Articles on the cement industry from Global Cement

The cement industry worldwide is facing growing challenges in the context of saving material and energy resources as well as reducing its CO₂ emissions. The International Energy Agency highlighted in its 'Road Map for the Cement Industry' that the main levers for the cement producers are the use of alternative materials, be it as fuel or raw material and in addition the reduction of the clinker/cement factor by utilisation of well tried and proven materials like slag, fly ash, pozzolanas or limestone fines. This underlines that in the years to come cement will depend on OPC clinker to a high degree. New cements will therefore most certainly first take into account higher amounts of main constituents besides clinker which show pozzolanic or latent hydraulic properties.

Artificial materials that originate from natural or industrial resources but require additional thermal treatment and/or activation may also have a role to play. It is not clear at present to what extent cements based on magnesia can play a role. On the other hand, sulphoaluminate cements may have a significant role to play. Unfortunately, due to their specific raw materials as well as their performance in concrete they will most probably not be able to substitute relevant parts of today's cement markets.

Cement production has undergone a tremendous development from its beginnings some 2000 years ago until the present day. While the use of cement in concrete has a very long history, the industrial production of cements started in the middle of the 19th century, at first with shaft kilns. These were later on replaced by rotary kilns as standard equipment worldwide. Annual global cement production has reached some 2.8Bnt/yr and is expected to increase still further to around 4Bnt/yr. Major growth is forecast in countries like China and India as well as the Middle East and Africa (MENA), (Figure 1). At the same time, the cement industry is facing challenges such as increased energy costs, requirements to reduce CO₂ emissions and problems of sourcing raw material of sufficient quality and quantity.

The World Business Council for Sustainable Development and its Cement Sustainablity Initiative (CSI), comprising global cement producers, has initiated a project called 'Getting the Numbers Right,' which for the first time provides a good database for most of the global cement industry with respect to CO₂ and energy performance. Figure 2 shows the energy performance of global cement productions covered by the CSI members.

Alkali-activated 'geopolymer' concrete has been commercialised in Australia under the trade name E-Crete^(TM) and is now finding acceptance among the end-user community and from regulatory authorities. E-Crete is derived from fly ash and blast furnace slag with proprietary alkali activators and is available in both precast and pre-mix forms. The pre-mix concrete is able to be placed using largely standard concrete processing equipment and expertise. A life-cycle analysis of E-Crete has shown savings of around 80% in CO₂ emissions compared to a standard OPC-based binder, which provides the primary driver for the uptake of this technology on a larger scale. Commercialisation of geopolymer technology by Zeobond has been linked closely with both scientific research in this area and a broad process of industry and stake-holder engagement. The combination of these two activities will be highlighted throughout this article.

Introduction

Concrete made from OPC including its blends with mineral admixtures is second only to water as the commodity most used by mankind today (1). Global OPC production in 2008 was around 2.6Bnt (2), corresponding to around 11Bnt/yr of concrete (3). The cement industry contributes conservatively 5-8% of global carbon dioxide (CO₂) emissions (4), mainly through decomposition of limestone and combustion of fossil fuels during cement production. Grinding and transport are lesser but also significant contributors to the environmental footprint of the cement industry. With rapidly increasing demand for advanced civil infrastructure in China, India, the Middle East and the developing world, the cement and concrete industries are expected to expand significantly (5).

The future cement industry is now coming to grips with the fact that meaningful production of alternative binders will form part of a carbon constrained industry, aiding to significantly reduce CO₂ emissions and provide some advantages in performance only offered by these alternative binding systems (6 - 7). Usually the driver for competition has been cost reduction, in which case alternative materials starting from a low volume basis can never compete against large-scale OPC production. Abatement of CO₂ and technical features are now forming a major role in growth of alternative binder systems.

There are various possible alternatives to OPC technology which have attracted attention in different parts of the world (8). Calcium sulphoaluminate cements are increasingly being used and studied and contain binding phases based mainly on Klein's compound (ye'elimite) (9 - 10). In addition, there are two major types of alternative binders that have not been commercialised widely, being the alkali activated material (AAM) system and also magnesium-based systems.

In AAM chemistry, the reactive aluminosilicate phases present in materials such as fly ash, slag, calcined clay or volcanic ash are reacted with alkaline reagents including alkali metal silicates, hydroxides, carbonates, and/or sodium aluminate (11) to form zeolite-like aluminosilicate gel phases of varying (but generally low) degrees of crystallinity. AAM concrete has been shown to be quite resistant to attack by acids and by fire and does not produce the high reaction heat associated with OPC concrete, which reduces cost and potential cracking issues when the material is placed in large volumes (12).

Magnesium-based cements (including oxide, phosphate, oxychloride and other specific types of phase assemblage) have been used in niche applications and can also give superior fire resistance, with much lower CO₂ emissions than OPC. Phosphate cements have not been used commercially and require more research and development, but in general the magnesium phosphate system has technical and economic limitations compared with AAMs. The focus of this article will therefore be on the commercial application of AAM technology, and in particular its development via the E-Crete^(TM) technology, which is now available in Australia in precast, pre-mix and in-situ cast formats.

When a cement plant in the UK experienced operational problems with the drag chain system supplying biomass-derived fuel to its kiln, UK-based John King Chains Ltd (John King) was able to help.

A UK cement plant operated a drag chain system for the transfer of a biomass-derived fuel into the kiln. The drag chain system pulled the material a total of 80m along a horizontal section and then up an inclined section.

Since the commissioning of the plant in the recent past, the plant operator had experienced operational issues with the drag chain system.

Site visit

A meeting with representatives of the cement plant operator was arranged to discuss the experiences of the existing and inadequate chain system, establish the symptoms and generate initial theories and hypotheses. This included a site visit to investigate the current system, physically examine drag chain components, gather information, identify the cause(s) of the operational problems and formulate solutions.