating it to the air permeability of a standard material.

Wagner Turbidimeter: Determines the surface area by measuring the rate of sedimentation of cement suspended in kerosene.

The finer the cement particles, the slower the sedimentation.

Fineness also measured by determining the percent passing the 0.045 mm sieve.

Specific Gravity of Portland Cement

Specific Gravity: Needed for mixture proportioning calculations. Specific gravity of portland cement is 3.15.

Cement quantities are specified and measured by weight rather than volume.

LEED notes - fly ash

a couple of notes on fly ash concrete - 

how is fly ash made?

from the combustion of coal.  fly ash is the residue gathered from the chimneys of coal-fired power plants.  it is one of two kinds of coal ash.  the other is bottom ash, which is, you guessed it, found on the bottom of the coal furnaces.

what is fly ash made of?

many, many different compounds.  silicon dioxide, calcium oxide, and other toxic constituents (including arsenic, beryllium, mercury, thallium, et cetera).

how is it stored?

at coal power plants and in landfills.  approximately 43% of the fly ash is recycled, which is utilized in the production of Portland cement.  it is also used in the synthesis of geopolymers and zeolites.

how is fly ash classified?

it is broken down into two distinct grades - class F fly ash and class C fly ash.  the difference between the classes are the trace amounts of calcium, silica, alumina, and iron in the coal ash.  these four elements all highly influence the way coal is burned during the process of fly ash formation.  

what is class F fly ash?

class F fly ash burns anthracite and bituminous coal.  it is pozzolanic (cementitious with calcium hydroxide) in nature, containing less than 10% lime.  it requires the presence of a cementing agent, typically portland cement with water, in order to cure.

what is class C fly ash?

class C fly ash is produced from the burning of lignite or sub-bituminous coal.  it is also pozzolanic, but also has some self-cementing properties.  it also contains 20% lime content, and unlike class F, does not require an activator for the hardening process.

how is fly ash recycled?

not easily.  with increasing landfill costs and with national efforts to push sustainable design, too many u.s. coal-fired power plants are reporting minimal tonnages of fly ash recycled.  this in turn causes unnecessary purchases of unwanted acreages in landfill space.  

what are the environmental benefits of fly ash?

the main benefit is the reduced demand for virgin materials that would need quarrying.  it can also easily substitute for similarly strong materials (such as portland cement).

what are the uses of recycled fly ash?

some include: portland cement and grout, embankments and other structural fill, waste stabilization, cement clinkers, soft soils, aggregate, and cellular units of concrete (geopolymers, roofing tiles, paints, etc.).

all in all, fly ash accounts for an innovation in design credit for the LEED AP exam, and it is fairly useful (not to mention enlightening) to know that even a sturdy unit of construction, which concrete is, can still be recycled economically and (somewhat) efficiently.