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The most common type of cement is a general-purpose type 1. Type 1 cement is used in most residential construction and is typically used if no other specification is called out.
Type 2 cement is sometimes specified for walkways where de-icing chemicals might be used. The sulfate-resistant properties of type 2 concrete make it ideal for this application. It is also used in applications where a slower cure is desirable. This includes summer pours when the temperatures are high and finishing the concrete is a race against time. It also includes large pours where the heat radiated from the concrete carrying can cause problems.
Type 3 concrete is sometimes called high-early cement because it achieves most of its strength within the first week of curing. Type 3 cement should be used when the concrete must be put into service as quickly as possible. This concrete can withstand a full load bearing within weeks of the pouring. This concrete is also used when the forms must be removed early.
Type 4 cement is a slow curing style that generates very little heat. It is used extensively for large mass concrete structures such as dams.
Type 5 cement can withstand severe sulfate action that occurs in heavily alkaline soils or groundwater. Standard concrete can have a chemical reaction between the sulfates and the compounds formed by the Portland cement. This can cause the concrete to deteriorate rapidly.
Another type of concrete is lightweight concrete. Using artificial aggregates or gas forming admixtures, the density of the concrete is reduced. There are basically four kinds of admixtures that can be added to concrete to change the properties. Air entraining agents are used in areas with hard freezes. This admixture this admixture stabilizes the bubbles formed in the concrete during the mixing process. The bubbles create tiny voids that act as shock absorbers allowing the concrete to better withstand freezing and thawing cycles. The air bubbles are typically 5 to 7% of the total volume of the mixture. Although the bubbles make the mixture slightly weaker, this mixture is more workable. The mixture requires less water which makes the mud more resistant to salts and produces a long-term durable concrete.
A set retarder may be added to extend the setting time by 30 to 60%. This is especially useful in warmer weather.
Some concrete companies may also add water reducing agents called plasticizers. Plasticizers reduce the water content by up to 15%. Reducing the water content reduces water related problems such as shrinkage, cracking, crazing, and dusting plasticizer can increase the concretes strength and allow it to better bond to reinforcing rods.
The most common admixture is calcium chloride. This chemical is an accelerator that is used to make concrete set faster and with more heat. Accelerator is typically added during cold weather pours. Pop out, scaling, and cracking can occur if the concrete freezes during the curing process. Accelerator is not an anti-freeze the accelerator allows the concrete to be poured finished and insulated the four the onset of freezing temperatures.
For most homeowners, Type 1 cement will meet most of their needs. The other types of concrete may be used in special situations based on whether and environmental restrictions.
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