Tag Archives: Portland cement

What is cement and its history?

Cement, the gray and fine powder, is the most used construction material on the world. However, how many people do they know what cement is? the answer for this question from Wikipedia is as follows,

In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word “cement” traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder.

The reason why cement can sets and hardens independently is its hydraulic property. When cement is mixed with water, the chemical reaction occurs and produces hydration products, such as C-S-H gel, calcium hydroxide, ettringite and monosulfoaluminate. The C-S-H gel is the main material to bind different particles and resulting in the engineering strength which is needed as a construction material.Since the set and hardening process of cement is chemical reaction, so we can get cement set under water.

What is cement?

Cement (Source: Wikipedia)

As for the chemical composition of cement, there are four essential elements in cement, which are Calcium, Silicon, Aluminum and Iron. The four elements form four clinkers, namely,

  • Tricalcium silicate (3CaO.SiO2), (50-70%)
  • Dicalcium silicate (2CaO.SiO2), (15-30%)
  • Tricalcium aluminate (3CaO.Al2O3), (5-10%)
  • Tetracalcium aluminoferrite (4CaO.Al2O3.Fe2O3), (5-15%)

If you would like to know the production process how cement is made, refer to this web page or the video below.

It is common to see another name of cement that is Portland cement, because concrete made with cement resembled natural stone from the Isle of Portland. It worthy to note that first cement is produced by early Greeks and Romans from volcanic ash mixed with slaked lime. Unfortunately, this art was lost during the Middle Ages. The modern cement, Portland cement, is developed in England by bricklayer Joseph Aspdin in early 1800’s.

Hydration products of slag in blended cement

Many XRD analysis results have shown the main hydration products of slag blended cement are essentially similar with that of pure Portland cement, except the amounts of CH found by this method or other are in varying degrees and less than those that should be given by the pure Portland cement constituent if the slag part did not participate the reaction.

The main hydration products of the slag-cement are C-S-H gel, Ca(OH)2, the sulpho-aluminate hydrate phases AFt and AFm and a Mg, Al-rich hydroxide phase.

TEM slag
Fig. 1. Transmission electron micrograph showing foil-like Op C-S-H in a water-activated slag paste hydrated for 3 1/2 years at 40°C (W/S = 0.4) (by I.G Richardson: The nature of C-S-H in hardened cements).

TEM cement slag
Fig. 2. A TEM micrograph that illustrates fine, dense Op C–S–H in the paste containing 75% slag. (by I.G Richardson: Composition and microstructure of 20-year-old ordinary Portland cement–ground granulated blast-furnace slag blends containing 0 to 100% slag).

In the case of C-S-H, its morphology and composition may be modified by partial accommodation of M and A within the micro- or nanostructure, its Ca/Si ratio is then lowered (e.g., 1.55) than that formed from alite and belite (e.g. 1.7). Hydrotalcite-like phase with approximate composition Mg6Al2(OH)16(CO3)·4H2O is formed from the MgO content of BFS, typically 5-9%.

As stated above, the hydration products can be classified into inner product and outer product. Inner product C-S-H from cement grains has a Ca/(Si+Al) ratio similar to outer product C-S-H, Meanwhile, Katoite (C3ASαHβ, α < 1.5) has also been suggested as a slag product, but it is less documented than the former mentioned species.

Activation of slag in blended cement

The latent hydraulic property of slag means no hydration products could be observed when slag is solely placed in contact with water, though surface analysis using X-ray photoelectron spectrometry showed that the surface of slag was modified as soon as it came into contact with water making a protective film lacking Ca2+ that prevent further reaction, thus, slag dissolves to a small extent.

The hydration of slag and cement is more complex than that of Portland cement; given that both slag and cement take part in the reaction with water.

The glass network of slag is mainly the cause of its latently hydraulic properties. Adding activators slag could be activated to hydrate. There are two methods known to activate slag, viz. the addition of high amounts of calcium sulphate and alkali activation. The drawback of the former method requires long curing period and is sensitive to carbonation and frost attack, whilst the latter has excessive shrinkage and efflorescence as well as the intensive energy consumption in the production of alkaline activator.

Blended with Portland cement, slag can be activated with a relatively high pH solution of blend system caused by the hydration phase CH, which is one of the main hydration products of C3S and C2S in Portland cement. slag has been successfully used with Portland cement to produce blended cements. As soon as water is added into the slag and Portland cement system, the latter begin to hydrate and a small amount of slag reacts as well due to the existence of gypsum in cement.

As the hydration of Portland cement proceeds, more and more alkali is available in the pore solution resulting in a high pH value (e.g. 13), which greatly activates the hydration of slag. Later, as the formation of CH proceeds due to the hydration of alite and belite, the hydration of slag is activated by CH, which results in microstructure development and strength gain of paste; however, the content of CH in the blended system increases initially and then decrease showing CH not only acts as an activator for the hydration of slag but a reactant.

It is noted that this method is only feasible with limited proportions of slag blended in cement. If very high proportion of slag is blended, the early strength may be markedly reduced, which is the major concern for application in the construction industry, and it is aggravated when a coarse slag is used.

Research conducted by F. Bellmann shows that activation is also possible by lowering the pH to a range between 11.8 and 12.2 by the addition of calcium hydroxide and soluble calcium salts which include calcium chloride, calcium bromide, calcium nitrate, calcium formate, and calcium acetate. By the mixture of the two additives, CH acts as reactant precipitating aluminium in an AFm phase, meanwhile, due to its presence, soluble calcium salts can decrease the pH value in pore solution as long as the equilibrium condition of calcium hydroxide is maintained.