In 2008 Ramboll was approached by a UK architect who was interested in building a doubly-curved shell roof out of lime-concrete. There was little available to suggest whether or not a structural grade lime-based concrete was feasible. The Pantheon in Rome, however, with its 42 metre diameter lime-concrete dome, was testament to the strength and durability of this ancient material. On this basis, EngD Research Engineer Ellen Grist was tasked with finding out what material strengths were attainable today.
Work undertaken at the University of Bath has demonstrated the feasibility of producing a lime-pozzolan concrete with a 28-day cube strength of around 50 MPa and an elastic modulus of 20GPa. Furthermore, production and testing of two reinforced lime-concrete beams has demonstrated the possibility of producing structural elements with a finished appearance and flexural behaviour similar to Portland-cement concrete.
What are lime-concretes?
Lime is a durable, abundant and versatile binder that has been used extensively in construction for thousands of years. Lime used for building (calcium hydroxide) does not occur naturally but is manufactured by burning limestone (calcium carbonate), a raw material which is abundantly available across the globe. Burning the limestone produces quicklime (calcium oxide), which is then slaked, by addition of water, to produce the lime binder.
Before the advent of Portland cement in 1824, lime was the predominant binder used for making mortars, renders and concretes. Lime-pozzolan concretes were used by the Greeks and Romans to build walls, floors, baths, aqueducts, vaults and domes. Two-thousand-year-old structures including the Pantheon, the Baths of Caracalla and the Pont du Gard aqueduct, to name a few celebrated examples, are a testament to the strength and durability of this ancient material. Despite a long and rich history of lime in construction, Portland cements are now specified almost exclusively.
This new lime-pozzolan concrete is not a return to a former technology as it exploits carefully controlled pozzolanic materials, by-products of current industrial processes, and takes advantage of the significant advances in the development of concrete technology, specifically the performance of cutting edge water reducing admixtures. As a result the speed of set, workability and compressive strength of this lime-pozzolan concrete is unprecedented.
The manufacture of Portland cement is an energy and carbon intensive process and widely acknowledged to be responsible for 5-8% of the total global anthropogenic carbon emissions. In 2011 over 3.4 billion tonnes of cement were produced worldwide, enough to produce over 11 billion m3 of concrete (or 1.6m3 for every man, woman and child on the planet). Although often vilified on environmental grounds cement, as the principal binding constituent of concrete, continues to be a key driving force of human development. Hydraulic-lime pozzolan cements are one of a number of ‘second generation’ low-carbon cements being developed to meet the on-going demand for concrete in construction.
The manufacture of lime is not dissimilar from the manufacture of Portland-cement, as both are produced by burning limestone. The environmental benefits of lime-based building materials lie in potential process energy and carbon savings, associated with lime production. The vertical-shaft kilns used for production of NHL5 tend to operate at 1000°C. This is substantially lower than the 1450°C which is needed for the calcination of limestone to produce Portland cement. Also quicklime tends to disintegrate during slaking, substantially reducing the demand for the energy-intensive finish grinding.
The environmental benefits of lime are realised when it is used in conjunction with pozzolanic materials, including alumina-siliceous waste ashes, which govern the set and mechanical strength of lime-pozzolan concretes.
Future lime-pozzolan concretes
Lime-pozzolan concrete is not a single material but a new binder technology that could be the basis of a whole range of concretes with different performance characteristics including structural strengths. Future lime-pozzolan concretes could use different combinations of pozzolanic additions and aggregates to create optimised mix designs for specific applications.
Project specific testing and development
After a period of laboratory testing, a polished lime-concrete floor screed was developed for the ground floor of a school building in the UK. This application facilitated the use of site-sourced weathered limestone aggregate, which was revealed in the polished surface. The embodied carbon of the polished lime-pozzolan concrete floor was 60-70% less than that of a CEM1 polished concrete floor.
Re-emerging technology demonstrated at Ecobuild
New technologies cannot realise environmental benefits whilst sat in University labs, only in the appropriate use in the ‘real world’. As with any innovative technology, further testing will be necessary to address project specific questions about the mechanical and thermal performance of this material.
Working in conjunction with the University of Bath, the organisers of Ecobuild and a host of industry partners: EJ Lazenby Contracts, The Traditional Lime Co, St. Astier, Elkem, Hanson Regen, BASF, Aggregate Industries, Raymond Brown Minerals & Recycling Ltd and Hatcher Prichard Architects, Ramboll produced a pair of lime-concrete benches for the Future Materials Gallery at Ecobuild 2013. The first of their kind, these two lime-concrete benches have polished seats which reveal the recycled aggregates (crushed demolition waste and glass cullet) with which they have been made.