SUPPORTING A SUSTAINABLE FUTURE WITH MICROSILICA CONCRETE Des King*, Elkem Materials South East Asia, Singapore 32nd Conference on OUR WORLD IN CONCRETE & STRUCTURES: 28-29 August 2007, Singapore Article Online Id: 100032005 The online version of this article can be found at: http://cipremier.com/100032005 This article is brought to you with the support of Singapore Concrete Institute www.scinst.org.sg All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information www.cipremier.com
32 nd Conference on OUR WORLD IN CONCRETE & STRUCTURES: 28 29 August 2007, Singapore SUPPORTING A SUSTAINABLE FUTURE WITH MICROSILICA CONCRETE Des King*, Elkem Materials South East Asia, Singapore ABSTRACT This paper discusses the use of silica fume to enhance the properties of high performance concrete. It highlights the different benefits from manufacture, through plastic properties, strength and durability. Through project examples it demonstrates improvements in construction sustainability. Keywords: silica fume; microsilica; pozzolanic reactions; sustainability; high strength; properties of concrete. INTRODUCTION TO THE MATERIAL The term microsilica is the one normally used to describe the very fine powder, which is extracted from exhaust gases of silicon and ferrosilicon smelting furnaces and utilized in concrete to improve the properties of the concrete. Other terms for the same product are silica fume and condensed silica fume (CSF). The main purpose of incorporating the material in concrete is to make use of the very fine and reactive particles to produce a denser cement matrix. The microsilica particles have a pozzolanic reaction with calcium hydroxide from the hydration of the cement, thereby increasing the total product of hydration and reducing the amount of calcium hydroxide. When properly used, microsilica increases the strength and reduces the permeability of the concrete providing a more durable and more sustainable product. A small quantity of microsilica can be effective in a concrete mix, a typical dosage being in the range 5 to 10% by weight of the cement. SOURCES AND AVAILABLE FORMS The benefits of the material as an addition to concrete were first realised in Norway and microsilica concrete was used there as long ago as 1952. A Norwegian Standard for the incorporation of microsilica in cement was issued in 1976. The material is readily available in countries where the smelting furnaces are located, in particular Scandinavia, Canada, South Africa, Australia and China. The material used in South East Asia is imported from a number of countries, including Scandinavia, Iceland and China. The international standards commonly used in specifications are ASTM C1240 and EN 13263. Microsilica is an extremely fine powder with a mean particle size between 0.1 and 0.2 microns and a specific surface area of between 15,000 and 20,000m /kg. The particles are spherical in shape. For comparison, the specific surface of Portland cement is between 350 and 500m /kg: microsilica particles are about 100 times smaller than those of cement. The chemical composition of microsilica is largely silica (more than 90%), with small amounts of other metallic oxides and some carbon. The material extracted at the smelting plant is available in two grades, standard and refractory, the standard grade being normally used for concrete. Refractory grade is even finer with a higher silica content and is used in other specialised products.
The extremely fine particle size of microsilica would make it difficult to handle, transport, store and dispense without some adjustment to its original form. This is achieved either; by making larger particles from the powder, or by mixing it with liquid in the form of slurry. The material can be supplied in a densified form, in which the particles are loosely agglomerated and will break down under the action of the concrete mixer. The bulk density of microsilica is 200-300kg/ in the undensified form and 500-600kg/m when densified. Microsilica is available in slurry form. Only the manufacturer or a capable supplier should carry out mixing with liquid and special stabilising agents to obtain stable slurry. Microsilica slurry has a Particle Density of about 1.4. Accurate dispensing and homogeneous mixing can be achieved using a slurry and this form of the material has been used in ready mixed concrete supplied to many construction contracts around the world. Densified powders, are also being used increasingly and perform equally well. However, it is normally necessary to modify loading and mixing procedures to ensure proper dispersion of the microsilica when densified powders are used. Fig. 1 Microsilica particles EFFECTS ON FRESH CONCRETE The inclusion of a quantity of extremely fine particles in a concrete mix inevitably creates a much more cohesive material although, being spherical in shape, the particles of microsilica do have a lubricating effect on the concrete. The water demand to maintain workability is increased by compensation is normally provided by the inclusion of a superplasticiser, high range water reducer or plasticiser. The resultant concrete will still appear cohesive in comparison with an ordinary concrete mix. However, the microsilica concrete does behave in a thixotropic manner and, when it is vibrated, it will have the mobility required for easy compaction. Microsilica concrete is normally designed to have high workability for easy placing. In such cases the workability should be checked using the British Standard method for determination of flow. Such tests are recommended for concretes with very high workability. The slump test is a static test and does not give any indication of the way in which the concrete will behave when it is being placed and compacted. The slump test should not be used for high workability concretes or those which are thixotropic in nature, such as concrete containing microsilica. Being more cohesive than ordinary concrete, microsilica concrete is less prone to segregate even at very high workability's and there is a virtual absence of bleed water. This property of the material makes it very suitable for grouts and pumped concrete and for these applications microsilica can be classed as an anti-bleed agent. The inclusion of microsilica makes it possible to pump mass concrete which has a very low cement content: concrete with only 50kg of cement per cubic metre has been pumped successfully. The high cohesion and stability of microsilica concrete enables it to be placed under water with minimum risk of segregation during placing. In addition it is used an effective viscosity modifying
material in the production of self-compacting concrete. Linked with advanced admixture technology SCC with long workability retention for hot weather environments is achieved in practice. Setting times are similar to those of ordinary concretes. Figure 2. Fresh concrete performance Figure 3. The Roman Pantheon Pozzolanic concrete MIX PROPORTIONING The proportion of microsilica included in a concrete mix is usually expressed in terms of the percentage by weight of cement. It is normally regarded as an addition to the cement or cementitious content of the mix, not as a cement replacement. The amount included in the concrete for a particular use should be determined to suit that situation and the suitability of the concrete checked by means of trial mixes. As with other cementitious materials, microsilica will function differently with different types of admixture and trial mixes are essential. As a guide, the following amounts of microsilica are normally used: Normal concrete 5-10% of cement by weight High strength and high performance concretes 5-15% of cement by weight As a pumping aid 2-5% of cement by weight
Generally a plasticiser or superplasticiser is included in the mix to ensure that the concrete is sufficiently workable. Some reduction of the sand content is necessary and the use of coarser sands is preferred. Microsilica is compatible with both pfa and ggbs and can be used to improve the properties of concretes containing either of these materials. The use of these ternary, or triple blend cementitious cements with silica fume provide the best results for long term durability and have been successfully used in most of the world s monumental structures since the early 1990 s. The Roman Pantheon is the largest (43.4m dia.) un-reinforced solid concrete dome in the world. It was built by the emperor Hadrian almost 2,000 years ago. It is probably the most sustainable-formed concrete structure in the world and is a wonderful example of the earliest pozzolanic concrete made from natural pozzolanic volcanic ash mixed with lime. This is similar to the reaction that microsilica has with the lime liberated from the hydration of modern Portland cement, thus producing a denser cement paste matrix making the concrete stronger and more durable. In Asia and most other countries the microsilica is specified and used for its technical benefits, and savings in the initial cost of materials are not so important. STIFFENING AND HARDENING BEHAVIOUR Due to its very large specific surface, microsilica is extremely reactive compared with other cementitious materials such as fly ash and slag. The reaction of the microsilica is with calcium hydroxide generated during cement hydration to produce further amounts of calcium silicate hydrates, in addition to the hydration products of the cement itself. The physical effect is to create hydration products emanating from each particle of the microsilica thereby helping to fill the cement matrix, making it dense and impermeable. As with Portland cement, some unreacted material may remain in the concrete, even at later ages, but this will not detract from its dense and impermeable nature. Microsilica does increase the amount of heat evolved in concrete in the early stages of hydration but not to the extent that might be expected from the rapid rate of reaction. In high cement content concrete this effect will be appreciable but microsilica in concrete with medium cement content is not likely to have a noticeable effect on the heat evolved. The material is more susceptible to changes in temperature than Portland cement, pfa or ggbs and particular care is needed at both low and high ambient temperatures to ensure that microsilica concrete is properly cured. Microsilica is used to produce lower heat concrete, and this is made possible because the Portland cement element in the concrete mix can be significantly reduced for the same strength. Cement grain www.concrete.elkem.com Cement grain making concrete sustainable Figure 4. In a blended cement with microsilica there are about 125,000 silica fume particles for every grain of cement The finishing characteristics of microsilica concrete are very good and it is suitable for power trowelling and power floating. Because there is virtually no bleeding, there may be a tendency for the concrete to
crack due to plastic shrinkage. This is attributed to the rate at which moisture migrates to the surface being less than the rate of evaporation. Accordingly, greater care must be taken to cure the concrete properly, immediately after the finishing operation, to avoid cracking and/or surface desiccation. STRENGTH OF CONCRETE The use of microsilica is often associated with high strength concrete although its beneficial effects on the durability of the concrete are equally, if not more, important in many cases. Realisation of this additional strength does depend upon uniform mixing of the material in the concrete and adequate workability for compaction followed by thorough compaction and proper curing of the concrete. For concrete work with a large exposed surface, such as a floor slab, it is particularly important to ensure that curing is properly carried out. Strengths of 100 to 130 N/mm have been achieved in practice on several projects for the construction of tall buildings in North America, Europe, Asia and Hong Kong. Tensile strength is similarly increased by the inclusion of microsilica and this can be of benefit in floors and paved areas. The refined structure of the cement paste in microsilica concrete results in improved bond between the matrix and the aggregate particles, which increases the strength of the concrete and also has beneficial effects on durability. Bonding to steel reinforcement and to fibres is also greatly improved. There is an improvement in bond to old concrete and this makes the material very suitable for mortars and concretes used for repair work. SUSTAINABLE BENEFITS OF HIGH STRENGTH CONCRETE Because of the need to reduce CO2 emissions and the association between the manufacture of cement, a reduction in the volumes of concrete produced may be beneficial. The design of building structures with very high strength concrete will provide double benefits. Firstly by reducing the volume of structural concrete necessary, subject to design and secondly by increasing the value of the building in locations of expensive rentable floor space. Examples are shown in Hong Kong where Grade 100 and grade 90 concrete is specified and used in columns and core walls of two new tall buildings. Actual results of the properties of the concrete are shown in tables 1 to 4.
Fig 6 ICC Building in Hong Kong with G90 concrete supplied by Alliance Concrete Fig 7. In the One Island East Tower Grade 100 was used in these columns and the core walls. Concrete supplied by Gammon Construction MOVEMENTS The elastic modulus of microsilica concrete is similar to that of ordinary concrete for similar strengths. Very high strengths do result in more brittle concrete, whether or not microsilica is included, and this is considered in the design codes. The higher modulus associated with very high strength concrete is beneficial in tall building design and this may encourage the use of concretes with compressive strengths in excess of 100N/mm. Drying shrinkage of microsilica concrete has been found to be very little different from that of normal concrete. Cube compressive strength (MPa) No. of 7-day 28-day results Mean SD Max. Min. No. of results Mean SD Max. Min. 1,051 88.5 3.5 101.5 80.0 4,465 113.5 3.4 129.5 105.0 Table 1. Summary of cube compression test results for G100 concrete Estimated in-situ strength (MPa) No. of specimens M ean M ax. M in. 27 103.0 113.0 90.5 Table 2 Estimate of in-situ strength (MPa) for G100 concrete
The amount of shrinkage that takes place in particular concrete depends upon the aggregate volume and the properties of the aggregate itself. Careful selection of the materials and proper design of the mix will ensure that the shrinkage is at an acceptable level. Little information is available about the creep of microsilica concretes but some published data on high strength concretes containing microsilica indicate that the amount of creep is low. Examples of these properties in practice have been measured recently in a new Hong Kong tall building where Grade 100 has been specified. Table 3 Comparison with code of practice Statistical data Design parameters given by the Structural Use of Concrete 2004 Concrete strength grade C100 C100 Concrete cube comptressive strength (MPa) Avg. 113.5 Min. 100 Elastic modulus (GPa) Avg. 43.7 37.8 Creep coefficient at 162 days after loading Shrinkage at 162 days after loading (microstrain) 0.56 0.72* 180.6 343.2* * Calculated base on w/c = 0.38 and cement content = 500 kg/m 3 due to limitation of f CURING Curing is the process of preventing loss of moisture from the concrete whilst maintaining a satisfactory temperature regime. Curing and protection should start immediately after compaction of the concrete to protect it from premature drying out, high internal thermal gradients and low ambient temperatures or frost. Suitable measures include leaving formwork in place, covering the surface or spraying the surface with a curing membrane. The proper curing of concrete, which contains microsilica is particularly important, to prevent cracking, surface damage and possible loss of strength. The length of time for which the curing should be carried out depends upon the ambient conditions but, for microsilica concrete, should generally not be less than 7 days. Table 4 Mix design data for G100 Target grade strength (MPa) 100 Workability - slump flow (mm) 650 Water to cementitious ratio 0.25 Cementitious content (kg/m 3 ) <600 Binder paste volume 37% Fine / total aggregates ratio 0.39 As per manufacturers Superplasticiser dosage advice to obtain flow Table 5 Workability retention for G100 Time elapsed after mixing (hr) Slump flow (mm) Slump (mm) 0.0 795 280 1.0 795 290 2.0 715 280 3.0 600 270 4.0 595 255 5.0 545 240 PERMEABILITY AND DURABILITY The permeability of properly cured microsilica concrete is considerably less than that of ordinary concrete and some tests have shown the concrete to be virtually impermeable to water under pressure. Fig 8 This reduction in permeability is primarily due to the refinement of the structure of the cement paste by the presence of very small particles of microsilica acting as additional nucleation centres for hydration.
The reduction in permeability has beneficial effects on the durability of concretes which are exposed to attack by sulfates, chlorides, acids, carbonation or frost. The reaction of the microsilica with calcium hydroxide reduces, and may virtually eliminate, the calcium hydroxide content of the cement matrix and this also greatly improves the durability of the concrete. However, successful resistance to any type of aggressive action does depend upon adequate curing of the concrete. Prevention of chloride-induced corrosion depends upon the alkalinity of the concrete surrounding the steel reinforcement, as measured by a high level of ph in the concrete. In microsilica concrete the ph value is lower than in ordinary concrete but the greatly reduced permeability of the concrete compensates for this and there should be less risk of corrosion caused by chlorides. Fig. 9 Chloride diffusion coefficient Assume Fick's 1st law sq.cm/sec Chloride diffusion coefficient 4E-09 3.5E-09 3E-09 2.5E-09 2E-09 1.5E-09 1E-09 5E-10 0 Values for field cured specimens (cores) Tests by Taywood 2.1E-09 OPC 3.0E-11 OPC+MS 5.0E-10 OPC/GGBS 4.0E-11 OPC/GGBS+MS 3.3E-09 SR Experimental work in Norway has shown that microsilica concrete can have sulphate resistance as good as that of concrete made with sulphate resisting cement and, when the microsilica is used in conjunction with pfa or ggbs, the sulphate resistance is even greater. Fig. 10 Percent expansion 3 2.5 2 1.5 1 0.5 0-0.5 Sulfate expansion 0 20 40 60 80 100 120 140 Weeks in solution OPC OPC+MS OPC/GGBS OPC/GGBS+MS SR
RESISTANCE TO ACIDS Resistance to attack by acids is generally improved by the inclusion of microsilica in the concrete and this makes microsilica concrete particularly suitable for agricultural applications. The storage of agricultural products and silage can give rise to the formation of organic acids, which are aggressive to concrete. The use of microsilica in the concrete for storage containers, farm roads and other paved areas helps to resist this aggressive action. The rate of carbonation of microsilica concrete is generally higher than that of ordinary concrete at low strength levels. At strength levels above 40N/mm, the permeability of microsilica concrete is considerably less than that of ordinary concrete and the carbonation rate is correspondingly low. ENVIRONMENTAL ISSUES AND SUSTAINABILITY Because benefits are realised through the use of other supplementary cementitious materials, such as fly ash and blast furnace slag in triple, or ternary blends, the Portland cement element in concrete can be reduced. It is established that one tonne of cement clinker produces almost a tonne of CO2. Additionally all these SCM s including microsilica are by-products of other processes. Microsilica in particular does not contribute to CO2 emissions in its production. Fig 11. Dubai Airport Resistance of concrete to freezing and thawing conditions is usually achieved by air entrainment and this is also possible with microsilica concrete. Generally the dosage of air entraining agent has to be increased to compensate for the effects of the very fine particles of microsilica. Proper curing of the concrete is essential to ensure satisfactory frost protection. Microsilica concrete is used extensively for new concrete floors and overlays to existing floors where constant abrasion is experienced. In many cases steel fibres have been incorporated in the mix to improve abrasion resistance still further. The material has also been used to repair other structures, which are subject to abrasion, such as sea defence works, and spillways. Fig 12. Eastsea Bridge Shanghai Fig 13. Burj Al Arab Hotel, Dubai Fig 14. High quality acid resistant floors in the UK
MICROSILICA CONCRETE - SUMMARY OF HANDLING AND PLACING RQUIREMENTS AND RECOMMENDATIONS FOR THE USE OF MATERIAL Trial mixes are essential before determining the final mix proportions for a particular use. Concrete containing microsilica will be cohesive and appear to have relatively lower workability. However, it will respond to vibration and its finishing characteristics are good. Exposed surfaces must be cured by covering or spraying with curing compound immediately after finishing or there will be a risk of plastic shrinkage cracking. The concrete will bleed very little but the absence of bleed water must not be taken as an indication of early stiffening and hardening. Proper curing must be continued for seven days or more to ensure that there is no loss of potential strength. Microsilica can be used to produce high strength and high performance concretes, provided that a suitable admixture is incorporated in the mix to reduce the water content whilst ensuring adequate workability. Microsilica concrete has high cohesion and is particularly suitable for self-compacting and underwater concreting. Microsilica can be used as a pumping aid. Microsilica concrete has good bond with steel bar reinforcement and fibres. Microsilica mortars and concretes have good bond with old concrete and are suitable for repair materials and shotcrete. Microsilica concrete has good resistance to sulphates, acids, chlorides and abrasion and is suitable for use in situations where such attack may occur. References Microsilica in concrete. Concrete Society Technical, Report No.41, 1993. Condensed silica fume in concrete. FIP State of the Art Report. Thomas Telford, 1998. Condensed silica fume in concrete. Malhotra VM. Ramachandran VS, Feldman RF and Aitcin P-C, CRC Press, 1987.5. Microsilica concrete, Part 1: the material. Concrete Society Current Practice sheet No.104 and Concrete, October 1985. Microsilica concrete Part 2: in use. Concrete Society Current Practice sheet No.110 and Concrete, March 1986. Guide for the Use of Silica Fume in Concrete, ACI Committee 234, 200 Properties of Flowing Very High Strength Concrete. Gabriel W K Chan, K C Ng, Lawrence W H Leung, Gammon Construction Ltd. Hong Kong