Raman Mangabhai reviews some developments in cements

Transcription

1 Raman Mangabhai reviews some developments in cements Concrete is the most widely consumed man-made material in the world. The production of cement is responsible for 3-5% of total global carbon emissions the world s second largest emitter of carbon dioxide (CO2). Traditionally, Portland cement has been manufactured using natural geomaterials (calcareous rocks such as chalk or limestone and argillaceous rocks such clay or shale). The materials are heated in a kiln at a temperature of 1450 C, which forms clinker. The clinker is cooled, ground and blended with gypsum to regulate the setting. During the clinkering process CO2 is emitted from the decomposition of limestone and burning of kiln fuel. It is estimated that kg of CO2 is emitted per tonne of cement. The raw materials are becoming difficult to access with demand for Portland cement increasing worldwide. Indeed, whilst the world annual cement production in 1970 was about 500 million tonnes by 1994 Chinese production alone had reached 400 million tonnes, with the total world production being at 1000 million tonnes. Chinese production was estimated to be 2400 million tonnes with the world total of 4200million tonnes in In order to reduce the CO2 from the manufacture of Portland cement new developments in cement manufacture have taken place,1,2,3 which include (i) modern cement plant (precalciner, milling technology) (ii) use of alternative raw materials (pfa/ waste materials) and (iii) alternative fuels ( biomass/tyre chips). This paper briefly describes a few developments in cements (i) calcium sulfoaluminate cements (ii) alkali activated (iii) calcium metasilicate, (iv) Calcium hydrosilicate cement (v) MgO based cements (vi) blended cements Calcium sulfoaluminate cements (CSA) Calcium sulfoaluminates were developed in the 1970 s in China by China Building Materials Academy. Its initial intended use was manufacture of self-stress concrete pipes due to swelling properties. Ettringite is formed during hydration and depending on the presence of calcium hydroxide the Ettringite swells On the absence of calcium

2 hydroxide the ettringite does not expand and contributes to the strength of the material. Subsequently it has found many uses such as fast setting mortar, tile adhesives and concrete. The manufacturing process is similar to Portland cement but limestone is partially replaced with sulphur and aluminium bearing minerals and clinkering temperature range is 1200 to 1300 C. Aether and ALI CEM Green have been produced by LafargeHolcim and Italcementi respectively. Ecobinder project is developing a standard for CSA cement Part 1, Composition, Specifications and conformity criteria for CSA cements whilst China already has standards (National Standard of Calcium Sulfoaluminate Cement. Calcium metasilicate Solidia Cement 5, developed a non-hydraulic cement composed primarily of lowlime, calcium silicate phases such as wollastonite /pseudowollastonite (CaO SiO2), where clinkering temperature is reduced to 1200 C This contrasts with the high-lime phases that comprise ordinary Portland cement (OPC). The setting and hardening characteristics of Solidia Cement are derived from a reaction between CO2 and the calcium silicates. Alkali-activated cementitious materials (AACM) AACM are defined as a substance consisting of an alkali activator (e.g. sodium or potassium based) and AACM powder or blend of such powders (fly ash, ground granulated blast furnace slag) with or without the inclusion of subsidiary constituents, with or without the incorporation of Portland cement, which under aqueous conditions, react to produce a hardened monolithic material6,7. AACM materials have been used in building in China8 and non structural applications. Calcium hydrosilicate cement Calcium hydrosilicate cement has been developed by Karlsruhe Institute for Technology (KIT) in Germany which use less lime and burning temperatures can be significantly reduced. It is marketed by Celitement.de however to date commercial production is low2. MgO based Magnesium Oxide (and Magnesium Oxy Chloride) were used extensively up until the 1930 s especially in terrazzo floors. The promotion of more cost effective Portland cement led to a decline or stop of MgO cement for structural and convention uses

3 MgO based cements have been developed and are mainly for specialist applications such as repairs and fire protection although varieties are available for self-levelling floors and mortar. Generally these products are more expensive than Portland cement variations and so they are likely to be used in technical situations which justify extra cost. Blended cements Fly ash, ground granulated blast furnace slag, limestone, silica fume, pozzolana, etc have been blended to produce blended cements such as CEM II10 where clinker can be substituted by 35 % with PFA or slag, or 20 % with limestone, where CEM III clinker can be substituted by 95 % and for CEM IV clinker can be substituted by 55%. Such blended cements are widely recognised in standards and national regulations. CO2 reduction Figure 1 compares the CO2 reduction with various types of binder with CEM1 (taken from MPA1). The use of CEM1 only concrete has been reduced significantly in many markets by the use of blended cements with fly ash, ground granulated blast furnace slag, limestone, pozzolans etc. Comparison with CEM1 indicates that Aether and CEM II/A-L reduces CO2 whereas (i) the use of 30 % PFA reduces by 30 %, (ii) 45 % PFA and 50 % slag reduces by 46 % (iii) for Solidia and Celitement the reduction is below 46 % and (iv) for AAMC the range is uncertain as the raw materials may need calcinations and activation at higher temperatures. However research is under way to reduce the activation temperatures.

4 Fig 1 CO2 for common cements vs alternatives2,3 Permitted uses and regulations Portland cement varieties have many permitted uses both structural and nonstructural. Specific requirements are detailed in national building regulations and laws. National regulations may vary in what product can be used in an application because of different local conditions. Portland cement varieties including blended cements, such as those in EN are widely accepted in building regulations because they have a long history of use in comparable situations. Some of the developing cements such as CSA have some standards and some acceptance partly because actually they have been around for some time9 and their properties typically comply with well used standards such as ASTM and EN. The absence of long term performance information is a barrier for new cementitious materials. Durability has to be established over time and before a material can be accepted in regulations and regulated applications. Some cement such as Alkali Activated Cementitious Materials have had local Publicly Available Standards (PAS) developed6 but this will not mean acceptance in regulations or authoritative standards until performance can be proven over time. Of course many products can find acceptable uses but not so easily in replacement of conventional concrete. CONCLUSIONS Portland cement will be the major use in construction however it will be blended with fly ash, slag etc. and methods of manufacture will change to reduce CO2 emissions. Alternative cements will find a niche market which will depend on its commercial availability and value for money. Specifications for the use of alternative cements will need to be developed for use in construction. This will require verification of performance and durability over time. The main driver for new cements is the reduction in carbon footprint and raw materials conservation. The use of Portland blended cements using by products such as slag and ash produce carbon footprints similar to even the best performing alternative cements and better than some others. WCA believes that Portland type cements will continue to be the predominant binder for concrete but the industry is continually improving eco performance. REFERENCES

5 1 2 McCague, C. (2016), Next generation of cements, Concrete Quarterly Summer, pp McCague, C. (2017), Modern cements, CISF Conference, 4-5 May, UK 4 Gartner, E and Sui, Tongbo. (2017), Alternative cement clinkers, Cement and Concrete Research, DOI: /j.cemconres Schuler, T and DeCristofaro, N. (2016) Sustainable innovation on the road to market: Moving from the lab to global impact for the cement and concrete industries, Editors: Jones etal 9th International Concrete Conference 2016 Environment, Efficiency and Economic Challenges for Concrete, 4-6 July 2016, Dundee, Scotland, UK 6 PAS 8820:2016, Construction materials- Alkali-activated cementitious material and concrete-specification, BSI, London. 7 Provis, J L and Van Deventer, J S J (Editors), (2014), Alkali-Activated Materials : State of the Art Report, RILEM TC 224-AAM), Dordrecht:Springer/RILEM 8 Yang, C, Zhang, Pan Q, Yang K, Yu L and Bai Y (2016), Structural use of cast- in- situ alkali-activated slag concrete in an office building-chongqing Jianke, China, Paper presented at ICT Seminar on Alkali-activated cementitious material and concrete PAS 8820: 2016, 21 September 2016, University College London, UK BS EN 197-1:2011, Cement. Composition, specifications and conformity criteria for common cements, BSI, London.