What Does Cementitious Material Mean?
Cementitious materials are those substances that have cement-like properties. The primary
cementitious material present in concrete is portland cement. Today, most concrete mixtures also incorporate Supplementary Cementitious Materials or SCMs. These make up a share of the cementitious component in concrete. These materials are generally by-products of other processes or natural materials.
What Are The Physical Properties Of Cement?
Cement is a well-known binding material having physical properties that make it a popular choice for civil engineering or construction projects. Its properties include:
- Fineness
- Soundness
- Consistency or uniformity
- Strength
- Curing time
- Heat of hydration
- Low inflammability
- Bulk density
- Specific gravity
Types Of Cementitious Materials
There are two types of cementitious materials:
- Hydraulic Cement
- Supplementary Cementitious Materials
Hydraulic Cement
Hydraulic cement is an inorganic or mineral substance that reacts with water under ambient conditions to form a hard, water-resistant product. This process is called “Hydration.” It is a non-reversible chemical reaction. During the reaction, heat is generated as the water-cement paste hardens and glues the aggregate particles together. The most common hydraulic cements are those that are based on calcium silicates, like the Portland cement.
Blended hydraulic cement is more suited for hot weather applications. One can obtain it by making a composite of reactive materials like Portland cement clinker, pozzolanic materials, granulated blast furnace slag, silica fume, burnt shale, or fly ash.
Common Hydraulic Cement
- Ordinary Portland Cement – Ordinary Portland cement is also known as OPC. It was so named because it has a strong resemblance to the Portland rocks found in England. Controlled amounts of powdered shale, limestone, and other calcareous materials are heated together in a furnace at very high temperatures (more than 2500°F or 1370°C) to get OPC. This produces small-sized hard spheres, which are known as clinkers. Subsequently, the cooled clinker is ground with gypsum to get a white or gray powder-like cement called the Ordinary Portland Cement. The cement’s chemistry and fineness determine the Portland cement’s reactivity and the hardened properties of concrete made using it.
- Portland-Limestone Cement – Portland Limestone Cement is also known as PLC. Sustainability concerns primarily drive its increasing use across the US. Manufacturing Ordinary Portland Cement produces nearly the same amount of CO2. Therefore, PLC is considered its greener alternative. It is a blended cement that lowers the carbon footprint by nearly 10%. PLC is made up of cement clinker ground together with around 15% limestone and produces the same set times, strength, and finish without any notable differences.
Supplementary Cementitious Materials
Supplementary cementitious materials (SCMs) are a partial replacement for Portland cement. It makes the concrete easy to work with and adds to its hardened properties through the hydraulic reaction with water or pozzolanic reaction with calcium hydroxide or CH. It reduces thermal cracking in structures by lowering the heat of hydration. Generally, SCMs are added during cement manufacturing to make blended cement that is more consistent. This may also happen at a batch plant.
Common Supplementary Cementitious Materials
- Fly Ash – The most widely used SCM in concrete is fly ash. Historically, it was obtained from coal-fired power plants. But with green initiatives becoming a norm, there has been a definitive shift from such power plants. This has led to a decrease in the supply of fly ash, while there has been a consistent increase in its demand. Therefore, efforts are being made to reclaim the previously disposed fly ash and use it in the foreseeable future to meet the demand.
Some of the properties of fly ash that make it such a sought after SCM are:
- Adding fly ash to concrete makes it easy to work with, even with less water.
- It delays the setting time.
- Delayed setting time makes for a stronger structure.
- It reduces the heat of hydration.
- Adding fly ash to the concrete mix improves the resistance to sulfate attacks and alkali-silica reactions.
- It reduces water permeability in cement.
- Slag Cement – Earlier slag cement was called ground, granulated blast furnace slag. It is a by-product that is found in blast furnaces where the extraction of iron from iron ore is carried out. While still in its molten state, the slag is sprayed with water in a granulator. It becomes glassy sand-like particles of amorphous calcium, magnesium, aluminum, and iron oxides. These particles are then ground finely to become equal to or smaller than the Portland cement particles.
The impact of using slag cement with concrete are:
- Less water is needed to achieve workability.
- Curing (drying and hardening) time can be delayed.
- The early strength of the build might be less, but the latter strengths are much improved.
- It improves the concrete’s resistance to chloride penetration significantly.
- Silica Fume – Silica fume is also known as condensed silica fume or micro silica. It is a pozzolanic by-product of the silicon and ferrosilicon industries. The particles of silica fume are mostly glassy spheres of silicon dioxide, which are very small in size. The water requirement of silica fumes is relatively high, due to which sometimes it becomes necessary to use superplasticizers. The concrete formed as a result has very high strength and lower permeability. However, due to insufficient bleeding, it poses a higher risk of shrinkage and cracking. Typically, silica fume is used only in special applications like making gutters and curbs.
Summing Up
Concrete or mortar on their own does not have all the qualities that are required in any construction. Therefore, it becomes necessary to add cementitious materials which lend it all the desired qualities. The proportions in which these materials are added determine the quality of the end result. Needless to say, the cementitious materials have to be added in precise proportions to get precise results.
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