Articles on the cement industry from Global Cement

This article, based on a paper presented at the 13th International Congress on the Chemistry of Cement at Madrid in July 2011, describes a radical, step-change approach to reducing CO₂ emissions from cement manufacture.

A highly novel method of cement mineral synthesis using a molten salt process yields a fine-grained product that requires little or no grinding. The specific sustainability impacts that may arise from a molten salt route have been assessed and quantified against the conventional manufacturing route in terms of resource efficiency (energy consumption and waste output), supply chain influences and management of the production process.

World-wide, mankind produces over 3Bnt/yr of cement,^1,2 most of which is used to make an estimated 10km³ of concrete – more than an order of magnitude greater than the combined volume of all other man-made materials (Figure 2). Although the carbon dioxide (CO₂) embodied in concrete is low (0.4t/m­³) compared to other common construction materials such as glass (2t/m³), steel (10t/m³) and polymers (40t/m³),³ the sheer volume of cement produced world-wide presents a serious climate change challenge simply because of the amount of CO₂ emitted during its manufacture - almost 1t of CO₂ per tonne of product.

Introduction

A cement is any substance which binds together other materials by a combination of chemical processes known collectively as setting.¹ Cements are dry powders and should not be confused with concretes or mortars, but they are an important constituent of both of these materials, in which they act as the 'glue' that gives strength to structures. Mortar is a mixture of cement and sand whereas concrete also includes rough aggregates (Figure 1).^2,3

Because it is a major component of both of these building materials, cement is an extremely important construction material. It is used in the production of the many structures that make up the modern world including buildings, bridges, harbours, runways and roads. It is also used for facades and other decorative features on buildings. The constant demand for all of these structures, increasingly from the developing world, means that cement is the second most consumed commodity in the world after water.

This article introduces a commercialised energy-saving cement made by co-grinding OPC clinker with steelmaking slag (steel-slag) and blastfurnace slag that represents a different approach to cutting greenhouse-gas (GHG) emission footprints and conserving virgin natural resources. The cement uses only 15-30% of OPC clinker, 30-40% of steel-slag and 40-50% of blastfurnace slag (BFS), allowing for great reductions in GHG emissions, consumption of virgin natural resources and energy use by between 70% and 85%. The characteristics of steel-slag as a supplementary cementitious material are also discussed.

Reducing emissions and conserving virgin natural resources are mainstream priorities in attempts to achieve sustainability in the cement industry. Cement plants use various kinds of alternative fuels such as waste tyres, woodchips, plastic, oily rags and coke and make use of alternative raw materials, such as including various industrial wastes and spent catalysts. The application of different supplementary cementitious materials (SCM) in cement production has already been proven as the most effective way to reduce greenhouse gas (GHG) emissions from cement production by replacing the same amount of clinker. Producing 1t of clinker emits about 1t of GHG. Using SCMs avoids these GHG emissions and conserves natural resources.

Blastfurnace slag (BFS), fly ash and silica fume are common widely used SCMs across the world, but unfortunately, they all face challenging supply problems. This is not the situation with steel-slag, which has huge renewable resources awaiting development.

The energy intensive cement industry faces high and rising energy costs and must meet requirements for reducing CO₂ emissions. One remedy for these challenges is to increase energy efficiency by recovering waste heat and converting it to electricity.

Modern cement plants can be thought of as huge heat exchangers. After firing the kiln (and eventually the precalciner), the heat in the stack gas and from the hot clinker can be used to a large extent in the plant for:

• Secondary air preheating,
• Tertiary air preheating,
• Preheater tower with 3–6 stages to preheat raw meal,
• Preheater waste gas to dry raw meal in the raw mill,
• Waste heat recovery to dry and preheat conventional or alternative fuels,
• Use of clinker cooler waste heat to feed districtheating systems.

Waste heat recovery is the most important measure towards decreasing heat losses and therefore has the highest priority. It is the most efficient way to increase energy efficiency because the efficiency of heat recovery is high, 1kWh of waste heat reused in the plant ends in 1kWh of useful heat.

2011 sees the 100th anniversary of the start of the Turkish cement industry. Since 1911 the cement sector has developed rapidly from a production capacity of just 20,000t/yr in 1911 to over 66Mt 100 years later.

In terms of production, the Turkish cement sector is ranked number one in Europe and number four in the world after China, India and the US. In terms of cement exports it is first with a global share of 12%, leaving China and many countries of South Eastern Asia behind.

By 2010 the sector became a vital branch of industry for the national economy, with turnover of around US$4.5bn, exports valued at US$1bn and direct employment of 15,000 people. Based on achievements during 100 years, the sector entered the new millennium with a new vision and mission enabling to serve as a model all over the world.