Blended cement

The activation theory of slag in blended cement

As it is well-known, the activation of latent slag in blended cement is implemented by calcium hydroxide (CH, Ca(OH)2) which is the main cause maintaining the high pH value of pore solution.

The activation happens as the break of glass layer of slag particles. However, how does the break take place in detail? Researchers have already proposed a theory to explain the process, which will be discussed in the post. The main composition of glass is SiO2, thus the break of glass layer is actually the break of chemical bond between Si and O (Si-O). In the first step, the bond Si-O is broken by hydroxyl,


Then the other part of broken bond Si-O further react with water (H2O), as the following reaction equation,


By the second step, hydroxyl is returned. The break of Si-O bond is finished. Therefore, the two steps can be combined into one equation,


From the overall process, it can be seen that hydroxyl acts as activator and is not consumed.

This is the theory which explains the activation of slag by CH in slag blended cement.

Blended cement

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.