Concrete with Mineral Additions

Some types of concrete have special properties that are useful with regard to the durability of reinforced-concrete structures. For example, conventional concrete with mineral additions, high-performance concrete (HPC) and self-compacting concrete (SCC) are briefly discussed here. 

Studies in the early 1970s [16,17] demonstrated that the addition of natural poz-zolana to Portland cement could reduce the chloride diffusion coefficient of concrete by three times. Additions of ground granulated blast furnace slag (GGBS) and fly ash (PFA) have an even more marked effect on the diffusion coefficient 

Polder and Larbi [19] estimated depassivation times, and thus service fives, almost 7 times greater for cements with 70% GGBS than for Portland cements (Table 12.5). 

Type of cement Recommended strength class for cover of  

Bamforth [23] suggested the recommendations given in Table 12.6 that show how the use of normal Portland cement does not produce concrete adequate for a service Hfe of 75 y in a chloride-containing environment, unless class C50/60 concrete and concrete covers of 100 mm are used. Cements with a high percentage of mineral additions allow the use of more reasonable concrete cover thicknesses and lower classes of strength. 

---

First of all the C/S molar ratio changes depending on the saturation of the liquid phase in concrete, in relation to Ca(OH)2 (Fig. 3.17). Especially in the case of cements with mineral additions, particularly siliceous fly ash or ground granulated blastfurnace slag, this ratio decreases and the sections of chains composed of [SiO ] " tetrahedra become longer [32]. 

The sulphate attack with C-S-H gel decomposition and thaumasite formation is particularly harmful for the durability of concrete, because it occurs with the destraction of the most important binding constituent of concrete and leads to the collapse of this material. Because of the relatively low temperatures of thaumasite formation, the concrete foundations, and elements of the underground sewage systems, as well as the concrete road elements are especially susceptible for this type of destructioa In order to prevent thaumasite formation the classic approach should be applied, first of all the permeability of concrete should be reduced, and the use of cement with mineral additions should be considered. Bensted [273] suggested the lowering the C3A and alite content in cement. 

Berry and Malhotra [118] conclude that in any high quality concrete with fly ash the carbonation process is comparable to this in concrete without mineral additions. Concrete with low cement content, not snbjected to the proper curing at early age (stored at low humidity) will be undoubtedly susceptible for the action of various corrosions physical and chemical, including carbonation [118]. 

As aforementioned, the efflorescence is not harmful for the durability of concrete. When the mechanism involved in efflorescence formation is known, the remedies of effective preventing its occurrence are also possible. For example cements with mineral additions can be used in the production of concrete elements. Silica fume has particularly beneficial effect, which prevents efflorescence already from 5 % addition. The groimd granulated blastfurnace slag or siliceous fly ash are good additives too. The latter one should be groimd when added to concrete or taken from the last section of electrical precipitator, where the finest fractions are collected. 

Finally, the so-called fillers constitute the last group of additives, which are, as a rule, inert and do not react in cement paste. The hmestone, added on a laige scale in Italy and in France was classified to this group a time ago [3]. As it is commonly known, the hmestone cannot be considered as an inert concrete component however, as compared with mineral additions from the former two groups, degree of CaCOj reaction in cement paste is rather poor. Nowadays, however, hmestones are recommended in the EN 197-1 standard as mineral additions, together with pozzolanic additives (see types of cements. Table 1.3). In 2007 the ratio of limestone cements produced in Europe and Turkey was 21.4 % (members of Cembureau) this corresponded to the annual output on the level of 56.2 million tons, in majority class 42.5. The ground quartz sand can be considered as typical filler, which does not react practically with calcium ions in cement paste, at ambient temperature. 

Slag cements 32.5N should not be used when eonereting proeeed at low tem-peratirres, particularly below 5 °C, beeaitse of very slow strength development, as compared to Portland eement concrete. This is related also to the Portland cement class 32.5 with mineral additions (Table 7.2) [83]. 

The other fine powder components, added to control the workability of concrete mixture are fly ash, limestone flour and ground granulated blastfurnace slag. The content of these mineral additions can be substantially high when cement type CEM 1 is used they substitute up to 50% cement, particularly that of class 42.5. One can conclude that the Portland cements with mineral additions can be also applied. The effect of limestone flour on the properties of concrete mixture was studied, among the others, by Gizeszczyk [32],... 

Nowadays blended cements are normally used, which are obtained by intergrinding or blending Portland cement with particular mineral substances. Among these, those with the addition of pozzolanic materials or ground granulated blast furnace slag are of particular interest with regard to durabihty of reinforced concrete. 

The similar reduction of water permeability was found in the case of a paste from cement with silica fume, rice hush ash and slags [138, 141], Also concretes produced of cement with 35 % fly ash addition show 2-5 times lower permeability, as compared to concrete from cement with no mineral additions [139]. 

Linking of the preventing action of mineral additions, with the sodium and potassium concentration lowering in the liquid phase, is obvious. The progress of alkali silica reaction is strongly dependent on this concentration, as well as on the related pH of this solution. Low alkali content is a warrant of the lack of concrete expansion with reactive aggregate. 

The concretes produced from cements with natural pozzolana addition are particularly resistant to the chemical corrosion, but the resistance to physical factors is only slightly changed. However, they should be cured for a longer time in humid condition than the concrete produced from Portland cement without mineral additions. They are useful in these conditions where a low heat of hardening and high resistance to chemical corrosion is required [5]. 

The authors [142] are reporting that the same advantage can be achieved in concrete production using cement with fly ash addition, with simultaneous warranty of its quality and optimum properties, which are under constant control. These authors are reminding also that the new ASTM C 1157 Standard edited in 1998, does not restrict the type and percentage of mineral addition, under the condition that cement complies with other standard requirements [142],... 

Metakaolinite is the high quality mineral addition to cement or concrete because of the excellent pozzolanic properties. In reaction with caldinn hydroxide C-S-H, C AHjj and gehlenite hydrate ate produced [183], These reactions are rapid and as soon as after 3 days these phases are detectable by XRD [184], The gehlenite hydrate is imstable in the presence of calcium hydroxide, while CjAHg is the stable phase, however, the rate of CH reaction with C2ASH3 is low and the non-equilib-rimn state is existing [185]. 

After 24 h of curing, the bars were placed in water at 80 "C for another 24 h to gain a reference length. They were then transferred to a solution of 1 N of NaOH at 80 C. Readings were then taken every day for 20 days. The concrete bars containing different amount of fiber and additives were tested. Also some specimens without any fiber or additives were tested as controls. The comparison with the control is an indication the pozzolanic ability of additives to suppress the ASR and from the solution. It also manifests if the mineral additives used are able to suppress the expansion by consuming more lime in concrete. 

A variety of materials, ranging from mineral binders to polymers, can be used as binder. However, materials based on polymers, such as epoxy or polyester resins, which are often used for concrete structures, should undergo careful assessment before use in historical masonry, due to possible issues regarding the mechanical and physical compatibility. Much more common for the repair of historical buildings are mineral binder systems based on cement or hydraulic lime with the addition of admixtures and fillers or aggregate. To inject bore holes, usually pure water/binder systems are used with typical w/b values of 0.8-1.0. However, the w/b ratio has to be adjusted according to the volume to be injected and to the moisture content of the substrate. 

---