We thought you might like to get the “hot skinny” on the rudiments of portland cement hydration since that is the basic phenomenon that keeps us all in business. It is simple, yet extremely complex, because concrete has to accommodate billions of chemical changes to the portland cement and any supplementary cementitious materials in order to exist.
These changes begin the moment portland cement contacts water, when the heat of solution of that meeting causes a flash increase in heat that is followed by a dormant period when, for practical purposes, nothing happens. Sometime later, chemical reactions take off and calcium hydroxide (Ca(OH)2, abbreviated CH), begins to precipitate in water-filled spaces where it is accommodated without any pushing or shoving.
The lime (CaO) needed for that reaction product is from the two calcium silicates in the portland cement; tricalcium silicate (3CaO·SiO2, abbreviated C3S), and dicalcium silicate (CaO·SiO2, abbreviated C2S), that release lime as they react with water to form calcium silicate hydrates (3CaO·2SiO2·xH2O, abbreviated CSH). The lime and silica end up as ions “floating” in the water solution, and combine and precipitate from the solution to form the primary solid components of the paste, CH and CSH. Interestingly, as the CH precipitates from the water solution, it, in reality, becomes a calcium sink that renders the solution deficient in calcium. That, in turn, is replenished by calcium from the silicates, which then react to form calcium silicate hydrates. Curing is needed to ensure that water consumed by these reactions remains available for continued reactions.
Meanwhile, even before the calcium silicates hydrate, there are reactions involving calcium aluminates in the portland cement. These calcium aluminates are generally considered to exist as tricalcium aluminate (3CaO·Al2O3, abbreviated C3A). Because C3A almost violently reacts with water and causes very rapid set (with the liberation of a lot of heat), the reactions are negated by adding sulfate (SO4), usually in the form of gypsum (CaSO4·2H2O), to the portland cement when it is made. The sulfate reactions result in the formation of beneficial, weak, ettringite needles that are easily broken up during mixing.
The addition of gypsum can lead to premature setting problems because usually it becomes partially dehydrated during the grinding phase of cement production, resulting in a more readily soluble form known as hemihydrate (CaSO4·½H2O) that is more commonly known as plaster of Paris—the same stuff used for casts. Actually, the dehydrated sulfate form can be anywhere between the hemi-hydrate and a complete loss of water—the latter is called soluble anhydrite (CaSO4). Because these are more readily soluble forms of calcium sulfate than gypsum, they sometimes precipitate and create a false set without the development of heat. False set is usually unrecognized because it is easily worked-through during mixing, unless the mixing period is short. There also have been instances where low alkali portland cement, low cement sulfate levels, and lignosulfonate-containing admixtures have resulted in greatly prolonged set. It is always desirable to pretest portland cements with chemical admixtures at dosage rates that may be used.
Some years ago, ACI sponsored a session in New Orleans that became known as the “Slump-Loss Symposium.” Some of the presented papers are published in ACI's Concrete International, January 1979 issue. During that symposium, everyone was supposed to air their slump-loss problems to a panel of slump-loss experts. Some of them did. One group even commented that there are no slump-loss problems in Germany—whatever that may mean.
With the advent of differently formulated water-reducers, and midrange and superplasticizers, and the widespread use of ground granulated blast-furnace slag cement and fly ash, perhaps the time is ripe for another ACI session. And along with it, or separately, something on changes in the rhelogical properties of concrete, which includes placement and finishing problems. If you have problems of this type, you should let someone know—even us through this column.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 53 years ago.
Bernard Erlin is president of The Erlin Company (TEC), Latrobe, PA, and has been involved with all aspects of concrete for over 47 years.