Full Description of Types of Concrete AdditionsArticles > Full Description of Types of Concrete Additions
These are materials which are permitted to be added to a concrete or mortar mix in much larger quantities than admixtures. In BS 5328: Part 1, the following materials are included under the heading of Additions in Clause 3.6:
- pulverized fuel ash (pfa) complying with BS 3892;
- ground granulated blastfurnace slag (ggbs) complying with BS 6699.
Pulverized fuel ash (pfa)
Pulverized fuel ash is also classed as a cement replacement and in fact that is its principal use in the concrete industry. This material is a by-product of pulverized coal-fired electricity generating stations. It is a fine powder, the particle size being in the range 1–50 microns. The approximate composition is:
50% silicon (SiO2)
28% alumina as Al2O3
11% iron oxide as Fe2O3
11% oxides of calcium, magnesium, sodium and potassium
The relevant British Standard is BS 3892: Parts 1 and 2.
Part 1 covers pfa for use as a cementitious compound in structural concrete; Part 2 covers pfa for use in grouts and for miscellaneous uses in concrete. Pfa exhibits pozzolanic activity and BS 3892: Part 1 contains a test method for determining the pozzolanic activity index. The Standard limits the sulphate (as SO3) content to a maximum of 2.5%.
Part 2, which deals with pfa for use in grouts, specifically states that this does not apply to grouts used in ducts for prestressing tendons.
- reduction in heat of hydration;
- improved workability with constant water/cement ratio;
- increased resistance to sulphate attack;
- reduced permeability to liquids;
- long-term increase in compressive strength.The main advantages claimed for the inclusion of pfa in concrete are:
There is some reason to believe that the inclusion of pfa in the concrete may render it more resistant to alkali-silica reaction. Reference should also be made to BS 6588: Specification for Portland pulverized fuel ash cement which lays down requirements for composition, strength, physical and chemical properties for two combinations of Portland cement and pfa. As far as the author is aware, it is not possible to determine by chemical analysis the proportion of pfa in hardened concrete or mortar. The presence of pfa in concrete can be confirmed by microscopic examination of thin sections.The pfa content in fresh concrete can be determined by the chemical method described in BS 6610, or by the particle density method described in Annex D in Part 128 of BS 1881—Methods for Analysis of Fresh Concrete.
Ground granulated blastfurnace slag (ggbs)
The relevant British Standard is BS 6699: Specification for ground granulated blastfurnace slag for use with Portland cement. The slag is a waste product from steelworks; the raw materials going into the blastfurnace are iron ore, limestone and coke. The products of the blastfurnace are iron and slag. The slag can be used as an aggregate or as an addition to Portland cement for concrete. When used in combination with OPC it increases the resistance of the concrete to sulphate attack, and to ASR by limiting the alkali content of the binder (cement plus ggbs). The proportions used with OPC depends on the required effect on the resulting concrete. Generally, mixes containing 40% ggbs and 60% OPC to 65% ggbs and 35% OPC are used.
Condensed silica fume
Condensed silica fume is a waste product of the ferrosilicon industry. It consists of 88–98% of silicon dioxide (SiO2) with very small percentages of carbon, ferric oxide, aluminium oxide (alumina) and oxides of magnesium, potassium and sodium.
It is a very fine greyish powder with a specific surface about fifty times that of normal Portland cement, the particles having a diameter of about 1 micron, and is a highly reactive pozzolan. The addition of condensed silica fume to concrete and mortar has a significant effect on the properties of the plastic mix, as well as on the hardened material. The dosage is generally in the range of 2% to 10% by weight of cement.
Its presence imparts a number of beneficial characteristics to the concrete and mortar:
- increased cohesion;
- reduced permeability;
- increase in compressive strength;
- increased resistance to sulphate attack (except possibly ammonium sulphate);
- increased resistance to a number of aggressive chemicals; including some ammonium-based fertilizers.
The very small particle size increases the water demand of the mix and can result in premature stiffening if placing and compaction is delayed. It is normally used with a superplasticizer. When used with selected Portland cement, selected aggregates and a superplasticizer, compressive strengths of over 90N/mm2 can be obtained; this involves careful mix design and strict control of all aspects of the concreting procedure.
In the UK and USA condensed silica fume is marketed as a stabilized slurry which contains a plasticizer or superplasticizer. The Agreement Certificate for the ‘Elkem Microsilica’ gives the pH of the silica/water slurry as 5.5 plus or minus 1.0. As far as I am aware, it is not possible to determine by chemical analysis, the proportion of condensed silica fume present in hardened concrete or mortar. There is no British Standard for condensed silica fume.
The term ‘polymers’ includes a wide range of materials, but here epoxies, polurethanes and polyesters, are dealt with under Reactive Resins. The polymers described in this section are mainly available in liquid
or powder forms. The liquids are dispersions (also referred to as latexes), and are generally whitish in colour. The solid content and viscosity vary; the solid content is generally in the range of 40–70%. The polymers available in powder form are mainly:
- modified polyvinyl acetates (PVAs);
- ethylene vinyl acetate (EVAs);
The dispersions include styrene butadiene rubber (SBR), Neoprene (an artificial rubber), styrene acrylics and natural rubber latexes. In my opinion, the manufacturers of polymers for use in concrete or mortar mixes should comply with Code requirements for the disclosure of information on ingredients.
While each of the polymers mentioned above have their own specific characteristics, they are all intended to impart some useful properties to the concrete/mortar in which they are incorporated, and these properties are summarized below:
- improved workability of the mix with constant w/c ratio or reduced w/c ratio with constant workability;
- increased bond with the substrate;
- reduced permeability and absorption;
- improved resistance to carbonation;
- some limited increase in resistance to attack by aggressive chemicals.
A great advantage in the use of synthetic polymer dispersions is that they can be tailor-made to meet the requirements of the proposed end use. The particle size of the SBR solids in an SBR latex is in the range 0.13 to 0.23 microns which can be compared with Portland cement of 1–50 microns. When these polymer latexes are added to the mix on site, the proportions recommended by the suppliers should in general be followed. In the case of SBRs, the proportions are usually in the range of 5% to 10% by weight of cement; the amount of emulsion added depends on the reason for its addition.
All polymers are expensive and therefore their use should be given careful consideration. Proprietary repair mortars consisting of prepacked cement and aggregate often have a selected polymer latex included as a gauging liquid to which water must be added to provide the required workability. Other prepacked mixes have the polymer included in powder form.
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