08-17-2017, 12:05 AM
The cement industry faces a number of challenges that include depleting fossil fuel reserves, scarcity of raw materials, perpetually increasing demand for cements and concretes, growing environmental concerns linked to climate change and an ailing world economy. Every tonne of Ordinary Portland Cement (OPC) that is produced releases on average a similar amount of CO2 into the atmosphere, or in total roughly 6% of all man-made carbon emissions. Improved production methods and formulations that reduce or eliminate CO2 emissions from the cement manufacturing process are thus high on the agenda. Emission reduction is also needed to counter the impacts on product cost of new regulations, green taxes and escalating fuel prices. In this regard, locally available minerals, recycled materials and (industry, agriculture and domestic) waste may be suitable for blending with OPC as substitute, or in some cases replacement, binders. Fly ash, Blast furnace slag and silica fumes are three well known examples of cement replacement materials that are in use today that, like OPC, have been documented and validated both in laboratory tests and in practice. The first is a by-product of coal combustion, the second of iron smelting and the third of electric arc furnace production of elemental silicon or ferro silicon alloys. This paper presents a concise review of the current state-of-the-art and standards underpinning the production and use of OPC-based cements and concretes. It outlines some of the emerging green alternatives and the benefits they offer. Many of these alternatives rely on technological advances that include energy-efficient, low carbon production methods, novel cement formulations, geopolymers, carbon negative cements and novel concrete products. Finally, the economics of cement production and the trends in the UK, US and the Gulf Cooperation Council (GCC) Region are presented, to help guide and inform future developments in cement production based on maximizing the value of carbon reduction.
Keywords
Cement Standards; Energy; Economics; Carbon
1. Introduction
Concrete is a basic building material that will continue to be in demand far into the future. A world without concrete, and its dominant precursor, Ordinary Portland Cement (OPC), is hard to imagine. Although there are different types of concrete that have been developed for use in different applications, their common virtues are familiarity, versatility, strength, durability, wide availability, fire resistance, resistance to the elements and comparatively low cost.
OPC is a vital construction material and also a strategic commodity (Vlasopoulos, 2010). Such is our dependence on OPC that the world currently produces nearly 3.6 billion metric tonnes of the material each year (USGS Mineral Commodities Summary, 2012), with volume predicted to rise to more than 5 billion metric tonnes by 2030 (M ller and Harnisch, 2008 and OECD/IEA and World Business Council for Sustainable Development, 2009). Although figures vary from country to country, around half of the world s OPC is used to make around 11 billion metric tonnes of concrete annually; the rest is used in mortars, screeds, stucco, coatings, soil stabilization and other applications. (New Zealand Cement Holdings, 1988 and Smith et al., 2002). Today, the OPC market is dominated by China, which is attributed to 57.3% of global consumption (CEMBUREAU, 2012).
The cement industry, like the rest of the construction industry, is facing unprecedented challenges relating to energy resources, CO2 emissions and the use of alternative materials. Worldwide, the cost of energy is rising inexorably as fuel sources deplete. This has clear, traceable impacts on the cost of producing cement and its market price; Green taxes are an additional cost that is incurred if emissions are not restricted, potentially leading to a doubling in the price of cement by 2030 (OECD/IEA and World Business Council for Sustainable Development, 2009).
Despite the incremental improvements in process efficiency that have been adopted by the cement industry in recent years, OPC production is still responsible for around 6% of all man-made global carbon emissions. The Cement Sustainability Initiative, developed by the World Business Council for Sustainable Development, brings together the major cement producers from across the world to try and tackle this problem ( OECD/IEA and World Business Council for Sustainable Development, 2009). An important part of the initiative is a database showing CO2 emissions and energy performance figures for many of the significant players in the global cement industry, to promote the sharing of ideas aimed at improving these values.
The push to reduce global CO2 emissions is backed by governments and corporations who understand that the present rate of release of this greenhouse gas into the atmosphere is a serious threat to future life and prosperity on the planet. Various authorities have introduced legislation and incentives (tax rises such as CO2 taxes, quarrying and extraction tax, etc.) in order to regulate and reduce the activities of the industrial sectors most responsible for greenhouse gas emissions. However, the rate of increase in emissions continues almost unabated as a result of population growth and increased industrialization and economic activity in developing countries, notably in Latin America, Africa, the Middle East, India and developing countries in Asia where a three to fourfold increase in demand is projected by 2050 (OECD/IEA and World Business Council for Sustainable Development, 2009).
If the cement and concrete industries are to become sustainable and effectively contribute to emission reduction then, in addition to improvements in process efficiency and reliance on OPC blends incorporating waste materials, moving to less carbon intensive fuels, developing clinker substitutions employing other low carbon materials with cementitious properties and new low carbon and carbon-reducing cement formulations and production processes are needed.
Carbon-reducing cements, if they could be developed for commercial-scale application, probably offer the safest, most economical and elegant Carbon Capture and Storage (CCS) technology. Other approaches to CCS that require piping CO2 emissions from cement production (and other polluting sources such as power generation), are viewed by some as the best way forward. However, widespread concerns relating to long-term reliability and high capital cost suggest that ideas such as pressurized, pumped storage of liquefied CO2 in geological formations may be neither technically nor commercially viable. They cannot be relied on to provide a permanent solution as the risk of containment failure is simply too great.
In order to appeal to major cement manufacturing companies, an alternative cement product has to be able to generate at least the same economic value as that from an OPC production plant. At 7.6% of world cement production, the cement industry in Europe represents around 56,000 direct jobs. The average cement plant will produce around 1 million tonnes of cement per annum and cost around 150 million. Advances in automation mean that a modern plant is usually manned by less than 150 people (CEMBUREAU, 2012). Each tonne of OPC produced requires 60 130 kg of fuel oil or equivalent, depending on the cement variety and the process used, and about 110 KWh of electricity. This accounts for around 40% of the average 0.9 tonnes of CO2 emissions per tonne of cement produced, with the rest attributed to the calcination process, other manufacturing processes and transportation.
Concretes, on the other hand, refer to mixtures comprising coarse aggregates (such as crushed rock, ranging in size from 5 to 20 mm), fine aggregates (such as sand, ranging in diameter from 63 microns to 5 mm) and a cement binder. When mixed with appropriate quantities of water and (where required) performance-enhancing admixtures, this produces an initial fluid phase that can be shaped or cast and sets to produce a solid phase comprising a very strong, rigid concrete element or structure. Conventional Portland-clinker-based hydraulic cements (products of the calcination process) use source-materials that are cheap and abundant almost everywhere. Finding suitable alternative cements would require an investigation into their cost and profitability as well as their structural characteristics.
As alternative low-carbon cements and concretes enter the equation, three conditions will determine their success or otherwise; firstly that they are useable and perform well both short term and long term, secondly that there is sufficient information validating the capabilities of the product so that they meet engineering standards for specific functions, ranging from the making of cavity blocks to ready-mix for in situ casting of foundations, and thirdly, that there is sufficient raw material that can be transported in bulk to processing plants.
Although the ingredients for making cement are readily available in most countries, there may be opportunities to use locally sourced specific raw materials such as industrial waste, recyclable material or even earth. These materials must of course have controlled characteristics and properties that are suitable for purpose, whether it is for blending ground granulated blast furnace slag for strength enhancement or soil for producing compacted earth blocks; from the point of view of the engineer or architect, such materials are generally selected on the basis of the added functionality that they offer and their cost-effectiveness.
The present paper examines the production of OPC (the benchmark cement against which all cements are measured) and applicable standards. It will summarize the waste substitutes that are currently being used to reduce the carbon footprint of a range of Portland-based cements. Co-incineration of waste-derived fuels (municipal waste, sewage sludge, animal meal, waste by-products, etc.), to reduce emissions and effectively dispose of these wastes, will also be briefly discussed. Some traditional replacement cements will be briefly discussed, and the emerging, next generation of green alternative cements such as Calcium Sulfoaluminate (C$A) cement, successfully developed and used in China since the 1970s, will be introduced.
The paper will conclude with a look at current and projected future demand for cement, highlighting the main players. A preliminary analysis of the economics of low carbon cements will be carried out and a valuation method for the carbon reduction benefits they offer is proposed.
2. Portland-based cements and other hydraulic binders
Ordinary Portland Cement (OPC) is the dominant contemporary cement. It is a mixture of compounds produced by burning limestone and clay together, in a rotary kiln, at a temperature of around 1,450 C. Approximately 40% of cement plant CO2 emissions are from the burning of fossil fuel to operate the kiln, 50% due to the manufacturing process and the remaining 10% are accounted for by indirect CO2 emissions relating to transportation of the finished product and front-end production processes (World Business Council for Sustainable Development, 2012 and Hendriks et al., 1999).