What’s the difference between Type III cement and CSA cement?

Type III cement is a form of portland cement. This article explains Type III cement, but basically, Type III is a high-early-strength cement. It is ground finer and reacts faster than Type I cement, so the early strength gains are greater. Note the word “early”.

Generally Type I cement based concrete reaches about 60% of its 28 day strength in the first 3 days;

Type III cement achieves about 70% of its 28 day strength after 3 days. That is indeed a little faster than Type I.

With either Type I or Type III portland cement, continued strength gain requires continuous wet curing for weeks. Concrete that dries out prematurely never reaches its full potential. (Note however that with concrete countertop mixes that achieve over 8000 PSI compressive strength well before 28 days, it is really not necessary for them to reach their full potential. They are strong enough after a few days, so I generally cure regular portland cement mixes for only 3 days.)

CSA cement typically achieves 80% or more of its 28 day strength in the first 24 hours, and usually close to 100% of its strength within the first 3-7 days. Because of the very rapid reaction, wet curing is necessary only for the first few hours, not continuously for weeks like with Portland cement based concrete.

 

In the graph below, note the steeper curve for Type III, meaning faster strength gain.

Strengths of type I vs type III

Strengths of type I vs type III cement

Figures taken from PCA, Design and Control of Concrete Mixtures, 2003.

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: Qwix is a CSA additive sold by Ultimax. Qwix, since it’s blended with portland cement, 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 Qwix 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.

Do pozzolans and CSA cements work together in concrete countertop mixes?

In short, no.

There is still a lot of confusion, even several years after CSA cements became popular in concrete countertops. The simple, chemical fact is that CSA cements do not produce calcium hydroxide, so pozzolans have nothing alkaline to activate them.

CSA additives are different. Materials such as the brand Qwix® must be blended with portland cement. Therefore, 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.

While it’s possible to make strong, rapid setting concrete using Qwix® , portland cement and pozzolan, this requires three ingredients, ingredients that each need to be sourced, shipped, dosed and weighed. I prefer simply to use a true CSA cement such as CTS Rapid Set® .

To test the interaction of a true CSA cement and pozzolans, in 2008 I performed compressive strength tests on a variety of concrete mixes that used Ultimax brand CSA 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 CSA cement. Rather than getting the strength increase seen with pozzolans and portland cement, you get a strength decrease.

Bottom line: Don’t use pozzolans with CSA cements.

VCAS strength tests

Various CSA cement brands and manufacturers

Currently there are three manufacturers of calcium alumino silicate (CSA) cement products in the United States: CTS Cement, Buzzi Unicem and Ultimax. Though they are all called CSA cement, the products are not the same, nor do they yield similar strength concrete for a given age.

CTS manufactures Rapid Set brand cement, which is a CSA cement that is used alone and is not made with nor blended with Portland cement.

Buzzi Unicem makes a CSA cement simply called “CSA”, but on their website it’s clear that it should be used with Portland cement:

“CSA® is a hydraulic, cementitious binder (per ASTM C219), high in Calcium Aluminum Sulfate Crystals. When used with Portland cement concrete, CSA generates a strong cementitious matrix that enhances the physical and chemical properties of the mix.” 

In this regard this “cement” is really a cement additive.

Ultimax sells a CSA cement additive called Qwix that also is meant to be blended with ordinary Portland cement.

“Qwix® is a mineral additive used with Portland cement concrete mixes to enhance the physical properties and the chemical durability related properties.”

According to Ultimax’s website, Buzzi Unicem manufactures Qwix under license. Qwix is a CSA cement additive that must be used with Portland Cement.

Ultimax also makes rapid hardening cements that use their “Ultimax cement technology”. It is not clear whether these rapid hardening cements are blended with Portland cement, but compression test data (shown below) clearly show a gradual strength gain similar to Portland cement, while CTS Rapid Set CSA cement does not.

In April 2009, The Concrete Countertop Institute conducted standard concrete compression tests to compare the early compression strength of concrete cylinders made using CTS Rapid Set cement, Ultimax cement and Portland cement. (Qwix was not tested, nor was Buzzi Unicem CSA, since both are CSA additives, not CSA cements.)

Cylinders made using Rapid Set and Ultimax Gray cement used only those products; cylinders made using ordinary Type 1 Portland cement used a blend of 90% (by weight) gray Portland cement and 10% VCAS pozzolan. All cylinders were prepared (C192) and tested (C39) in accordance with ASTM guidelines. All concrete cylinders used the same aggregate-based concrete mix design with water-to- cementitious ratios of 0.35 and a CSA retarder (Ultimax Delay) dose of 0.5%. Concrete compression tests were performed at 1, 3 and 7 days after casting.

CCI compression strengths are similar to or even a bit higher than strengths advertised by Rapid Set and Ultimax on their websites. Data for Portland cement and VCAS are shown for comparison purposes.

csa_compressive_graph

 

Differences between CSA cements and CSA additives in concrete countertops

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, besides water, 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.

Rapid Set and Ultimax CSA cement products

Rapid Set and Ultimax CSA cement products

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. This does not produce the same end product as the process I described above.

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.

QWIX by Ultimax

QWIX by Ultimax

Keep it simple: A recommendation for 100% CSA usage in concrete countertops

I have encountered many different approaches among concrete countertop professionals in using CSA cements. Some use portland cement, CSA cement and pozzolan blends to achieve high-quality concrete. I recommend against this approach, as it adds complication and negates the benefits conveyed by CSA cements.

Pozzolans are substituted for portland cement to address a host of problems caused by a by-product of the reaction of portland cement with water: calcium hydroxide. Calcium hydroxide makes concrete weak and causes effloresence. Pozzolans consume calcium hydroxide, thus improving the quality of the concrete.

When CSA cements react with water, they do not produce calcium hydroxide. Therefore, simply substituting CSA cement for 100% of the portland cement eliminates all of the problems listed above and obviates the need for pozzolans.

Let me say that again: Simply using 100% CSA cement (and 0% portland cement) completely eliminates all the problems that portland cement caused and pozzolans were trying to solve. Why in the world would you substitute CSA cement for only part of the portland cement, leaving all the problems that then need to be remedied with pozzolans? In my opinion this merely adds complication and expense without providing much benefit. It causes people to spend endless hours tweaking their mix proportions when they should be focusing on selling countertops. I recommend the simple “100% replacement” approach that saves time and money and maximizes benefits.

CSA cements in concrete countertops: Rapid strength with a low carbon footprint

This article begins a series on CSA cements. Bear with me – this article is highly technical, but it gives you a background on CSA cements. In subsequent articles, I will explain practical issues such as how to set retard CSA cements and whether to use pozzolans with them.

Developed in China in the 1970’s, calcium sulfoaluminate (CSA) cements are a class of specialty cements that are included in the family of rapid-setting cements. Rapid-setting cement is used in many applications such as bridge decks, airport runways, patching roadways, sidewalks, etc. where rapid strength development is necessary. Additionally, CSA cements are sometimes used in shrinkage compensated concrete by mixing with portland cement and for controlled low-strength materials (CLSM) used for diggable back-filling of utility trenches.

CSA cements have yet not seen widespread use in concrete countertop manufacture, but they should – they offer tremendous advantages over portland cement in terms of strength, speed and greeness.

Rapid Strength Gain

The primary advantage of CSA cements is that concrete made with CSA instead of portland cement often achieves compressive strengths of in excess of 5000 psi in 24 hours; with CSA’s, it’s possible to achieve 28 day strength in 24 hours. This is the main reason CSA’s are used in place of ordinary portland cement (OPC) for certain applications. Rapid strength gain is critical in situations where an airport runway, a bridge repair or a damaged freeway must be returned to service in a very short amount of time.

Low Carbon Footprint

Another key advantage is that CSA cements are also significantly greener. Portland cement is fired in kilns at temperatures of around 1500°C (2700°F), whereas CSA cements only need to be fired at temperatures of around 1250°C (2250°F). The resulting CSA clinker is softer than OPC clinker, requiring less energy to grind.

The cement industry represents a small yet significant proportion of total global carbon dioxide emissions. The chemical conversion of limestone to calcium oxide reveals the inherent production of carbon dioxide. For every 1000 kg of calcium trisilicate (C3S) produced from limestone a resulting 579 kg of CO2 gas is emitted solely from the chemical reaction, regardless of the process used or the fuel efficiency. Green Cities Competition. “Green Cement: Finding a solution for a sustainable cement industry”, Department of Civil and Environmental Engineering, University of California at Berkeley. April 22th, 2007. John Anderson. 

Calcium trisilicate (C3S) is the compound responsible for early strength gain in portland cement. The other compound, calcium disilicate (C2S), forms more slowly and is responsible for longer term strength. C3S makes up about 50-60% of portland cement composition, while C2S makes up a smaller fraction of OPC, generally around 18-20%.

As is evident in the breakdown of CO2 emission sources, the chemical conversion of limestone to calcium oxide contributes to about 48% of the CO2 emissions generated in the production of ordinary portland cement. Burning fossil fuels to achieve the high kiln temperatures accounts for an additional 42%. Combined, 90% of the CO2 emissions are directly associated with the chemical conversion of limestone into cement.

In contrast, producing 1000 kg of CSA results in only 216 kg of CO2, a reduction of about 62% relative to OPC. This reduction is far greater than that achieve by using industrial waste derived pozzolans as OPC replacements, such as fly ash and blast furnace slag, which are often used to replace only about 10% to 30% of the portland cement. Concrete made with 100% CSA is 2 to 6 times greener than OPC that has had a significant quantity of cement replaced with pozzolans, and that includes “green” pozzolans like fly ash and slag. In fact, CSA cements had the lowest carbon emissions out of nine alternative cements, including magnesia (Sorel cements), sodium metasilicate (water glass) and calcium aluminate cements.

Lower Alkalinity

The main mineral components in CSA cement are anhydrous calcium sulfoaluminate (4CaO·3Al2O3·CaSO4), dicalcium silicate (2CaO·SiO2) and gypsum (CaSO4·2H2O). The lime in CSA cement is bonded and not free so its alkali is lower. The pH value is only 10.5-11; the pH of ordinary portland cement (OPC) is around 13, which is 100 to 300 times more alkaline than CSA cement. The low alkalinity naturally minimizes the chance for alkali aggregate reaction. This is important when glass is used in the concrete and the concrete is exposed to moisture.

CSA cements do not work like portland cement. Because of the much lower alkalinity, they don’t work with pozzolans, so using a pozzolans like silica fume, metakaolin and VCAS as a cement replacement to boost strength or reduce cement content (and thus restore or even improve the strength relative to 100% OPC) just won’t work. Compression tests performed by CCI showed a 30% loss of strength at both 1 day and 7 days when 20% of the CSA cement was replaced with VCAS.

Lower Shrinkage

CSA cements get stronger, faster than OPC, and CSA cements demonstrate very low shrinkage characteristics. This due in part for two reasons. The first is that CSA’s require about 50% more water than portland cement for proper hydration. The minimum recommended water to cement ratio (w/c) is 0.35, whereas with OPC it’s around 0.22-0.25. Because of the higher water of hydration requirements, most of the mix water is consumed for hydration and less excess water is available to cause problems with shrinkage. The second reason is that the very rapid strength gain can prevent shrinkage cracks because the concrete strength increases more rapidly than do the concrete’s shrinkage stresses.

However, if w/c ratios below 0.35 are used significant shrinkage can occur. This not only can mean curling but also large cracks and discoloration. CSA cements have a strict minimum water requirement that should not be ignored.

Shorter Curing Time

Curing with CSA is important, but wet curing durations are often measured in hours, not days or weeks. Optimal hydration and slab stability are achieved when the CSA concrete is kept wet for at least 3 to 4 hours after casting. During the initial hydration phase, the concrete demands moisture and the rapid reaction generates significant heat. If sufficient moisture is not provided during curing cracking and curling are possible. When moisture is provided through ponding or repeated wetting during the first few critical hours, long term stability and strength are preserved and ensured.

Direct Portland Cement Replacement

CSA cements can and should be used as direct, 100% replacements for portland cement.

Because CSA’s don’t react with pozzolans, none are needed to achieve high strengths and eco-friendly concrete. This simplifies mix design and minimizes inventory. All you need to do is replace 100% of the cementitous material in your current mix design, eliminating the pozzolans. Using pozzolans with CSA cements can actually weaken the concrete, so it’s best not to use them at all.

Superplasticizers, especially polycarboxylates, and viscosity modifiers work the same with CSA’s as with portland cement. Other exotic admixtures like liquid silicates or acclerating agents are not necessary, and won’t work or are not compatible with CSA cements. Conventional cement retarders are not compatible. Only special citric acid based retarding admixtures made for CSA cements will work.

Color Considerations

There are white and gray versions of CSA’s, as well as a light tan/buff color. The gray color is lighter and more brownish/greenish than most portland, so blending white and gray with some iron oxide black pigment can create a convincing “natural” portland cement color. Be sure to experiment with your color formulas when you make the switch to CSA cements.

CSA cements are compatible with concrete pigments, and they can be dyed and acid stained just like OPC. Decorative aggregates, metal and glass are all compatible, so specialty embedments and exposed aggregate looks are possible.

Cost

While the rapid strength gain, high “green” value and low shrinkage are valuable assets, such high performance does come at a price. On average, an 88 lb bag of white CSA can cost well over twice as much as a 94 lb bag of white portland cement. Since time is money in the business world, saving days spent waiting for the concrete to gain strength may be worthwhile, especially if shorter turn around times is needed. Concrete cast today can be stripped and processed tomorrow, and in many circumstances it can be cast and stripped all on the same day. This increases the production rate of your casting tables, and it minimizes the number of tables you need, and thus the shop size required.

But if your production process is inefficient, if you take a long time to get things done, or if you are not experienced with from-scratch concrete mixes, then the benefits of CSA cements won’t be realized. Much like driving a very fast sports car while being in a traffic jam, making concrete that gains strength very rapidly is pointless if the whole production process is not optimized to take advantage of its rapid strength gains.

Special Considerations

Finally, CSA cements are not, in my opinion, for the beginner. Everything about them is magnified and accelerated. They are far more sensitive to temperature, w/c ratios, pozzolan replacements and the like. Everything happens faster, so if it’s hot out and you aren’t using the enough of the right retarder, most of the concrete you just made could very well become a solid mass before you’re able to place it. At summer temperatures (above 80-85°F), non-retarded CSA concrete made with a w/c of 0.35 can set in as little as 5 minutes. With the right retarder that can be extended to 15 to 20 minutes of working time, with a few more minutes before setting takes place. While this seems very short as compared to OPC based concrete, using CSA’s requires a well-practiced, highly organized mixing, cleaning and casting process. It forces you to become efficient and organized. And that’s just plain good for business.

Suppliers

Currently two companies in the U.S. are suppliers of CSA cements: Ultimax and CTS Cement Manufacturing. Ultimax produces white, gray and a light gray version, and has a specialized retarder that provides extended working time. (Do not confuse the Ultimax product “Quix” with a pure CSA cement. Quix is a CSA additive. Read more here.) CTS manufactures a range of blended rapid-setting products, one of which is called “Rapid Set” cement. At this time only a gray version is commercially available from CTS.

 

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