Precast concrete materials, manufacture, properties and usage - Chapter 4

4 FLY ASH This material, also known as pulverised fuel ash or PFA, is a by-product of electricity generation from pulverised coal firing. It is mainly of interest to those countries having this form of power production, but even in some of those countries it is not necessarily used everywhere because of transport costs. It has a beneficial action in many applications in in situ concrete where its pozzolanic (long-term cementitious effect in the presence of lime and water) and exotherm control properties, as well as its ability to give ordinary Portland mixes an improved sulphate resistance, have been used to advantage. As far as precast concrete product properties are concerned these benefits are of little value because of early strength requirements, generally small sections being cast, and good compaction, respectively. What is of interest to the precaster are the following questions: (a) Does the addition improve the early (0–10 minute old) handling properties? (b) Does the addition improve the early strength (6–18 hours old)? (c) Has the product better surface appearance and arrisses? (d) How are other relevant properties affected? (e) Does one get less wear and tear on machinery and plant? This chapter divides into several parts, the first part dealing with a description of fly ash, and the remaining parts dealing with specific process studies of applications researched by the author. There is one matter to note before proceeding, however, and that is a criticism (constructive) of the terminology ‘cement replacement’. Depending upon how one defines the control mix (the mix not containing fly ash) any addition of ash to the mix is a replacement of the cement and/or the aggregate. The only factor that is of interest is that of the concrete being Copyright Applied Science Publishers Ltd 1982 economical to produce as a function of materials price, the total cost of production and the number of rejects. 4.1 PROPERTIES OF FLY ASH Fly ash is a light slate grey to dark grey or brown powder extracted from the flue gases of a power station, usually by means of electrostatic precipitators. Its colour is governed mainly by the amount and particle size of the residual unburnt carbon, and secondly by the iron oxide. Table 4.1 gives the reader an idea of the ranges of chemicals in fly ashes internationally, bearing in mind that sources, other than those specifically selected, can be modern, old or standby power stations. TABLE 4.1 RANGES OF CHEMICAL MAKE-UPS OF FLY ASHES The large ranges shown arise not only from the varying efficiencies of the boilers but also from the fact that a single power station may well rely upon supplies from more than one colliery and that there could be several seams being worked in each colliery. Apart from the sulphate and carbon contents, precast concrete product performance is luckily quite insensitive to the chemical make-up of the ash. The first four chemicals, with the fluxing alkalis, form very small hollow glass balls, resulting in a low bulk density material. The presence of lime at high levels can result in cementitious properties and it is advisable to ensure that high-lime fly ashes are dry-stored otherwise they will slowly harden. The magnesia could cause expansive properties in the concrete if it is in the form of periclase. Although it is generally not in this form, Standards assume that it could cause trouble and specify limits. The sulphate is one of the troublesome ingredients because concretes Copyright Applied Science Publishers Ltd 1982 can generally tolerate a maximum sulphate level (SO3) of about 5% by weight of cement. Since cement already has up to 3% as SO3 from the gypsum used to retard the setting rate, the extra 2% or more needed to reach this can be easily obtained with an ash (2% SO3)/cement ratio of 1/1 by weight. Such concretes can suffer from long-term internal sulphate attack even though all their other properties may be acceptable. This is shown in Fig. 4.1 in five-year-old kerbs. Carbon is found as angular soft black particles which act as nominal voids and create a high water demand in the mix. Concrete colours tend to be darker than expected due to the carbon being ground finer in the mixer. Its presence is the reason why fly ashes cannot be used in light-coloured concretes. Carbon level is the factor leading to a loss of strength. Particle size can vary from 200 to 800m2/kg (Rigden or Blaine). Again, as for chemical composition, consistent material can generally only be obtained from a specified source. For in situ work the pozzolanic activity can be indicated by the passing 45 µm sieve but, as stated before, this is of little or no interest to the precaster. The acceptable range in precast processes is 300–600 m2/kg; if the ash is too coarse it has a reduced beneficial effect on properties and if it is too fine it becomes difficult to disperse and mix. The bulk density of fly ash can vary from 700 to 900 kg/m3. Compared to Portland cement’s range of 1300–1500 kg/m3 it can be seen that ash can result in dust nuisance and needs to be silo rather than bag handled and, in both cases, requires the installation of dust-extraction plant. Fig. 4.1. Internal sulphate attack in kerb containing fly ash. Copyright Applied Science Publishers Ltd 1982 This bulk density figure means that a fully compacted fly ash concrete can have a higher denseness coupled with a lower density compared to a control concrete. In the subsequent sections the following terminology is used: F Fly ash (Specific ashes F1, F2 and F3 used in some tests) C Ordinary Portland cement A Aggregate total W Water absorption at stated time (% on oven dry weight) I Initial surface absorption at stated time (ml/m2/s), F, C and A all on weight proportions. 4.2 WET-PRESSED PRODUCTS The process used here was the Fielding and Platt wet-pressed method where the initial water content of the mix is approximately halved under the action of pressure and taken out of the mix by a vacuum pressure box and a bottom filter. In some of the works tests three ashes with the properties shown in Table 4.2 were selected. The mix used was a uniformly graded, nominally dry 20 mm granite down to dust and Table 4.3 shows the mixes used in the pressed kerbs. Table 4.4 shows the 7 and 28 day flexural strengths in N/mm2 working to a national standard minimum limit of 5 N/mm2. Not only are the observed results recorded but they are also corrected for the financial gain bearing in mind that the mix becomes leaner in cement per unit TABLE 4.2 PROPERTIES OF ASHES USED IN THE THREE ASH-WORKS TESTS †‘Modern’ in 1963 when these ashes were sampled is no reflection on the later and improved boilers where a typical carbon content would be 1% or less. ‡The standby ash could not be air-permeability tested as its high carbon content did not enable one to make a bed in the cell. Copyright Applied Science Publishers Ltd 1982 TABLE 4.3 WET-PRESSED PFA MIXES TABLE 4.4 OBSERVED AND COST-CORRECTED FLEXURAL STRENGTHS OF WET-PRESSED KERBS volume as the fly ash proportion increases. As a comparative exercise a 5·9/1·0/1·30/2·00 mix is about 20% cheaper than the control mix and the corrections are based on a 1% economy for every 0·1 F/C increment. By this form of correction of the results one gets an idea of how much it costs to obtain strength in the product. The cost-corrected results are given in brackets. It can be seen that F3 detrimentally affects the strength at all loadings but that F1 and F2 have an initial benefit followed by a decrease in strength with increasing fly ash levels. The cost per unit strength numbers (given in parentheses) are interesting for F1 and F2 and indicate that up to or above equal cement weights fly ash concrete can produce economic and acceptable strengths. When one plots on a graph strength against fly ash concentration one obtains a pattern of points through which the imaginative person can draw what he or she likes. However, when one plots the strengths against carbon/cement ratio using Table 4.3 one achieves an interesting shape of Copyright Applied Science Publishers Ltd 1982 ... - tailieumienphi.vn