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It is important to distinguish between true PLC and Portland cement containing a small proportion of limestone as a minor additional constituent (MAC). BS EN 197-1: 2000(4) permits incorporation of 5% MAC by mass in a Portland cement. This may be finely ground limestone. If Portland cement contains limestone MAC, it is still classified as a Portland cement (CEM I in BS EN 197-1: 2000).
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Portland-limestone cements (PLCs) are produced at numerous plants across the UK(1). They have strong positive environmental credentials. This cement type accounted for more than 24% of the European cement market (approximately 40 million tonnes) in 2000(2). However, these cements have not been used to any great extent in the UK. The objective of this article is to update an earlier article in CONCRETE, explaining the properties and uses of factory-produced Portland-limestone cements. It is hoped that they will become more widely used as a consequence(3).
What is PLC?
Factory-produced PLC is produced by inter-grinding Portland cement clinker with limestone. They are covered by the cement standard BS EN 197-1: 2000 Composition, specifications and conformity criteria for common cements(4) where they are given the following designations:
* CEM II/A-LL where limestone content is 6-20% of cement mass
* CEM II/B-LL where limestone content is 21-35% of cement mass
* an LL suffix is replaced by L when the limestone has a total organic content (TOC) greater than 0.20%, yet below 0.50% by mass.
The only PLC currently manufactured in the UK is CEM II/A- LL (or A-L), containing between 6% and 20% limestone. It is available in both the 42,5 and 52,5 characteristic cement strength classes. The factory-produced cement conforms to BS EN 197-1: 2000 and is both quality assured and CE-marked(4). Prior to the introduction of BS EN 197-1: 2000, PLC containing up to 20% limestone was covered by BS 7583: 1996 Specification for Portland limestone cement.
It is important to distinguish between true PLC and Portland cement containing a small proportion of limestone as a minor additional constituent (MAC). BS EN 197-1: 2000(4) permits incorporation of 5% MAC by mass in a Portland cement. This may be finely ground limestone. If Portland cement contains limestone MAC, it is still classified as a Portland cement (CEM I in BS EN 197-1: 2000). Codes and Standards across Europe classify the mechanical properties and durability of concrete containing CEMI with a limestone MAC as identical to concrete containing CEM I without a MAC. The two cement types should not be confused. PLC is a unique cement type with different properties to CEM I, specifically manufactured to contain a higher proportion of limestone.
PLC concrete properties
Fresh concrete
When factory-produced PLC is used in concrete, setting time may be slightly reduced in comparison with CEMI concrete. However, the most noticeable differences in terms of fresh concrete properties are reduced water demand (enhanced workability) and significantly less bleeding(3). PLC is compatible with most admixtures at dosage rates corresponding to CEMI concretes.
Hardened concrete
Portland cement hydration is modified in PLC by the presence of finely divided limestone. This is thought to result from acceleration of the C3S hydration rate by increased nucleation and modification of the C3A hydration process(4). This results in a stable monocarboaluminate phase. There is also a reduction in cement paste porosity as a result of pore filling by fine limestone particles. Limestone is softer than cement clinker and is ground to a finer particle size during inter-grinding.
Although exact mechanisms are not fully understood, it is clear that limestone in PLC does not operate as an inert filler. Thus the term 'limestone filled cement', sometimes applied to PLC, is technically incorrect. The effect of modifying concrete is to increase early strength development relative to CEMI concrete of the same characteristic compressive strength class. However, seven and 28-day strengths will be similar to CEM I concrete. Further strength development is generally limited after 28 days. PLC concretes generally require a slightly higher cement content than a CEMI concrete to achieve the same 28-day strength. Strength development is typically comparable to concrete containing a 50% CEM 1/50% ground granulated blast-furnace slab (ggbs) combination (CIIIA), yet with improved early age strength (see Figure 1).
Concrete containing PLC is typically lighter in colour than CEMI concrete. The finer particles result in a smooth surface finish, making the concrete particularly suitable for visual concrete elements.
Durability
The most powerful evidence regarding PLC concrete long-term durability is the successful use of this concrete type over more than 20 years in a wide range of exposure conditions across continental Europe, particularly in France and Italy.
However, assimilation in the UK has been hindered by conservatism in the construction industry. Extensive, authoritative investigations in the UK have confirmed that PLC concrete performs in a similar way to CEMI concrete of a similar strength class, the only exception being resistance to aggressive sulfates(6).
Figure 1 : Comparison of strength development of Blue Circle PLC concrete with concrete containing a 50% CEM l/50%ggbs mixer combination (CIIIA). The concretes have equal slump and cement content of 425kg/m^sup 3^.
Figure 2: New buildings at the Cornwall College of Arts, constructed using Blue Circle PLC concrete.
Codes and Standards
On 1 December 2003, BS 5328: 1997 Concrete(7) was withdrawn. CONCRETE readers will be aware that concrete is now specified according to BS EN 206-1: 2000 Concrete -Part 1: Specification, performance, production and conformity(8) and the complementary BS 8500: 2002 Concrete -Complementary British Standard to BS EN 206-1(9).
It is recognised in BS 8500: 2002 that the behaviour of CEM II/A-LL (or A-L) PLCs is akin to CEM I Portland cement in most exposure environments. Unlike BS 5328: 1997(7), the new Standard BS 8500: 2002(9) states that PLC can be used in reinforced concrete exposed to chlorides and non-air-entrained concrete exposed to freeze/thaw cycles. There are no differences between the specified requirements for CEM I or CEM II/A-LL (or A-L) concrete in the new Standard relating to carbonation exposure (XC classes), de-icing salts (XD classes), seawater (XS classes) and freeze/thaw (XF classes). As previously stated, the only specific restrictions imposed on PLC refer to concrete exposed to aggressive ground conditions. PLC should not be exposed to soil with a Design Sulfate Class exceeding DS-1, as stated in BS 8500: 2002 and BRE Special Digest 1(10). Ongoing research by the Building Research Establishment (BRE Ltd) and the British Cement Association is, however, examining PLC concrete performance under the more aggressive DS-2 conditions. The results, to date, have been encouraging.
PLC use in the UK
PLC is produced by all the major UK cement manufacturers. Blue Circle PLC is extensively used in the ready-mixed concrete market of south-west England. Aggressive sulfate levels are generally quite low in the area and PLC can be used for both subterranean and above-ground concrete (see Figure 2). Concrete producers derive numerous benefits, including reduced setting times and good early-age strengths, particularly in winter, when compared with concrete containing cement replacements. The concrete has improved cohesion characteristics, even when the mix incorporates harsh or poorly graded aggregates.
PLC provides single product flexibility for customers with limited silo capacity, including those producing mortar and screeds in addition to concrete. Mortars benefit from improved plasticity and cohesion. Surface regularity of screeds can be improved by the fine limestone particles.
PLC has been readily used in the UK precast concrete products sector. Its high early-age strength, light colour and contribution to good surface finish are attractive to manufacturers of decorative concrete components and products. Factory produced PLC benefits the concrete producer technically and economically. The manufacturing process results in reduced carbon dioxide emissions, when compared with ordinary CEM I cements. Experience from mainland Europe suggests that PLC use will increase considerably after the benefits have been fully analysed and understood.
References:
1. PRICE, W. New cements - out of the ordinary, NCE Special Report - Concrete in the environment, November 2002, p.24.
2. MOIR, G. Gaining acceptance, International Cement Review, March 2003, pp.66-70.
3. KELHAM, S. Portland limestone cements, CONCRETE, Vol.32, No.5, May 1998, pp.30-31.
4. BRITISH STANDARDS INSTITUTION. BS EN 197-1: 2000 Cement-Part 1: Composition, specifications and conformity criteria for common cements.
5. BRITISH STANDARDS INSTITUTION. BS 7583:1996 Specification for Portland limestone cement (withdrawn, superseded).
6. MATTHEWS, J. Performance of limestone filler concrete (in) Euro-cements, impact of ENV 197 on concrete construction (eds) Dhir, R. and Jones, M., E & F.N. Spon, London 1994
7. BRITISH STANDARDS INSTITUTION. BS 5328: 1997 Concrete.
8. BRITISH STANDARDS INSTITUTION. BS EN 206-1: 2000 Concrefe - Part 1: Specification, performance, production and conformity.
9. BRITISH STANDARDS INSTITUTION. BS 8500: 2002 Concrete - Complementary British Standard to BS EN 206-1: Part 1 - Method of specifying and guidance for the specifier; Part 2 - Specification for constituent materials and concrete.
10. BUILDING RESEARCH ESTABLISHMENT. Special Digest 1: Concrete in aggressive ground, BRE Ltd, Garston, 2003.
Copyright The Concrete Society Mar 2004