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.
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.
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.
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.
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.
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.