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Cement:

Cement is a key ingredient in concrete. It is the binding agent that ‘Glues’ the concrete together. There are two main categories of cement:

  • Non-Hydraulic Cement:

The Process by which Non-Hydraulic cement is produced and used is known as the lime cycle:


Limestone is extracted from quarries and heated in a furnace where as a result of thermal decomposition, it is broken down into calcium oxide and carbon dioxide:  CaCO3 → CaO + CO2


The calcium oxide is then combined with water to produce slaked lime: CaO + H2O → Ca(OH)2


The slaked lime is then mixed with other ingredients like sand to produce mortars, plasters, lime washes and concrete. As these composites set the slaked lime reacts with carbon dioxide in the air to produce calcium carbonate and water: Ca(OH)2 + CO2 → CaCO3 + H2O


  • Hydraulic Cement:

This type of cement is similar to non-hydraulic cement however additives like Pozzolan’s (materials that if finely ground and combined with calcium hydroxide (Slaked Lime) react and become binding agents) allow for chemical reactions that produce hydrates.  As hydrates are composed of water, Hydraulic cement can be used underwater and in other wet conditions. These hydrates help protect concrete from water and chemicals as they do not readily dissolve in water. Due to the numerous and varied additives added to hydraulic cement, the exact chemical process is still being researched.




Varieties of cement:


There is a wide variety of cement however Portland cement is the most commonly used variety.  It is composed of calcium oxide, gypsum and other additives and is primarily used for concrete production.  However to reduce the environmental impact of cement product and to increase the strength of concrete other additives are being used more frequently:



  • Ground granulated blast furnace slag:


Ground Granulated blast furnace slag is produced by taking the slag from a blast furnace, rapidly cooling it with water, then grinding it down in to a fine powder. The use of GGBS in cement produces high strength concrete but the greater the content of GGBS the longer it takes for the concrete to strengthen however resistance to sulphates (acid rain) increases and decreases the amount of heat released by the exothermic chemical reaction that occurs when cement sets.


  • Silica Fume:


Silica fume is the by-product of arc furnaces used by the steel industry. As individual particles of silica fume are around 100 times smaller than cement particles it is a very good pozzolan. Concrete produced using cement that contains Silica Fume has increased strength (both compression and bond strength) and resistance to corrosion (from salt in particular).



  • Fly Ash/Coal Ash:


Ash has been used in cement since the roman times as it is a pozzolan. However in the absence of volcanic ash (that the Romans used) the ash that has been filtered in the chimneys of coal fired power plants can be used.  Fly ash is ash from the top of the chimneys and bottom ash is ash that has settled at the bottom of the chimneys.  There is a lot of variation when it comes to the composition of the ash as it depends on the coal that has been burnt. Despite the many benefits of using fly ash in concrete, there are concerns about its use due to dangerous impurities it contains like heavy metals.





Aggregates and their properties


Aggregates are used to provide strength to concrete, they take up about 60-75% of the concrete mix. They control the properties of the concrete and vary widely depending on what you are trying to achieve with your material. The change in properties of the material can include:

  • Durability

  • Surface texture

  • Toughness

  • Weight

  • absorption

The shape of the aggregates has a large effect on the properties and rough pieces require more water, this is due to holes being created within the mix which must be filled. Aggregates are the most mined material in the world and are usually a mix of small chips of rock, crushed stone, sand and gravel, but almost anything can be used as long as it does not interfere with the chemical composition.


Stone Aggregates


There are two types of stone aggregates, fine and course. Fine is usually made up of small gravel pieces and a high volume of sand, these are usually collected from river or lake beds. Course is made up of larger stones which are mined and this creates a much rougher surface to the concrete. Stone aggregates are cleaned and sometimes put through a process called heavy jigging which removes any lower grade pieces and improves the quality of the concrete.


Recycled Aggregates


Waste plastic and rubber can be used as alternatives to normal aggregates, and reduce the mass of plastic being thrown away as the material is recycled.

Glass can add a shiny effect to the concrete which makes it more aesthetically pleasing but due to the reaction between the glass and the cement it can weaken it and reduce durability.


Alternative Aggregates


Leca is the lightest available aggregate which is made up of small sections of expanded clay. It is mostly used on projects where the weight of the concrete is a big issue, for example, cladding. It also effects the colour of the concrete and produces a browner colour.


                                                   Leca



Concrete

What is concrete?


Concrete is made up of Portland cement, an aggregate and water, in general these are the only ingredients used as it makes the properties of the concrete easier to predict, the more the ratios are altered the more difficult it can be to control.

The most important factor is the water to cement ratio as this directly affects the strength of the concrete, the more water the more pores there will be in the cement paste portion and the larger these pores can be, obviously creating structural weaknesses and inconsistencies. The trade off for creating strong concrete with a low water concrete is the mixture can be quite stiff and difficult to place/ shape.

Another important detail to consider when mixing the concrete is the shrinkage which occurs as the mixture sets, though a low water content can create a strong and durable concrete it doesn’t necessarily reduce shrinkage, which can be problematic in decorative pieces or components that need assembling. the cement is also the most expensive portion of the mix, using the largest possible aggregates, less water and cement (stiffer mix) and finally using the optimum ratio of fine to coarse aggregate can help to reduce cost as well as improving the quality of the concrete produced by reducing shrinkage and pore size).


Accelerators

These can be added to make the concrete set faster and to accelerate early strength (if there are other jobs waiting). Adding more accelerator as would be expected speeds up the setting process proportionally. An example of a cheap and common accelerator is calcium chloride, however because concrete is so often used in conjunction with steel (reinforcement) a non chloride accelerator must be used to avoid corrosion.

Retarders

These perform the opposite function of accelerators and are often used in hot weather conditions to delay setting time (even setting to avoid fractures and cracks when concrete shrinks). Retarders are also used to delay the set of difficult jobs and for special finishes such as exposing aggregate. Retarders can also be used to reduce water content for the previously stated benefits.

Fly ash

This matter is a product of burning plants, this can replace up to 30% of the cement used in the mix, this is beneficial as it improves workability, is easier to finish, reduces heat generated by concrete yet costs the same as the cement it replaces. Reduction of cement use is essential as fly ash is much more sustainable than Portland cement which is rapidly running out.


Water reducing admixtures

These are exactly what the name described, when added to the mix they reduce the amount of water required strengthening the concrete. Low range water reduces can reduce the water content by up to 10% while more expensive reducers can go up to 30% though these are rarely used as they are not essential and can be expensive.


Air entraining admixtures

These are necessary when concrete is going to be exposed to freezing and thawing, air entraining agents entrains microscopic air bubbles in the concrete which the water can expand into when it freezes, instead of damaging the concrete.







Varieties of concrete:



  • Air Entrained Concrete

Prepared by adding an air entraining mixture, which contains lots of air pockets.

This relieves the internal pressure on concrete, providing chambers for the water to expand into when it freezes.

Low strength compared to normal concrete.



  • Self Compacting Concrete

·        No vibration required as it is compacted due to its own weight.



  • High Strength Concrete

·        Has a low water cement ratio and silica fume is often added.

·        These aspects make it less workable so super plasticisers are usually added.








Precast Vs In-situ Concrete


  • Precast concrete: is a construction product produced by casting concrete in a reusable mould which is then cured in a controlled environment, transported to the construction site and lifted into place.


Precast concrete
  • In-situ concrete: the concrete that is produced by the method in which the liquid material is poured into forms at the building site.
                                             In situ concrete



Comparison between precast and in-situ concrete:



BIBLIOGRAPHY

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http://philipmarshall.net/Images/lime_cycle_1_640.gif

Builder Bill, ‘Concrete Basics’ <http://www.builderbill-diy-help.com/concrete-basics.html> [accessed 10 February 2015]

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Tool Base, ‘Concrete Aggregate Subsitutes’ <http://www.toolbase.org/Building-Systems/Foundations/concrete-aggregate-substitutes> [accessed 11 February 2015]

Dennis Day, ‘Alternative aggregates in asphalt and concrete’ <file:///C:/Users/Alice/Downloads/april_2004_-_alternative_aggregates_in_asphalt_and_concrete%20(1).pdf> [11 February 2015]

Hering International, ‘Glass Concrete’ <http://www.heringinternational.com/en/products-services/architectural-concrete/special-surfaces/glass-concrete/glass-concrete-006/> [accessed 11 February 2015]

Alibaba, ‘LECA’ <http://www.alibaba.com/product-detail/LECA-PearLeca-Light-Expanded-Clay-Aggregate_137397635.html> [accessed 11 January 2015]


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The lime life cycle






Ground granulated blast furnace slag  


Silica Fume   


Fly ash/Coal ash






Stone aggregates   



Recycled aggregates  

 

Leca in concrete

































Concrete





















Fly ash  






























Air Entrained Concrete







Subpages (1): Cement
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