CSA Explained: CSA cements, CSA additives, pozzolans, citric acid and ice
By Jeffrey Girard, P.E., President, The Concrete Countertop Institute
There is a great deal of confusion about CSA (calcium sulfoaluminate) cements. Much of this stems from the fact that there are two different products that are CSA based, and both of these are being used and recommended by various people in various ways for concrete countertops.
Differences between CSA Cements and CSA Additives
CSA-based cements, which I refer to as CSA cements, such as Ultimax’s UC Precast rapid hardening cement and CTS’ Rapid Set cement, are true cements. They don’t need anything else to work. CSA-based cements are added to sand, gravel and water to make concrete, exactly the same way portland cement is added to those ingredients to make concrete.
CSA cements are made using pure CSA clinker that is blended with portland clinker and fired in a kiln, similar to the way portland cement is fired in a kiln. Because the CSA and portland clinker are fired together at high temperatures, they combine to form CSA-based cements. There is no portland cement left after firing. It is chemically transformed into a rapid hardening CSA-based cement. This is NOT the same as simply dry blending portland cement and CSA-based cement together.
Another CSA-based product, Qwix, is NOT a cement. Qwix is made from pure CSA clinker, the same precursor to CSA-based cements. It is ground like portland cement and then bagged.
Qwix is an additive, not a cement. I refer to it as a CSA additive. It is designed to be, and MUST be, blended with portland cement. In this way it is similar to pozzolans like VCAS or metakaolin. Pozzolans must be blended with portland cement. They won’t do anything alone.
Pozzolans and CSA Cements
Concrete made with only a CSA cement does not need pozzolans. CSA cements do not produce calcium hydroxide, so pozzolans have nothing alkaline to activate them.
CSA additives are different. Because Qwix must be blended with portland cement, the basic chemistry and the side effects that stem from using portland cement are inherited. Pozzolans can be used, and are dosed based on the portion of portland cement used in the concrete. So while it’s possible to make strong, rapid setting concrete using Qwix, portland and pozzolan, this requires three ingredients, ingredients that each need to be sourced, shipped, dosed and weighed.
To test the interaction of CSA cement and pozzolans, in 2008 CCI performed compressive strength tests on a variety of concrete mixes that used Ultimax cement and VCAS pozzolan. The cylinders were prepared according to ASTM C192 standards and tested by an independent concrete testing laboratory. Three different concrete mixes were made with Ultimax cement where 20% of the cement was replaced by VCAS. Test results showed that all of the mixes had a 30% loss in 1- and 7-day compressive strengths versus mixes that used only Ultimax cement.
Rapid hardening CSA cements such as Ultimax and Rapid Set provide extremely high early strengths, often achieving 3000 psi in 1 hour and nearly full strength after only 24 hours. CCI’s own tests using Rapid Set’s cement yielded 10,000+ psi compressive strength after only 24 hours. These tests were performed in the same fashion as the 2008 compressive strength tests, and the cylinders were tested by the same independent test laboratory. By using a rapid hardening CSA cement in your concrete, you can achieve strengths higher than what portland cement concrete can provide AND you reduce your inventory and simplify your mix design.
Set Retarding with CSA Cements
Qwix is a pure form of CSA that must be blended in portland cement, often replacing portland cement at about 15% by weight. Qwix may be used from 10% to 50% by weight of cement for replacement amounts. Qwix functions as a catalyst in the rapid hardening process of concrete, and the more Qwix that is used, the higher the early strength. However, higher replacement dosages will significantly reduce the set time.
All CSA products are inherently fast setting. In order to boost the work time, chemical retarders are often used. CSA cements can use ordinary citric acid, often at doses of 0.2% to 0.4%. Straight CSA cements (not blended with portland 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.
Qwix, since it’s blended with portland cement, may use citric acid to extend working time, but it may also reduce early strength development of portland cement. Dosages vary depending upon Qwix amounts and upon the amount of working time desired.
Another way to increase work time 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 is 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.
Recommendation for 100% CSA Cement Usage
Keep it simple! There is no significant benefit to clients in using a blend of portland cement plus CSA additive (Qwix), retaining all of the problems of portland cement and necessitating pozzolans, when you could simply use a 100% CSA-based cement (Ultimax or Rapid Set).
Blending portland, Qwix, a pozzolan will result in slightly lower material costs and greater working times, but the mixture requires more ingredients, longer time to batch and a more in depth understanding of how the three cementitious ingredients work together.
All you need to create high quality concrete is sand/aggregate, CSA cement, water and superplasticizer. The occasional admixture such as a set retarder (citric acid and/or ice or a chemical set retarder) may be necessary due to shop conditions. Seal with a reliable, tested, stain-resistant sealer, and you’ve got countertop-quality concrete that is highly functional in your clients’ kitchens or bathrooms.