Options for the Future of Cement

TECHNICAL PAPER SPECIAL ISSUE - Future Cements Options for the future of cement Karen L. Scrivener Options for more sustainable cementitious materials are reviewed. The most viable option involves blending Portland cement clinker with increasing levels of supplementary cementitious materials (SCMs), but the availability of common SCMs is low compared to the demand for cement. Calcined clays are the only other materials that are potentially available in large enough amounts to continue the trend of reducing clinker content in blended cements. Recently we demonstrated that partially replacing clinker by calcined clays combined with limestone (LC3 blends) can be used to achieve blended cements with good performance at much lower levels of clinker. Such blends can make a significant contribution to the reduction of CO2 emission associated with cement production. Keywords: Supplementary cementitious materials; sustainability; calcined clay; limestone; CO2 emissions. As global demand for concrete follows the growth of the enormous volumes that overall the production of cement middle class in the 21st century, the concrete industry and concrete is estimated to account for around 5-8% of IDFHVRQHGHÀQLQJFKDOOHQJHÀQGLQJZD\VWRLQFUHDVHWKH man-made CO2 emissions. availability of concrete while at the same time reducing LWVHQYLURQPHQWDOIRRWSULQW7RKDYHDVLJQLÀFDQWJOREDO 7DEOH VKRZV UHODWLYH ÀJXUHV IRU WKH HQHUJ\ DQG &22 impact, solutions have to be abundant, affordable, and emissions of some common building materials [1]. Here, adapted to the users in the developing world, where most typical values for concrete are shown, as this is the growth is expected. ÀQDO SURGXFW XVHG LQ WKH ÀHOG $V GLVFXVVHG HOVHZKHUH CONCRETE IS AN ECO-EFFICIENT MATERIAL Cement and concrete are essential to the infrastructure Table 1. Embodied energy and associated CO2 emissions for common construction material [1] RIWKHPRGHUQZRUOG1RRWKHUPDWHULDOLVDEOHWRIXOÀO Material Embodied CO the growing demand for building materials with such a 2 Energy (MJ/kg) (Kg CO2 /kg) low environmental footprint. Its widespread availability Normal concrete 0.95 0.130 and low cost make cement by far and away the most used Fired clay bricks 3.00 0.22 material on earth, with reinforced concrete accounting Road & pavement 2.41 0.14 for more than half of all the manufactured materials Glass 15.00 0.85 and products we produce. It is only because of these Wood (plain timber) 8.5 0.46 Wood (multilayer board) 15 0.81 The Indian Concrete Journal, July 2014, Vol. 88, Issue 7, pp. 11-21 Steel (from ore) 35 2.8 The Indian Concrete Journal July 2014 11 TECHNICAL PAPER SPECIAL ISSUE - Future Cements TECHNICAL PAPER [2], there is a huge scope also for reducing the amount fuels, reducing the need for primary fossil fuels to below of cement in concrete and it should always be borne in 20% in some modern plants. This versatility, the fact that mind that sustainability can and should be considered at WKHFDORULÀFYDOXHRIVXFKDZLGHUDQJHRIZDVWHSURGXFWV DOO VWDJHV RI WKH SURFHVV 7KH ÀJXUHV LQ 7DEOH DUH SHU can be exploited in a safe manner, should be seen as kg and the strength of concrete is not the same as the another advantage of cement production. VSHFLÀFVWUHQJWKRIVWHHORUZRRG7KHVHLVVXHVKDYHEHHQ considered by several authors [e.g. 3]. Nevertheless, the Any attempt to reduce the remaining 60% of “chemical” advantages of concrete remain clear in most construction emissions coming from the decarbonation of limestone will related application; even more so when the availability have the inevitable consequence of changing the cement’s of materials to substitute concrete is considered. As an chemistry. Consequently, the reactions and performance example, although the amount of wood used worldwide of new materials will not necessarily be the same as the is around one tenth that of concrete, our consumption reference Portland cement. It took more than 100 years of of wood is already considered to be unsustainable; we empirical testing to develop the basis for use of Portland are cutting down more trees than we are planting. It cement. The use of the most common blended materials follows that, independent of any other considerations; ²EOHQGVRI3RUWODQGFHPHQWFOLQNHUZLWKVODJDQGÁ\DVK ZRRG FDQQRW PDNH D VLJQLÀFDQW IXUWKHU FRQWULEXWLRQ WR – has taken more than 30 years to become established. meeting the increasing demand for building materials We do not have the time to go through this long testing worldwide. phase for every new material that comes along. That is ZK\ZHPXVWQRZPRYHWRZDUGVDPRUHVFLHQWLÀFEDVLV DEMAND FOR CEMENT IS INCREASING which can only come (on a reasonable timescale) through a systematic understanding of cementitious processes Figure 1 shows the projected increase in demand for and materials at the nano-scale, extended across all the cement. Most of the increase in the coming decades will scales involved in cement and concrete production, to be in emerging and developing countries, which already provide the multidisciplinary assessment and prediction make up over 80% of global production. Today, China’s tools needed to assess the functional and environmental production alone exceeds that of the whole world 10 years performance of new materials. ago. India is the country in which the increase is likely to be the largest – today the consumption per capita in India is only 1/6th that in China. OPTIONS TO REDUCE ENVIRONMENTAL IMPACT Cement production accounts for the overwhelming majority of the CO2 emissions associated with concrete. Unlike other materials, less than half (~40%) the CO2 emitted during cement production is related to fuel and electricity. The remaining 60% comes from the decomposition of the main raw material – limestone, or CaCO3. Great improvements have been made in the last few decades in lowering energy-related CO2. Today, the production of Portland cement clinker is one of the PRVW HIÀFLHQW LQGXVWULDO WKHUPDO SURFHVVHV LQ H[LVWHQFH DSSURDFKLQJ RI WKHRUHWLFDO HIÀFLHQF\ DQG LW LV XQOLNHO\WKDWVLJQLÀFDQWIXUWKHUJDLQVFDQEHPDGHKHUH as discussed in more detail by Gartner [4]. Furthermore cement plants can now use a wide range of substitute 12 The Indian Concrete Journal July 2014 SPECIAL ISSUE - Future Cements TECHNICAL PAPER SPECIAL ISSUE - Future Cements Table 2. summary of suitability of common oxides for cementitious materials. oxide hydrates geology CaO *RRGPRELOLW\K\GUDWHVFDQÀOOVSDFH 9 Limestone, widely distributed 9 Less soluble but still a major component of space SiO 9 Most common oxide, everywhere 9 2 ÀOOLQJK\GUDWHV With silica in feldspars, clays, etc. Al O 9DULDEOHVROXELOLW\K\GUDWHVÀOOVSDFH 9 9 / ? 2 3 also more concentrated and localised as bauxites Low mobility / solubility in alkaline solutions, poor Fe O ? Very widely distributed 9 2 3 FRQWULEXWLRQWRVSDFHÀOOLQJ Low mobility / solubility in alkaline solutions, poor Mainly as impurity in limestone, more concentrated MgO FRQWULEXWLRQWRVSDFHÀOOLQJ sources localised. The options for new cement chemistries are ultimately magnesium oxide, on the other hand, lead to the opposite limited by the composition of the earth. Just eight elements problem. These oxides do form insoluble hydrates, but constitute more than 98% of the earth’s crust – oxygen, because their mobility in the alkaline pore solution of silicon, aluminium, iron, calcium, sodium, potassium, cements is very low, these hydrates are almost exclusively and magnesium. The relative abundances are shown deposited within the boundaries of the original grains and ORJDULWKPLFDOO\LQ)LJXUH>@$VDÀUVWDSSUR[LPDWLRQ GR QRW FRQWULEXWH WR ÀOOLQJ WKH VSDFH LQLWLDOO\ RFFXSLHG cost will be closely related to availability, leaving only 7 by water or to binding the grains together. Even in very oxides as possible candidates for cement making. These old cement pastes (even > 100 years [6]), we see bright DUHVXPPDUL]HGLQ7DEOHZKLFKEURDGO\FRQVLGHUVÀUVW areas corresponding to the original ferrite phase in the WKHLU SRWHQWLDO WR IRUP VSDFH ÀOOLQJ K\GUDWHV DQG WKHQ cement, as in Figure 3 (a). In slag cements, the magnesium their geological distribution. component of the slag remains within the boundaries of the original slag grains and does not contribute to binding 7KHSRWHQWLDOWRIRUPVSDFHÀOOLQJK\GUDWHVLVWKHNH\WRD hydraulic cement. First, the anhydrous cement is mixed with water. The solid content of the resulting mixture cannot exceed around 50% (W/C ~ 0.3) if the mixture is to EHÁRZDEOH7KHQWKHDQK\GURXVPDWHULDOPXVWGLVVROYH and precipitate new solids incorporating water with a higher solid volume than the original anhydrous material. The phases present in Portland cement clinker are ideal in this respect as there is roughly a doubling in solid volume, which enables most of space initially occupied by water WREHÀOOHG\LHOGLQJDVWURQJVROLGZLWKORZSRURVLW\ The alkali oxides retained from Figure 2, Na2O and K2O, cannot produce hydraulic compounds as these have a very high solubility and will not deposit hydrates. Nevertheless, small amounts of these compounds play an important role in Portland cement, where they nearly all end up in the pore solution, conferring the concrete with the high pH needed to protect reinforcement. Iron and The Indian Concrete Journal July 2014 13 TECHNICAL PAPER SPECIAL ISSUE - Future Cements TECHNICAL PAPER the materials together (Figure 3 (b)). This brings us back Beyond these approaches the CSI study imagines the to the three oxides that dominate Portland cement – lime, remaining reduction in CO2 to come from Carbon Capture silica and alumina. and Storage (CCS) (grey area), which

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