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TECHNICAL PAPER COLLECTOR’S EDITION

anhydrous material, then per ml of hydrates that the total amount produced annually is less than
can be formed, this latter is related to their space 1/1000 of the production of Portland cement
filling capacity and so to strength development. The clinker, and only a small proportion of this is used
volume of hydrates for the aluminate compounds in construction. This is because the cost of CAC is
(monocalcium aluminate and ye’elimite) is shown more than five times the cost of Portland cement.
with and without added sulfate. Here it can be The higher cost of raw materials and production
seen that the figures for the two most important accounts for only part of this huge discrepancy.
Portland cement clinker minerals – C S and C S The other part lies in the number of people needed
3
2
– have fairly similar values. Notably we see that to support the use of this special material, in terms
C S only presents a saving of around 10% in CO of factories producing smaller amounts, and sales
2
2
compared to C S. This means that for belite rich and technical assistance for a specialised material
3
cements to represent a “real” CO saving, they that does not behave in the same way as the
2
have to have more than 90% of the performance reference Portland cement. Consequently the use
of alite rich cements. Despite intensive research of CAC can only be justified where their special
efforts, we are far from achieving this in terms of properties justify the higher cost.
the performance needed in modern construction.
This table clearly shows that the most interesting Calcium sulfo aluminate cements have gained a
minerals from the point of view of their associated lot of attention recently, as, in addition to the low
CO values are monocalcium aluminate (CA), the chemical CO of ye’elemite, the firing temperature
2
2
main component of calcium aluminate cements is lower and they are more easily ground. CSAs
(CACs), and ye’elemite or Klein’s compound (C A $), are a very diverse family of cements. They may
4 3
the main component of calcium sulfoaluminate contain from 70% ye’elemite down to 30% or less.
cements (CSAs or SACs). It is therefore worth The main other phase that is present is usually
discussing the viability of clinkers based on these belite. This leads to a two-stage development of
mineral in some more detail. properties, with the fast reaction of the ye’elemite
giving early strength and the belite hydration
The main problem is that while alumina is very giving later strength gain. Another complication
abundant in the earth’s crust, it exists mainly in is that the behaviour can be changed from rapid
feldspars and clays, where it is present alongside hardening to expansive by the amount of calcium
silica with Si:Al ~ 2 similar to that found in most sulfate added during grinding. Due to this diversity
portland clinkers. To obtain the desired alumina it is difficult to generalise, but it can certainly be
rich compounds, a more concentrated source of said that today, these are specialist products along
alumina is needed, typically bauxite. But bauxite the lines of CACs rather than cements for general
is not nearly as widely distributed as clay minerals construction. They have been produced in China
and is highly sought after for the production of for several decades and have been used for some
aluminium, making it is a much more costly raw large-scale constructions, yet to this day, the
material. But this is not the end of the story, as represent less than 0.1% of the cement produced
will be explained from the perspective of calcium in China and are mostly used for internal partition
aluminate cements. walls.

Calcium aluminate cements (CACs) have been in Personally, I am doubtful that CSA cements can be
commercial production for nearly 100 years, yet competitive with blended cements discussed below

136 The Indian Concrete Journal | November 2018