Between the linings

During the last few years, the cement industry has undergone many changes in supply, demand and technology, all of which have impacted refractory supplies to the industry.

It is easy to see why cement is such an important market for refractory producers when considering the following: about 0.7kg refractory material is used to produce one tonne of cement. It will therefore come as little surprise that the cement industry, which manufactured 2,800m. tonnes in 2009, is one of key demand drivers for the refractories sector after the steel industry and accounts for 7% of total demand.

One of the main concerns for cement manufacturers is the lifetime of kilns, with refractory maintenance constituting nearly 50% of all kiln interruptions. Shutdowns, especially unplanned ones, destabilise the kiln operation resulting in unproductive hours. As a result, cement manufacturers look for refractories which ensure uninterrupted kiln operations and improve the performances.

Without carefully manufactured refractories, in terms of raw material selection for different bricks, the kilns cannot operate effectively. Much of the industry’s battleground is in this area and new minerals are central to this.

France’s Lafarge, the world largest cement
manufacturer, uses rotary kilns in its 121 plants
worldwide, including Cauldon in the UK (pictured).
Paul McMullin - Lafarge


Raw material

Cement kilns use basic and acidic refractories, consuming a variety of refractory minerals. Basic bricks are made either with very pure magnesia (MgO) or dolomite in all kiln zones where liquid phase (incipient molten calcium silicate) is present. The reason is that these raw materials offer enormous resistance to the corrosion of the silicates. As pure basic bricks are very brittle, aggregates are added to improve their structural flexibility against thermal shock such as fused spinel (MgO.Al₂O₃), zirconia, hercynite, and chromite.

For other areas, high alumina and fireclay bricks are used. If the temperatures are higher, a higher refractoriness and abrasion resistance is needed, therefore 70-85% high alumina bricks are the right choice.

The main acidic minerals are sillimanite, kyanite, bauxite, synthetically made mullite and andalusite.

“The choice of everyone will be a function of how those bricks have to resist to alkali attack,” Mauricio Ferreira de Pinho, global commercial vice president of Brazil’s leading magnesite and refractories producer Magnesita Refratorios SA, explained to IM.

On a volume basis, fireclay bricks/castables are the most commonly used. They are used in static kiln parts. It is important that they offer the lowest Al₂O₃ content possible. The closest they are to 35% Al₂O₃, the better their performance. Low alumina bricks are very resistant to alkali attack and the main minerals used are flint clays, high grade kaolins, and plastic clays.

Some special castables with silicon carbide additions are required for areas where there is simultaneous abrasion and alkali attack. The main mineral is silicon carbide formed out of a mixture of silica sand and petroleum coke in special furnaces.

The insulating bricks, used in the static part of the kiln, are calcium silicate based or use ceramic fibres.

The industry is seeing a new trend of using some new industrial minerals in cement refractories, “especially after the massive use of alternative fuel”, revealed Ferreira de Pinho. The main minerals are zirconium, fused spinel with big crystal size, hercynite, galaxite, zirconia, and silicon carbide.

One of the challenges in raw material supply remains the price and quality of product.

“If on the one hand the availability of good raw material is decreasing, on the other, the prices are constantly increasing,” Ferreira de Pinho said.

For instance, some of the biggest challenges are to get access to pure magnesia with low silica (<0.4%) and a CaO/SiO₂ molar ratio from 2.5 to 3.0; or getting access to a doloma with low iron contend.

There are also major concerns over the availability of some refractories minerals. “Bauxite is available in very few countries, such as Guyana, China, and is in the hands of some specific suppliers. Andalusite is also a problem as very few countries have deposits,” he added, also pointing out that finding chromite of good quality was a problem since the Philippines mines were exhausted.


Market comparison of specific consumption of refractories 1950-2008



Source: Shinagawa Refractories Australasia Pty Ltd


World top 10 cement producers

Doloma derived from dolomite was the first chrome-free basic product that was used on a regular basis in the burning zones of rotary cement kilns. Their use was well established before hexavalent chrome issues associated with the magnesia-chrome brick became a major concern in the early 1980s. As these concerns grew, many cement plants converted to the use of doloma brick, a proven chrome-free alternative.

At present, doloma bricks account for about 70% of all bricks used in the sintering zones of cement kilns in North America. More recently, their use has expanded into lower transition zones and parts of the upper transition zone of rotary cement kilns.

Donald Griffin and Richard Knauss, from LWB Refractories USA, highlighted in a presentation at the Unified International Technical Conference on Refractories (UNITECR) 2009 in Brazil last October the latest improvements in the technology.

The new developments use a range of industrial minerals including magnesia enrichment (both fused and sintered), and zirconia additions. These minerals are reported to enable improved performance through lower porosity, to reduce permeability and to improve resistance to corrosion by alkalis and sulphur compounds.

Zirconia for thermal shock resistance

As Griffin and Knauss reported, the introduction of zirconia to improve thermal shock resistance was one of the first major improvements which boosted the use of dense doloma bricks in rotary cement kilns.

Magnesia and doloma-based refractories have a high thermal expansion coefficient, which results in a lower thermal shock resistance. A spall inhibitor that is chemically compatible with doloma is required, which excludes additives such as spinel, alumina, or chrome ore owing to the reaction with the lime phase in doloma and the formation of low melting compounds.

Zirconia aggregate additions were found to be an ideal material for improving the thermal shock resistance of doloma-based brick. Zirconia reacts with the lime phase to form calcium zirconate which is a very refractory material (mp ~ 2,340ºC). This reaction during the firing of the doloma brick causes micro cracks to form, resulting in a significant improvement in thermal shock resistance.

“Before the use of zirconia, doloma brick could not be pressed to a high density without significant sacrifices in thermal shock resistance. Additions of zirconia to the doloma-based brick allowed for the production of denser product with lower permeability and porosity levels and good thermal shock resistance,” Griffin said.

The loss of coating can cause the brick to be suddenly exposed to higher temperatures, resulting in a thermal shock. This development was the first critical step in extending the use of doloma brick into areas of the kiln where less stable coatings are encountered.


Main minerals used in cement kiln refractory bricks

Source: Adapted from Refratechnick data


Magnesia additions

Griffin and Knauss also reported that magnesia could be added to the doloma brick in order to improve volume stability and corrosion resistance.

A unique property of the doloma brick is its ability to deform without cracking. In a compressive, high temperature load test (1,550ºC, (0.17 N/mm²) a doloma brick can deform more than 10% without cracking while magnesia based systems will crack at over 1% deformation.

A magnesia doloma brick with zirconia was developed to improve the volume stability and prevent this phenomenon. The addition of magnesia to the matrix of the brick dilutes and disrupts the continuity of the lime phase and reduces the amount of deformation that takes place.

“Magnesia-doloma based products having an MgO level of 60% have performed very well in lower transition zones, particularly in kilns where bricks are exposed to high thermal loads,” Griffin explained.

While doloma bricks are compatible with lime, reaction with the alumina, silica and iron oxide components of cement clinker or coal ash can take place. These components can react with and flux the lime phase in doloma in addition to reacting with the calcium silicate secondary phase in the magnesia that is used in both doloma and magnesia-spinel bricks.

To further enhance the refractoriness and corrosion resistance of magnesia-doloma bricks, fused magnesia is added. The addition of sintered and fused magnesia also improves the corrosion resistance of magnesia-doloma bricks to alkali sulphates and chlorides as a result of the overall reduction of the lime phase. The improvement in resistance to corrosion by alkali sulphates and chlorides allows magnesia-doloma brick to be used in the lower part of the upper transition zone where the stability of the clinker coating is susceptible to shifts in the burning zone and condensation of volatiles may occur.

Potassium and sodium are basic in nature and do not cause adverse reactions with doloma bricks but they can form sulphate and chloride salts that condense in the porosity of the brick. Problems can occur when substantial amounts of free sulphate and chloride are present and react with the free lime associated with the doloma grain, with an adverse effect on the direct bonding within the brick.

The magnesia addition to the brick can also result in a reduction of the level of recarbonisation of the lime that is observed in doloma-based bricks during the process. Recarbonisation of the lime phase can result in densification of the brick and make it more prone to thermal shock.

Other approaches taken to limit the reaction between alkali salts and lime in doloma bricks are the use of an in situ calcium zirconate bonding phase and tar impregnation.


Refractory markets by consuming sector



Source: Ian Wilson


Asian opportunities for cement

The last two years have been tough for the cement industry which remains the second biggest market for refractories.

Since 2008, the cement industry has been severely impacted by the global downturn which saw construction suffer badly. World consumption decreased by 1.7% between 2008 and 2009 to 2,650m. tonnes last year.

But world consumption patterns were not uniform. Major developed economies such as Europe, the USA and Japan were bogged down in 2009, resulting from the depressed housing markets and other related industries. As a consequence, top cement producers such as France’s Lafarge, Switzerland’s Holcim Ltd and Mexico’s Cemex SA all had to reduce output between 2008 and 2009 (see table p. 47).

In addition, heavy snowfall at the end of 2009 and at the beginning of 2010 restrained construction activity in Europe and North America, as Holcim reported in its Q1 2010 results.

However, Asia and the Middle East resisted quite well even growing consequently during this period.

In India, cement production grew by 12.37% in 2009-2010 to 160.3m. tonnes against 142.6m. tonnes the year before. The Cement Manufacturers Association reported that cement sales were up by 12% to 159.4m. tonnes as compared to 142.2 m. tonne in 2008-2009. In March 2010, production and sales of cement increased by 9% and 9.3% respectively over the same month in 2009.

According to Research and Markets, Chinese cement production amounted to 1,630m. tonnes in 2009, rising by 17.91% on a year-on-year basis. In 2009, Chinese cement export volumes reached 15.61m. tonnes and a value of $687.19m., dropping by 40% and 37.5% compared to the year before ; the average export price was $44.02/tonne, rising by $1.83/tonne on a year-on-year basis.

Thanks to strong Chinese domestic demand for cement and robust profitability of cement enterprises, Chinese cement industry continued high-rate of expansion in 2009.

By the end of 2009, there were about 420 cement production lines under construction and over 140 production lines waiting to be started in China. After the completion of the production lines, its cement production capacity will be increased by 800m. tonnes and the annual cement production capacity will be 2,700m. tonnes.


Cement rotary kiln showing typical refractory lining options



Source: LWB Refractories GmbH, IM


Outlook: slow recovery and R&D focus

“We are seeing some recovery on the cement market but this varies from region to region,” Magnesita’s Ferreira de Pinho said to IM.

A recent economic forecast from the Portland Cement Association (PCA) suggests that increased public construction activities will pave the way for improved cement consumption in the USA and Canada in 2010.

The association, which represents cement companies in both countries, is anticipating a modest 5% increase in consumption over the severely depressed levels of 2009 (IM 11 May 2010: Pick up for N American cement).

Edward Sullivan, PCA chief economist, commented: “The 2010 recovery in cement consumption lays largely on expectations for public construction activity. Spending from the stimulus bill will more than double to $12bn. and that spending is expected to reflect an increased share of major highway construction and bridge projects high cement-intensive projects.”

The 3-5m. tonnes gain in cement use will materialise during the second half of the year estimates the organisation which predicts a 13.3% jump for 2011, followed by an 18.7% increase in 2012.

Mike Betts, Jeffries’ UK-based building materials analyst, remains cautious in his outlook for the cement industry this year, saying 2011 will be the earliest before any significant recovery will start. “The economy is recovering and improving its core fundamentals. However, recovery for the construction markets will be slowed by the continuation of tight lending conditions, high foreclosure rates and weak job markets,” Sullivan said.

On the refractories side, manufacturers will have to deal with the fact that cement manufacturing is becoming more and more aggressive, with some alternative fuels that bring different behaviour and corrosion systems to the refractories.

The industry could be seeing the beginning of an increased focus on the cement market. Therefore, refractories companies will have to centre their attention on research and development in this sector in order to fulfil the more demanding expectations of the market. With cement rebounding quicker and considering its integral use as an industrial material, the cement refractories sector could become more competitive in the medium term.


Cement at a glance

World production

• 2,800m. tonnes (2009)

World consumption

• 2,650m. tonnes (2009^e)
• 2,700m. tonnes (2008) (-1.7%)
• China and India account for about 58% of world consumption
• Forecast 2010: 3.7% increase to 2,748m. tonnes expected

Main producers

• China, India, USA, Japan, Korea

Main consumers Europe

• Spain, Italy, Turkey, Germany, France

World refractories consumption for cement

• 1.96m. tonnes (2009^e)

Top world cement producing countries

* Excluding Puerto Rico
Source: CEMBUREAU, IM