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