In  this  paper  the  use  of  fly  ash  for  particular  High  Strength  Concrete  (HSC)  realizations  is investigated.  The  use  of  fly  ash  as  a  partial  replacement  of  Portland  cement  in  HSC  seems  a valid  solution  in  particular  for  a  sustainable  construction  design,  considering  the  interesting HSC  performances  and  the  economic  aspects.  Obviously  the  advantages  demonstrated  for  the underground  constructions  can  be  also  evaluated  for  the  structures  in  elevation.  In  this  paper the  most  important  technical  regulations  about  the  use  of  fly  ash  for  the  concrete  mix  design are  discussed  and  it’s  shown  the  increase  of  the  mechanical  resistance  and  the  better  protection against  the  chemical  agents  attacks  and  against  the  freezing  and  thawing  cycles  using  fly  ash. Moreover,  in  comparison  to  the  fresh  concrete,  it’s  presented  the  benefits  related  to  the  minor ratio  water/cement  (defined  by  w/c)  in  the  same  conditions  of  workability.  Additional considerations  are  carried  out  about  the  hydration  process:  the  mixture  produces  less  heat  for the  lowering  of  the  C3A  and  C3S  percentages  using  fly  ash.  Finally,  a  greater  concrete impermeability  obtained  using  fly  ash,  due to  the presence  of  a  lower  hydrolysis content, which leads to an increment of  the  cement paste porosity, has  been evidence

G. Silvestro

M. Zucca

N. Giordano

N. Longarini

P. G. Crespi

English

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Longarini N., Crespi P.G., Zucca M., Giordano N., Silvestro G.: THE USE OF FLY ASH IN HIGH STRENGTH CONCRETE MIX DESIGN   101 THE USE OF FLY ASH IN HIGH STRENGTH CONCRETE MIX DESIGN  Nicola LONGARINI2, Pietro Giuseppe CRESPI1, Marco ZUCCA2, Nicola GIORDANO2, Giuseppe SILVESTRO3  1 Politecnico di Milano, Department of Architecture, Built environment  and Construction engineering (ABC), Milan, Italy. pietro.crespi@polimi.it 2 Politecnico di Milano, CISE Consortium, Milan, Italy. nicola.longarini@polimi.it;  m.zucca@aol.com; nicola_giordano@hotmail.it 3MSc Civil Engineer, Milan, Italy. giuseppe.silvestro@aol.it      Abstract In this paper the use of fly ash for particular High Strength Concrete (HSC) realizations is investigated. The use of fly ash as a partial replacement of Portland cement in HSC seems a valid solution in particular for a sustainable construction design, considering the interesting HSC performances and the economic aspects. Obviously the advantages demonstrated for the underground constructions can be also evaluated for the structures in elevation. In this paper the most important technical regulations about the use of fly ash for the concrete mix design are discussed and it’s shown the increase of the mechanical resistance and the better protection against the chemical agents attacks and against the freezing and thawing cycles using fly ash. Moreover, in comparison to the fresh concrete, it’s presented the benefits related to the minor ratio water/cement (defined by w/c) in the same conditions of workability. Additional considerations are carried out about the hydration process: the mixture produces less heat for the lowering of the C3A and C3S percentages using fly ash. Finally, a greater concrete impermeability obtained using fly ash, due to the presence of a lower hydrolysis content, which leads to an increment of the cement paste porosity, has been evidenced  Keywords: fly ash, mix design, high strength concrete, admixture  (received 14 April 2014, accepted 12 May 2014) Fly ash classification Nowadays, despite the increase of renewable energy sources, in some countries use of coal is the best way for electricity power production. It results in the generation of large quantities of fly ash. In this field, there are also different types of coal and the boiler used in this process produce different types of fly ashes, such as siliceous, calcareous or silica-calcareous fly ash.  The fly ash could be defined as fine powder of mainly spherical, glassy particles, obtained from burning of pulverised coal, which has pozzolanic and hydraulic properties, and consists essentially of SiO2 and Al2O3 (Table 1).  It is obtained by electrostatic or mechanical precipitation of dust-like particles from the flue gases of the power plants. This product has got pozzolanic properties and, for this reason, it isn’t only waste material but it may be used to improve performances of other products as concrete. Before use, fly ash may be subjected to industrial processing, for example by selection and classification, sieving, drying, blending, grinding or carbon reduction, to optimize its fineness or reduce its water demand.  When using fly ash, it should be noted that, apart from the effect from the pozzolanic property, certain properties of fresh and hardened concrete may be affected.   18th Conference on Environment and Mineral Processing    102 Table 1. Normal range of chemical composition for fly ash produced from different coal types  (expressed as percent by weight). Component Bituminous (%) Sub-bituminous (%) Lignite (%) SiO2 20-60 40-60 15-45 Al2O3 5-35 20-30 10-25 Fe2O3 10-40 4-10 4-15 CaO 1-12 5-30 15-40 MgO 0-5 1-6 3-10 SO3 0-4 0-2 0-10 Na2O 0-4 0-2 0-6 K2O 0-3 0-4 0-4 LOI 0-15 0-3 0-5  Figure 1. Common applications of fly ash in the US. - American Coal Ash Association (ACAA). 2006 coal combustion product (CCP) production and use. Aurora, CO: American Coal Ash Association; August, 2007. Fly ash use in concrete mix design Generally, pozzolanic additions in concrete offer undoubted advantages compared to using of cement (Portland or admixture) only. These additions are usually defined as "type II" and it’s different from those of "type I" which are mainly inert, sand and gravel properly. The first advantage is constituted by the increase of fresh concrete pumpability in a case where it used a low content water and, in this way, using a low content of cement, to maintain constant the water – cement ratio. In later time, the possibility of improving other performances by using of fly ash, compared to concrete mixes with cement only, has been evaluated. In this way, the “type II” addition can be used to replace (or in addition to) the cement so that it can modify the water – cement ratio, the principal parameter of concrete design. For instance, it is possible reduce the cement content with an increase of fly ash keeping constant the water – cement ratio and the final strength of hard concrete. For this reason, in concrete mix design procedure it is possible speck about “equivalent binder” rather than simple cement. Obviously, in concrete production standards such as the European standard EN 206-1, the correct procedure for maximum content and homogenization coefficient (KFA) is illustrated as the purpose of fly ash to cement assimilation, according to an equation like:  equivalent binder [kg ] = cement [ kg ] + KFA × fly ash [kg] . Longarini N., Crespi P.G., Zucca M., Giordano N., Silvestro G.: THE USE OF FLY ASH IN HIGH STRENGTH CONCRETE MIX DESIGN   103 The European standard EN 206-1 suggests the maximum fly ash content, in order to define a limit of fly ash use as “type II” addiction. Exceeded that limit, 1/3 of the cement content, the remainder is considered as of type I addition only, so aggregates. Regarding the coefficient (KFA) value for concrete admixture made with cement CEM-I, Portland cement conforming to EN 197-1, the European standard EN 2016-1 admits that:  CEM I – 32,5: KFA = 0.2; CEM I – 42,5: KFA = 0.4. There is another tip suggested by the standard about fly ash use: the minimum content of cement, defined according to the class of environmental exposure, can be reduced and replaced with fly ash, at maximum as:  k × (minimum cement content - 200) kg/m3. Furthermore, the application of this coefficient KAF to concrete for exposure classes “XA2” and “XA3”, with a risk of sulphate attack, is not recommended. Determination of mechanical strength If a concrete is made using a quantity "c" of cement and a quantity "p" of fly ash, it is defined as equivalent cement (ce) the amount of cement required to obtain the same contribution to the compressive strength given by p. The sum of the value of ce and c of the amount of cement determines the total content of cement (ctot), as shown by the following relationship:  ctot = c + ce + kp The compressive strength of a concrete depends mainly by  the water / cement ratio and the concrete hydration degree (a) which is, however, difficult to assess and, therefore, it is preferred to consider the time of hydration (indicated by t). Considering that the temperature remains constant during the period of hydration, either when additives don’t modify the kinetics of hydration either when the curing is developed in a wet environment, the concrete  compressive strength,  given at the hydration time t*, can be expressed as :  Rc = A (a / c) ^ B where a = water present in the mixture, c = cement present in the mixture, A and B = constants to be determined experimentally dependent on the hydration time and the type of cement.  In the presence of fly ash it is:  Rc = A [a / (c + kp)] ^ B where k = efficiency factor of fly ash in relation to the compressive strength, p = content of fly ash. Workability The workability is the property of fresh concrete and mortar of determining the easiness that they are mixed, packaged and casted. It does not depend on the properties of the concrete only, but is also strongly influenced by the nature of inert and by the specific shape of casts.  The addition of mineral particles improves the properties of concrete but it reduces the workability.  Indeed, the addition of fine dust raises the surface area and consequently the water request. However, it is verified, in some cases, the addition of fine dust such as the blast furnace slag (made in certain proportions) is able to decrease the amount of mixing water. As told before, using fly ash in the mix design for concrete, leads to a significant improvement of fresh concrete properties, such as workability and pumpability so that allows to optimize the water – cement ratio. In fact, the obtained mixtures are more homogeneous with a low risk of particles segregation and water bleeding. This improvement is due to the ash spherical shape that enables particles to flow each other more easily, reducing the friction between the particles. Furthermore, that form is able 18th Conference on Environment and Mineral Processing    104 to minimize the surface – volume ratio of the particles reducing the demand for water. It depends by the particles form because the spherical shape allows obtaining a higher packing density than irregular shapes particles, which follows a lower water demand in equal condition of workability. The spherical and regular geometry, as well as the small size, of vitreous ash allows the mobility of particles better than the others inside the mix, admitting to reduce water demand, in contrast to what happens with other fine dusts, with the exception of the cement. This effect rises if the content and fineness of ash increase. It can be said that fly ashes bring benefits to concrete mixes, in terms of workability and surface finish, when used in mixtures poor in fine aggregates. Heat of hydration The hydration of Portland cement is an exothermic process, in which the heat determines an increase of the temperature of the fresh cement. A partial replacement of cement with fly ash, when the kinetics of hydration appears to be much slower than that of the cement, implies a lower heat development and a consequent raise in temperature. Ashes from bitumen, characterized by a low content of calcium, are more effective than those with a high content of calcium (called sub-bituminous ashes) reducing the rate of temperature increase. In massive civil constructions, a partial substitution of cement with fly ash, in percentages between 40% and 70%, leads to a great reduction of the thermal stress generated during the curing tasks of the concrete. Hydration and mechanical strength Addition of fly ash in mix design of concrete leads to a change in the rate of hydration and in the subsequent development of mechanical strength. Partial replacement of cement with ashes generates a slowdown in the hydration process of the cement during the initial stages of that process. Reactions involving pozzolanic fly ash take place only when hydration process of cement has already started (Figure 2 – after the second step); it happens when a sufficient amount of calcium hydroxide, which reacts with the silica contained in the ashes, is generated.   Figure 2. Three steps of concrete hydration – (Enco Journal, prof. M. Collepardi) The period of curing where ashes assume behaviour of a material inert practically depends on the reactivity of the system. Precisely, it depends on the content of amorphous silica and on the specific surface area, as well as on the curing temperature of concrete dough. Accordingly, all the beneficial effects, brought by the introduction of ashes in terms of hard concrete mechanical strength and reduction of permeability, are only manifested at later time. The rapidity with which concrete with ashes develops an increase of resistance, depends on various factors, of which the most important are the fineness of the ash used, the content of amorphous silica, the average size of particles, the proportions in which they were added, the environmental temperature and the possible use of chemical additives modifying other characteristics. In many researches it is shown the concrete with ash additions get an higher mechanical strength than the others, even though it needs more time to reach the final strength. Moreover, the addition of mixtures of ashes rich in fine fractions leads to an increase of the final strength without affecting the short term development rate. There are many factors which influence in a negative way the strength development and impermeability of concrete, for instance the presence of large dimensions pores Longarini N., Crespi P.G., Zucca M., Giordano N., Silvestro G.: THE USE OF FLY ASH IN HIGH STRENGTH CONCRETE MIX DESIGN   105 in the hardened concrete and some micro-cracks generated at the interface between the cement paste and aggregates.  Using of fly ashes tends to minimize these effects because they act both from physical and chemical points of view. The first acts through the filling of large pores with the fine and very fine particles, the second  products of the pozzolanic reaction tends to redefine the structure of pores by reducing the development of micro-cracks at the transition zones. Mechanical properties and durability of concrete are conditioned by density and microcracks formation at the contact zone between the cement paste and aggregates. In conventional concrete, based on Portland cement, the transition zone has a density less than the remaining mass of material and an higher content of calcium hydroxide crystals. For this reason, the formation of micro cracking phenomena is easier than in other areas because the shrinkage induced by changes in temperature and humidity, between cement paste and aggregates is different. It is clear from the foregoing points, that the interface area of cement paste - aggregates is the most critical in terms of hardened concrete mechanical strength. Using of fly ash improves the mechanical strength and durability of concrete as it leads to an increase of the mechanical strength of the area of the interface by changing the structure. Resistance to chemicals attacks The resistance to aggressive chemicals is strongly influenced by the decomposition state of the cement paste which is due to internal chemical reactions occurring to the paste already hardened. Acid solutions react with the calcium hydroxide and generate a water-soluble compound which leaches from the concrete getting an high-porosity damaged material. As result of the reactions, there is a drastic decrease of the chemical resistance of the concrete and the water-soluble sulphates reacting with the Ca(OH)2 lead to the formation of calcium–sulpho–aluminate, also called “ettringite”. This reaction is characterised by a volume increase and, when it occurs in the matrix already hardened, it involves in a physical disgregation of the concrete. The addition of fly ashes leads to an increase of the resistance of concrete to attack by particularly aggressive chemicals as water-soluble sulphates. In fact, the reaction of pozzolanic ashes, at first, consumes the calcium hydroxide, which is one of the main causes of the vulnerability of concrete to chemicals, and secondly leads to a significant porosity of the paste decrease resulting in a decreased permeability to aggressive chemicals. By replacing about 15% of the weight of cement with fly ashes, like subbituminous type, the resistance of the concrete exposed to the action of solutions by Na2SO4 and MgSO4, in concentrations of less than 10%, is considerably increased. Resistance to freeze-thaw cycles The resistance of concrete to freeze-thaw cycles depends on several factors including the formation of an appropriate system of micro-pores that ensures entrapment of an appropriate amount of air, the cooling rate, the curing conditions, the type and conformation of the cement matrix and the possible presence of anti-freeze salts. Adding of fly ashes inside concrete, results in a lowering of the freezing temperature of water into the pores, because of the redefinition of these pores, resulting in a raise  of concrete durability. However, ashes inside the concrete tend to reduce the amount of entrained air and, as the amount of ashes represents a fundamental parameter, it is good to keep under observation the distribution of pores in which the air remains. Conclusion In this paper the main advantages in the use fly ash in concrete admixtures is described. As it has been seen, the use of waste ashes made by thermal power stations turns out to be not only in an improvement of intrinsic properties of concrete as mechanical strength, workability, durability and resistance to chemicals attacks and freeze cycles, but also an interesting solution in the perspective of a sustainable design which uses waste materials of a production process hardly reusable. Finally, it is important to say that when fly ash are used in concrete, the potential for leaching of trace elements is very low. This is due to the constituents of fly ash being encapsulated in the matrix of the concrete. 18th Conference on Environment and Mineral Processing    106 References [1] VANCE, Kirk et al. The rheological properties of ternary binders containing Portland cement, limestone, and metakaolin or fly ash. Cement and Concrete Research 52, Elsevier Science, p. 196-207, ISSN: 00088846. [2] GOYAL, Shweta et al. Effect of relative proportion of pozzolana on compressive strength of concrete under different curing conditions. International Journal of Engineering, Vol. 2 : Issue 1, p. 20-34, ISSN: 1985-2312.  [3] COLLEPARDI, Mario. The New Concrete. Grafiche Tintoretto, 2010, pp 436, ISBN: 978-88-903-7772-3. [4] COPPOLA, Luigi. Concretum. McGrow Hill, 2007, pp 700, ISBN: 978-88-386-6465-6. [5] UNI EN 197 – 1. Cement – Composition specifications and conformity criteria, 2011. [6] UNI EN 206 – 1. Concrete – Specification, performance, production and conformity, 2006. [7] UNI EN 450. Fly Ash for concrete – Definitions, requirements and quality control, 2007. 

The use of fly ash in high strength concrete mix design

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The use of fly ash in high strength concrete mix design

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