Ultrafine particles to save cement and improve concrete properties
2004 (English)Report (Other academic)
Fine particles play an important role in both fresh and hardened concrete and are often added as filler to give concrete specific properties. For example the development of more efficient superplasticizer allows increased amount of fillers to be used, which in turn has resulted in the development of self-compacting concrete (SCC). In SCC particles less than um are regarded as filler. Due to the new generations of very effective superplasticizers, it is today possible to include large amounts of even finer particles, particles much smaller (< 10 μm) than cement in size. These fillers here called ultrafillers. These ultrafillers will increase the strength of concrete and can thus act as a cement replacement. This is basically the effect behind CEM II/A-LL cement where the co-ground limestone due to the milling process will be an ultrafilier. In these cements the limestone filler has an efficiency factor of 1, i.e. the cement does not loose strength when substituted with UF. It is obvious that UF have other effects on concrete properties than ordinary fillers. It is also known from earlier work that different minerals have different influence on hydration properties. Thus several series of experiments with ultrafillers were conducted in order to obtain a better understanding of the reasons behind the effects and how they can be used in concrete production. The effect of different ultrafiller on hydration, paste structure, shrinkage and strength has been investigated. In the main experimental series quartz was chosen, as this mineral is easy to access and may act pozzolanic. Quartz in the size of cement, quartz finer than cement and ultrafine cristobalite quartz were tested. To compare it with other minerals, commercial ultrafine wollastonite and nepheline syenite (rock composed of nepheline and k-feldspar) were also tested. As quartz dust may result in silicosis, wet ground quartz was used in many of the experiments. All the ultrafine particles resulted in a pronounced increase in strength. It is possible to replace up to 40 % of the cement and still obtain similar strength. The best effect is achieved, when the cement is replaced but the water/cement is kept constant. The main effect on the hardened concrete seems to be due to the fact that the small mineral grains become an integrated part of the hydrated cement paste instead of discrete particles. Moreover, the ultrafine particles result in a more homogeneous interfacial zone. Mercury intrusion tests show that the ultrafiller gives a finer pore system. Basically the strength increase is due to the fineness, but increases in long-term strength also indicate that fine quartz also acts as slow pozzolana. With an optimised recipe and only 150-kg cement concrete with a compressive strength of 80 MPa can be produced. With a combination of ultrafiller, cement and silica fume concrete with more than 100 MPa in compressive strength can be produced with only 180 kg of binder. The incorporation of ultrafine particles accelerates the cement hydration. The effect increases with the fineness. Some minerals like wollastonite and calcite influence the onset of the acceleration period but the major influence is on the heat release during the acceleration period. Comparison between different minerals and production forms of the ultrafilier shows that the surface of the ultrafiller and aging is of major importance. The results show that ultrafine fillers can replace a substantial amount of cement, which will reduce the environmental impact by reducing energy consumption and CO2 release. Much of the potential is, however, already taken into account by the introduction of CEM II/A-LL cement that contains ultrafine limestone, but it is possible to reduce the amount of cement even further. In this case cement is replaced with ultrafiller while the water/cement is kept constant. This, however, demands superplasticizer and the result is that cement is replaced by a combination of ultrafine filler and superplasticizer. The concept with ultrafiller is currently probably not economical for the bulk production of concrete. This is due to the cost of grinding and handling of the fine materials in concrete production. By the use of wet ground material as slurry it is possible to avoid silicosis and other dust problems. If ultra fine particles are available as a by-product, however, it may be commercially interesting. The ultrafine particles are, however, interesting when one want to produce concretes for special applications or environments. Interesting areas are where one wants a lower energy release (heat development), dense concretes and special high performance concretes or other cementitious products where one wants to keep the amount of cement down. For example one can produce high strength concrete with relatively small amounts of cement to avoid thermal cracking and shrinkage. Ultrafine fillers were found to be an important ingredient in formulating ultra high performance concretes (reactive powder concrete).
Place, publisher, year, edition, pages
Stockholm: Cement- och betonginstitutet (CBI) , 2004. , 40 p.
, CBI report, ISSN 0346-8240 ; 1:2004
Cement replacement, Concrete, Microstructure, Strength, Ultrafine filler
IdentifiersURN: urn:nbn:se:kth:diva-81635OAI: oai:DiVA.org:kth-81635DiVA: diva2:497701
QC 201202282012-02-102012-02-102012-02-28Bibliographically approved