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Optimizing mixed cements

The major component in cement is limestone sourced calciumoxide. It is produced by milling limestone and heating it to 1400-1500 degree Celsius, which results in the limestone entering the melting phase. At this temperature it transform chemically into calciumoxide, the active ingredient having the cementitious property that is needed when making e.g. building materials.

As the energy cost is very high for making limestone-based cement, supplementary materials are actively sought after, in order to reduce cost of the final cement product, while also reducing the CO2 emissions and incresing the durability of the final concrete product.

Many different kinds of materials are used and have been tested over time, and they are commonly known as Supplementary cementitious Materials - or SCM's.

Dilution with nonactivated limestone

A simple, but effective, way of decreasing the amount of activated limestone in the final cement compound, is to use fine milled non-activated limestone. Adding this to the activated part, will effectively just dilute the cementitious property, but it works well for substitution rates up to about 20%. However, it still requires access to limestone, and it does not increase the durability of the concrete product.

Clay / Kaolin

Metakaolin is an artificial pozzolan which forms during de-hydroxylation of the clay mineral kaolinite. It is used in special applications where high strength and/or low permeability are required. This compound is more commonly used as an additive to concrete and not as a substitute for cement.
Metakaolin helps reach high compressive strength and workability of the concrete together with high alkali–silica resistance. Metakaolin is reference to ASTM D5370 – 14 (Type M) Standard Specification for Pozzolanic Blended Materials in Construction Applications.

Fly ash

Fly is a waste product formed during combustion at coal driven power plants. Before the 1970’s and 1980’s, fly ash was released into the atmosphere. However, the cement industry at large recognised the importance and desirable properties of fly ash in cement products. Today, coal driven power plants uses particle filtration systems to capture the fly ash before it reaches the chimneys.
The composition of fly ash varies extensively relative to original coal composition. Fly ash is widely used as a supplementary cementitious material due its low cost relative to clinkers. Typical, substitution rates range between 15 to 25 percent to the overall Portland cement content.
Fly ash is categorized as Class F fly ash and Class C fly ash, as defined by the ASTM C-618 and ASTM D5370 – 14 (Type F and C) standards based on the proposition of silica, alumina and calcium. Class F and class C fly ashes hold different cementitious properties.

Shale / Diatomacious earth

Diatomaceous earth is another natural pozzolan and consists of opaline (silica-rich) marine microfossils. High quality diatomite contains over 80% silica and it is added to the cement mixing process to boost the silica content of the product. Advantages in the uses of diatomaceous earth in Portland cement mixes have been recognized for almost a century and were used as an adjective in the concrete which was used to build The Golden Gate Bridge in San Francisco. Diatomaceous earth increases workability of the wet mix and improves the water tightness, increases the strength and decreases the rate at which the concrete is attacked by sea water which is being placed in the cured cement concrete. The physical and chemical properties of volcanic ash could be referenced with an ASTM C517 – 71 Standard Specification.

Silica fumes

Silica fumes are another waste product from other industries. Silica fumes are an amorphous polymorph of silicon dioxide which holds desirable cementitious properties. It is formed as a by-product during production of silicon and ferrosilicon alloy as an ultrafine powder with individual particles less than 1 μm in diameter. As for fly ash, silica fumes were released into the atmosphere it the 1970’s but was collected by manufacturers due to changes in stringent environmental regulations. Norwegian research showed the advantageous cementitious and identified the pozzolanic properties.
Silica fumes as concrete additive results in higher durability and strength developments of the mixed products. Typical substitution Portland cement substation rates range between 5 to 7% of the overall volume. Silica fumes are specified by ASTM C-1240, ASTM D5370 – 14 (Type SF) and AASHTO M 307 standards.

Slag

Slag cement (ground granulated blast-furnace slag) is another by-product originating from the production of iron. Slag is removed periodically from the blast furnace in order to secure on-going production. As for fly ash, slag cement properties are dependent on input raw materials together with the physical properties of the slag and how the molten slag is cooled. Slag cements are available in three forms; air cooled, granulated and expanded. Granulated blast-furnace slag is present in the form of pellets and requires grounding to form slag cement. Slag cement holds moderate strength developments of the concrete product but is a cheap additive. Numerous factors such as fineness, particle shape, chemical composition, temperature during slag formation determine the performance of slag mixed cement. Slag cement is specified by ASTM C-989, ASTM D5370 – 14 (Type S) and ASTM C-595 standard in order to be viable as a blended cement component.

Volcanic ash

Volcanic ash is a natural pozzolan which is formed during volcanic eruptions. The major pozzolanic component is a highly porous glass (silica-rich) and it was the Romans who first discovered the advantageous cementitious properties of volcanic ash and was e.g. used to build the Pantheon in Rome. Volcanic ash steps up setting times and compressive strengths of the concrete products whilst lowering the ricks of alkali–silica reactions and autoclave expansions. The physical and chemical properties of volcanic ash could be referenced with an ASTM C618-93 Standard Specification.