Can I Use Non-Shrink Precision Grout to Create Concrete Countertops?

Don’t even think about using non-shrink precision grout to create concrete countertops. While its properties may make it seem like an ideal choice at first glance, avoid the temptation to use it. I’ll explain why…

One reason that people are tempted to use non-shrink precision grout (also called NSPG) on concrete countertops is its name.  “Non-shrink” could have some big advantages. Secondly, its compressive strength is often very high. Typical compressive strength values are 8,000 psi to 14,000 psi at 28 days, with 1 day strengths in excess of 3,000 psi. Other factors are its availability and reasonable price.

However, “non-shrink” is a misnomer. This article explains why and what applications are appropriate for NSPG. Concrete countertops are not one of them.

What is Non-Shrink Precision Grout?

NSPG is an all-sand mix characterized by good flowability, very high compressive strength and under ordinary use no net shrinkage. Working time is often short, with set times of about an hour or so, but this can vary with different formulations.

NSPG becomes “non-shrinking” due the use of Type K expansive cement. Type K cement expands soon after setting. The “non-shrink” properties are a bit of a misnomer and can be confusing. All concrete shrinks to some degree or another over time as it dries out. However, the net shrinkage that occurs is what matters in the normal applications of NSPG. The expansive cement expands for a short period of time after it sets. This initial positive expansion exceeds the anticipated shrinkage that will occur over time when the NSPG is used for what it is intended. Thus there is still some expansion remaining. The concrete first expanded a lot and then shrank a bit less, so the net result is nearly no shrinkage. Hence “non-shrink”.

NSPG is used when setting machinery bases, generators, mills, presses, compressors, etc., structural grouting of precast columns, steel columns, and anchoring of sign posts, anchor bolts, and dowels. It is essentially a structural filler in these applications.

In the ordinary applications that NSPG is intended for, its expand-then-shrink behavior is tolerable. However for countertops, the fact that the NSPG shrinks at all is not tolerable. We will explain this later. First, let us consider the question: Why does “non-shrink” precision grout shrink at all?

How Does It Work?

Most NSPG is a blend of expansive cement, finely graded sands and other proprietary ingredients. The proportions of cement to sand are much higher than in ordinary concrete. When used as a structural filler, this blend creates a very creamy, highly workable (sometimes pourable) mix with high cementitous content (paste). When concrete shrinks, only the paste shrinks, so a higher paste volume results in higher shrinkage. NSPG attempts to counteract the anticipated shrinkage by first expanding.

NSPG and Concrete Countertops

This is part of the problem with using NSPG for countertops. With the oridnary applications that NSPG is intended for, usually not much of the grout’s surface area is exposed to the air, so drying occurs slowly. With a countertop made out of NSPG, there is a huge amount of surface that can dry out. Thus any early expansion that would normally counteract the inevitable shrinkage is overwhelmed by the excessive amounts of shrinkage caused by rapid and continuous drying.

Countertops are large, flat beams with large amounts of surface area exposed to the air, so drying becomes a serious issue. As concrete dries and loses moisture, the capillary tension created by the water in the fine pores causes volumetric shrinkage. It is the loss of internal water that causes the shrinkage. The longer the concrete is allowed to dry, the more shrinkage will occur. For larger pieces of concrete, drying shrinkage can continue for years. Even very thin pieces of concrete will continue to shrink over time, as will be seen below.

Testing

To investigate the shrinkage potential for the non-shrink grout, I cast several 5’ long, 1” thick beams with reinforcing steel extremely close to one face (the cast side).

The beam design was chosen to exaggerate the curling tendency of each mix. Long, thin slabs exhibit more curling than short, thick slabs. Shrinkage causes curling, either from differential drying or differential restraint, or both. Differential drying occurs when one side of a slab dries out faster or more than the other side. The side that is drier shrinks more, so the slab curls towards the dry side. Differential restraint occurs when one side of a slab is allowed to shrink more than the opposite side. The reinforcing steel keeps its side from shrinking much, whereas the side that has no reinforcing shrinks more. When both factors are working together, large amounts of curling occurs. (This is why it is important to moist cure slabs properly so that they don’t curl.)

The beams were first allowed to moist cure for 3 days before being stripped and then set out to air dry, reinforcing side down against a smooth impervious surface. These conditions are considered very extreme and were selected to maximize the amount of curl that would occur.

Curl measurements were periodically taken on the back side (the smooth cast side). A 6’ straightedge and a dial caliper were used to measure the growing gap between the slab ends and the straightedge. All measurements are changes relative to the slab geometry measured immediately after stripping.

In the graph, samples “D” and “F” are a commercial non-shrink precision grout mixed and cast according to the manufacturer’s instructions. Sample “D” had a water to cement (w/c) ratio of about 0.30, and sample “F” had a w/c of about 0.43.

NSPG GraphThe other samples, “AA” through “S”, represent comparable beams cast from various commercial concrete countertop mixes and my own from-scratch mix designs. These samples had w/c ratios that varied from 0.35 to 0.45.

Because the non-shrink precision grout had a higher cement to aggregate ratio than all of the other mix designs, the slabs made with the NSPG exhibited the most amount of curl, even though the w/c ratios were similar, or even lower than, the other mixes.

Two things become clear: first, lowering the w/c ratio is only part of solution to reducing shrinkage (and thus curling). Secondly, shrinkage (and therefore curling) occurs for a long period of time after casting, and that significant amounts of curling occur even after 14 days.

Better mix designs (those that minimize shrinkage tendencies through careful aggregate gradation and optimum cement contents) show lower overall shrinkage and stabilize sooner than the NSPG, which has large amounts of cement paste and a narrow range of fine aggregates. For these reasons and all of the reasons explained in this article, I do not recommend NSPG for use in concrete countertops.

Types of portland cement used in concrete countertop mixes

Portland cement comes in a variety of different types. In the United States, these types are classified as Type I, II, III, IV and V. Only Types I and III are necessary for consideration by concrete countertop fabricators; the benefits of Type II cement are generally irrelevant to the concrete countertop industry.

Type I is ordinary portland cement, and it is available in white or gray.

Type II is a moderate sulfate resistant cement, important when concrete is cast against soil that has moderate sulfate levels.

Type III is a high early strength cement. It is ground finer and reacts faster than Type I, so the early strength gains are greater. However the ultimate strength is not higher than Type I. Concrete made with Type III will have slightly higher 28 day strengths than concrete made with Type I, all else being equal. Type III is available in white or gray, but white Type III is difficult to find in small (less than pallet) quantities; it often has to be special ordered.

Type IV and V are often used in special construction applications where high sulfate resistance is required or a low heat of hydration is important. Neither of these types are practical choices for countertops.

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.

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 Buzzi Unicem CSA 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 Buzzi Unicem CSA, 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 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 CSA cement where 20% of the cement was replaced by VCAS. (Note: The cement brand used, Ultimax Cement, no longer exists. The only 100% CSA cement available now is CTS Rapid Set® Cement.)

Test results showed that all of the mixes had a 30% loss in 1- and 7-day compressive strengths versus mixes that used only 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 two manufacturers of calcium alumino silicate (CSA) cement products in the United States: CTS Rapid Set® Cement and Buzzi Unicem. Though they are both 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 USA makes a CSA cement additive 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. It cannot be used at a 100% replacement for portland cement; the recommended replacement rate is 5% to 20%.

In April 2009, The Concrete Countertop Institute conducted standard concrete compression tests to compare the early compressive strength of concrete cylinders made using CTS Rapid Set® cement and Portland cement. Buzzi Unicem CSA was not tested, since it is a CSA additive, not CSA cement. At the time, another 100% CSA cement was available, Ultimax Gray cement. This product is no longer available.

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 dose of 0.5%. (The retarder used was Ultimax Delay, which is no longer available. See this article on retarding CSA cements.) 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

Note that there used to be another CSA additive called Qwix. Buzzi Unicem CSA can be used exactly like Qwix.

Differences between CSA cements and CSA additives in concrete countertops

CSA-based cements, which I refer to as CSA cements, such as 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.

Another CSA-based product, Buzzi Unicem USA’s product called simply “CSA”, is NOT a cement. It is 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.

Note that another company, Ultimax, used to manufacture both a CSA cement and a CSA additive (Qwix). These products are no longer available.

 

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

CSA cement is available only in a light tan/buff color. White is not available. 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 portland cements. 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

Click here for an article about suppliers of CSA cement.

 

Using different mixes in a single concrete countertop slab

I recently got the following question regarding my blog post “Mix design for cast in place concrete countertop in Cayman“:

Q: Is it possible to use gray cement for the core and white cement as an outer layer?

White and gray portland cement are very similar to each other, and can be safely blended together. You can use one color cement for a core mix and a different color cement for a shell mix, provided the mix designs and the water/cement ratios are identical.

However, I would hesitate to use a gray cement core with a white cement “shell” for aesthetic reasons, especially for projects like the cast-in-place gazebo I did in Grand Cayman, and for regular precast “wet cast” projects. The simple reason is that it’s very impractical to pour a core mix without contaminating the form surfaces, contain the core mix so that it stays where you want it, and to pour the visible shell mix and still end up making good concrete without voids or weak zones. The minor cost savings in cement (white cement in Cayman is about US$20 per bag, vs gray cement at US$10 per bag).

The exception is when an ultra-expensive pigment is involved. In that case it is sometimes worthwhile in cost savings to take the extra trouble to use the pigment only in the outer layer.  I call this “buttering”. It is easier to do with a hand packed precast method, somewhat easy with GFRC, and hardest with wet cast precast or cast in place. Be aware, however, that some exotic pigments could affect the properties of the mix to the extent that you have issues with the 2 layers bonding.

Tips for “buttering”:

  • The first layer should be well compacted, of even thickness and fairly smooth. Loose, clumpy or uneven concrete will create weak zones that could lead to cracking or delamination.
  • The second batch of concrete should be the identical mix (but without glass or stone if used in the first mix).
  • It’s very important to use the same water content, and merely add more superplasticizer to make the second batch more workable.
  • It’s important to place the second batch before the first becomes hard or dries out.

For the gazebo job I did in Grand Cayman (8000 lbs of concrete), the extra price from using all white cement added up to only about an extra $100 in cost. Consider the need for 2 separate mixers, the extra labor and time needed to wrangle two mixes, and it’s easy to see that at times it’s simpler to bite the bullet and pay a bit more up front to make things simpler, and in the end, better and less expensive overall.

Here’s an example of a GFRC piece done with red outer shell (mist coat) and white core (backer). This was done just for illustration purposes. You can see in the second photo that some of the white showed through when the mist coat chipped.

gfrc red mist white backer

gfrc chip in mist coat

Curing: An essential step to create high quality concrete countertops

Curing. We all know it’s important, but what exactly is it, why is it important and what factors affect curing?

Adding water to portland cement starts a chemical reaction called hydration. As hydration proceeds over time, the portland cement and water are transformed into beneficial calcium silicate hydrate (CSH) compounds. These compounds are the glue that hold the aggregates together, creating the hard, solid material we know as concrete. There are other compounds that form during the hydration process, but they are not responsible for strength.

Portland Cement + Water = CSH (provides strength)

Curing is the process of maintaining moisture levels inside cast concrete so that hydration can continue. As long as free moisture and unhydrated cement exist inside the concrete, the strength, hardness and density will gradually increase. Practically speaking, curing is simply the process of keeping the hardened concrete moist so that it can continue to gain strength.

As the concrete gets stronger and denser, its porosity decreases. This is important, because early on the concrete is much more porous than when it’s older and has hydrated longer. Porous concrete loses moisture to evaporation quickly, and this can lower internal moisture levels and stop hydration. If the concrete dries out, it stops gaining strength. This is why it is so important to cover your concrete right after casting and keep it moist. When concrete dries out, it dies, just as a tomato seedling would die if it weren’t watered.

tomato seedlings watering can

When concrete is mixed, all the water needed for full hydration is present in the mix design. Often contractors add more to the concrete than needed for hydration, to make the concrete more workable. This extra water is called water of convenience. This extra water causes the cement particles to be too far apart to knit together into a strong matrix. It results in a longer set time and lower strength.

snowballs

Cement particles that are too far apart can’t knit together.

Very powerful superplasticizers make it possible to remove almost all of the water of convenience, leaving a little bit more than just the water needed for hydration. This is the ideal blend of just enough water for hydration, but not so much water that the cement particles are spaced too far apart.

It’s actually rare that concrete is cured until most of the cement is hydrated. This generally takes months or years to occur. Rather, the concrete is cured for as long as you need it to be to reach the desired strength. The length of curing time can vary widely depending upon the structure or item made out of the concrete, the mix design, the concrete’s temperature and the desired strength at a certain time, to name just a few factors.

For concrete countertops, clients are not willing to wait 28 days for their concrete to be delivered. Because of this, mix designs tailored for concrete countertops have high early strengths so that the concrete can be cast, cured, processed and delivered in a couple of weeks (or less). For example, ordinary construction grade concrete often achieves a compressive strength of 4000 psi in 28 days. It’s not unusual for a mix design for concrete countertops to reach that strength in only 2 days. With some advanced mixes, this can be achieved in a matter of hours. So for a particular design strength, different mixes require different curing times.

Another factor that will influence the curing time is temperature. Colder concrete gains strength much slower than warmer concrete. At 3 days after casting, concrete cured at 45 degrees F only has about 70% the strength of the same concrete cast at room temperature (73 degrees F). In contrast, concrete cast and cured at 90 degrees F has about 10% more strength than concrete cured at 73 degrees F. Over time these differences gradually become smaller, but often it’s the early (2 to 3 day) strength that is more important than the 28 day strength.

So remember, don’t use more mix water than you have to, keep your concrete evenly moist for at least the first couple of days, heat it if necessary. All this will allow your concrete to cure and strengthen. You will end up with higher quality concrete countertops and happier clients.

Portland Cement Type I, II, III: Which to use in a concrete countertop mix?

Portland cement comes in a variety of different types. In the United States, these types are classified as Type I, II, III, IV and V. Only Types I and III are necessary for consideration by concrete countertop fabricators; the benefits of Type II cement are generally irrelevant to the concrete countertop industry.

Type I is ordinary Portland cement, and it is available in white or gray.

Type II is a moderate sulfate resistant cement, important when concrete is cast against soil that has moderate sulfate levels.

Type III is a high early strength cement. It is ground finer and reacts faster than Type I, so the early strength gains are greater. However, the ultimate strength is not higher than Type I. Concrete made with Type III will have slightly higher 28 day strengths than concrete made with Type I, all else being equal. Type III is available in white or gray, but white Type III is difficult to find in small (less than pallet) quantities; it often has to be special ordered. Given this, it is best to stick with Type I cement.

Type IV and V are often used in special construction applications where high sulfate resistance is required or a low heat of hydration is important. Neither of these types are practical choices for countertops.

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