Concrete Countertop Mix Ingredients and Admixtures

In a previous article, I explained basic principles of concrete countertop mix design, such as the role of sand, cement, water, water-cement ratio and aggregate gradation. Click here to read that article and view the videos.

In this article, I provide details about two important specialty admixtures for concrete countertop mixes: superplasticizer and viscosity modifier. These admixtures work hand in hand to create the right mix consistency without compromising strength by adding water.


 

Superplasticizer (High Range Water Reducer) for Concrete Countertop Mixes

Polycarboxylate superplasticizers are a very powerful type of high range water reducer. This video shows how adding a small amount of a powerful water reducer, instead of water, can take a concrete mix from dry and crumbly to flowable.

Here is a video that demonstrates the dramatic effect superplasticizer has on mix consistency, then explains the science behind how it works.

 

The video uses a powdered superplasticizer, BASF’s Melflux 2651, in order to demonstrate that it is not the liquid in the superplasticizer that causes the increase in slump. Melflux 2651 is very difficult to find in reasonable quantities for a concrete countertop maker. Buddy Rhodes Concrete Products offers a similar powdered superplasticizer. However, powdered  superplasticizers are extremely powerful and very easy to overdose. For that reason, we recommend and sell a liquid superplasticizer, ADVA Cast 555.

Recommended Product

Water Reducer ADVA555ADVA Cast 555, manufactured by Grace and packaged by Buddy Rhodes Concrete Products, is a powerful high-range water-reducing admixture (superplasticizer) based on the next generation of polycarboxylate technology. ADVA Cast 555 is designed for use in precast, ready-mix and self-consolidating concrete (SCC) applications, and it is excellent for use in GFRC.ADVA Cast 555 provides excellent early compressive strength, faster setting strengths and fluidity in mix designs with substantially reduced water content.

Click Here for details of how to dose and use this superplasticizer, and to purchase it.


 

Viscosity Modifiying Admixture (VMA) for Concrete Countertop Mixes

The following seminar excerpt explains what viscosity modifying admixtures do. VMAs are used to reduce the slump of concrete, essentially performing the opposite of a high range water reducer (superplasticizer). They also help prevent segregation in aggregate-based mixes.

 

Recommended Product

Fritz-Pak’s Super Slump Buster is an easy to use, powdered viscosity modifying admixture (VMA).It is available in 8-oz bags, which is enough for dozens of average concrete countertop projects. VMA should not need to be used for every project, as explained below.

The manufacturer sells Super Slump Buster only in cases of 60 bags. We sell it by the bag.

 

Click Here for details of how to dose and use this VMA, and to purchase it.

 

Concrete Countertop Mix Design Principles

Here’s some information to help you understand important, fundamental principles of concrete countertop mix design that will help you be successful with make-your-own concrete countertop mixes. Learn about water-cement ratio, how admixtures work, and much more.

 

How Concrete Works Seminar – primary ingredients (18 minutes):

 

 

Admixtures Seminar – secondary ingredients (3 hours):

Did you like what you learned in just 18 minutes in the How Concrete Works seminar? Then click here to get FREE access to the 3-hour seminar “Everything You Ever Wanted to Know About Admixtures for Concrete Countertops”.

 

Mix Designs

We offer make-your-own mix designs for GFRC and precast, with calculators that remove all the math. These are strong, engineering-based mixes used for years by professionals all over the world. Our precast mixes achieve compressive strengths of over 4000 PSI (27 MPa) in 1 day, 6200 PSI (42 MPa) in 3 days, and 8200 PSI (56 MPa) in 7 days.

Click here to view our available mix designs.

mix calculator

 

 

How to use set retarders with CSA cements in concrete countertop mixes

All CSA products are inherently fast setting. In order to boost the work time, chemical retarders are often used. Unlike portland cements, CSA cements can use ordinary citric acid, often at doses of 0.2% to 0.4%.

Straight CSA cements such as Rapid Set Cement actually benefit from moderate retardation. The structural silicate crystal growth occurs more slowly, and like old-growth wood, results in better concrete with higher strengths.

Another way to increase work time when using CSA cement is to substitute ice for some of the mix water. Generally 1/4 to 1/3rd of the mix water is ice, and the melting ice absorbs heat from the other ingredients. The advantage of thermally retarding the mix with ice is that once the mixture warms up to ambient temperatures, its normal setting rate return, so early strength is far less affected than when citric acid is used.

Both ice and citric acid can be used together for very long work times. I have achieved over an hour of working time with Rapid Set Cement in 90 degree weather when citric acid was used along with ice. Ice can also be used with “plain” portland cement based concretes, as this is a time tested technique used in the construction industry for hot weather concreting.

Ice to cool and retard the concrete

Ice to cool and retard the concrete

Note: CSA additives are different. (See article here.) Since CSA addiitves are blended with portland cement, you may use citric acid to extend working time, but that may also reduce early strength development of the portland cement it’s blended with. Dosages vary depending upon CSA additive amounts and upon the amount of working time desired.

CSA cement vs. accelerators in concrete countertop mixes

A student asked me a question recently:

What’s the difference between CSA cements and accelerators? Don’t they both make the concrete cure faster?

The answer is no.

Accelerators don’t speed up strength gain, only set time. This means that concrete made with or without an accelerator will have developed the same 28-day strength at the same rate. The concrete with the accelerator just sets up faster so that it can be troweled or finished earlier.

CSA cements, on the other hand, actually speed up strength gain. They speed up strength gain so much that the concrete is as strong after 28 hours as it would have been in 28 days with portland cement! Plus CSA cements have a host of other benefits, such as elimination of efflorescence and reduction of carbon emissions.

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