There is a distinction between setting and hardening accelerators. Accelerators are concrete additives and can be applied as a powder or liquid. Their distinctive feature is that they are added to concrete in small doses, unlike admixtures such as limestone meal, silica meal or alumina, which are added in large amounts.
In setting accelerators, the additive influences the setting process, i.e. the fresh mortar is still workable. It reduces the working time and accelerates the setting process of the hydraulic reaction. In general, these agents increase the early strength, but simultaneously reduce the final strength of the mortar.
Hardening accelerators normally do not affect the processing time; however the increase of compressive and tensile strength is accelerated in the hardened mortar phase (mortar not workable any more).
In concrete technology, this term is used for liquid or powder ingredients which are added to concrete mixtures in small doses (<5% by weight of cement content). They influence the properties of fresh and hardened concrete by chemical and/or physical means. A distinction is made between 10 action groups:
|Action group||German Abbreviation||Colour code|
In addition, many agents are known in the construction chemistry which cannot be classified in these action groups, and which are, as a rule, incorporated in the dry mixtures as powder ingredients.
According to DIN 1045 concrete admixtures are finely separated ingredients which influence certain properties of the concrete and can contribute to the attainment of a higher content of mo grain (size fractions - 0.125 mm), if necessary. Concrete admixtures, also called fillers, have to be taken into account as volumetric components, as they are added to the concrete in larger doses than concrete additives are. Concrete admixtures basically must not be applied without a test for suitability. Examples of concrete admixtures are coal fly ash, silica flour, stone meal (quartz and limestone) and pigments.
Biogenic corrosion by sulfuric acid
Corrosion of concrete sewage pipelines above water level, caused by sulfuric acid which is produced by microorganisms (thiobacilli) during oxidation of H2S from anoxic or anaerobic sewage. This type of corrosion occurs where sewage systems are insufficiently aerated, for example in the transitions from pressure pipe lines into free-fall shafts, or in large unpressurized pipe lines with low flow rates. Biogenic corrosion by sulfuric acid can be significantly reduced by the use of Calcium Aluminate Cements rather than Portland Cements in the construction of new sewage pipelines.
A value determined by means of the Blaine instrument for quantifying the fineness of a powder. The measurement is based upon air permeability. Air is passed through a specimen, and the resistance of the specimen to the passage of the air is measured. Specimens of high fineness resist the passage of the air more than a comparable specimen of lower fineness. The measured values of the Blaine test are not absolute values for the specific surface of a powder but are relative values, relying on reference standards for calibration. This is why they cannot be directly compared with values of other measurement methods.
Bond strength, or shear strength, is the resistance to separation of a mortar on concrete. The bond strength is measured by adhering a metal stamp on the mortar and a subsequent pull-off in vertical direction from the base by means of a draught gauge. The measurement of bond strength is important for the determination of bonds between different layers such as wall/plaster or floor/screed systems. The bond between the materials can be improved by using bonding agents (primers) or adding celluloses to the dry mortar mixtures.
Ceramic bonding is strength development resulting from sintering of crystalline phases after dehydration, or by glass formation. In general, ceramic bonds are based on clay as carrier of the bond, and are used for consolidation of ceramic refractory aggregates. During the drying and heating process, up to approximately 110°C the adsorbed water will evaporate first, followed by crystalline water at 500 and 600°C. Finally at approximately 1000°C the aggregates will sinter with the clay binder. The actual sintering temperature depends on low-melting impurities (alkalis, alkaline earth, Fe2O3, etc.) or specifically applied sintering agents (clays or fluxes).
Quotient of ultimate load and cross-section of an axially loaded sample. The compressive strength of Alumina Cements is determined by means of mortar prisms sized 4x4x16 cm, which have recovered from tensile strength prisms. Unbroken, parallel faces measuring 4x4 cm are loaded. Standard test ages are 6 hours, 1, 7 and 28 days.
Value for assessment of workability, deformation and compaction of concretes and mortars. For concretes, 4 sectors are distinguished: stiff, plastic, soft and flowable. There are several procedures for the determination of consistency, which either assess the rheological properties (e.g. slump test for concrete), the compaction (compaction by means of vibration in a metal container; compaction test according to Walz) or the deformation (deformation test according to Powers). Consistency is a key value for assessment of workability and must be adapted to the particular conditions.
Conversion: (of Alumina Cements)
Transformation of the metastable initial hydrate phase CAH10 (hexagonal) into the stable cubic hydrate phase C3AH6. Above 20°C, transformation proceeds from 3CAH10 -> C3AH6 + 2AH3 + 18H. Conversion causes a reduction in volume, creating voids in the cement matrix as a consequence of the evaporation of disengaged free water. The strength of the cement stone is drastically reduced by this process. Moreover, this porosity can cause corrosion of reinforcing steel in reinforced structural parts, and subsequently lead to a failure of the whole structure. This reaction called conversion depends on the temperature, the density of the cement matrix, the applied w/c-ratio (water/cement) and the cement factor (cement:aggregate ratio). Higher temperatures accelerate the reaction, while the following rule of thumb may be applied:
- 20°C conversion proceeds in years
- 40°C conversion proceeds in days
- 60°C conversion proceeds in hours
As a consequence of the cement amounts in concretes and mortars with low w/c-ratios and of non-hydrated cement in the cement stone, new CAH10 will continuously be formed by disengaging water of conversion, under normal temperature conditions.This way, a kind of self-healing effect is achieved over a certain period, which however should not obscure the fact that some time the strength will be reduced. The problem of Alumina Cement in reinforced concrete parts is not mainly the reduction of strength but the increasing porosity and the corrosion of reinforcement steel. As a consequence of the very high level of strength of Alumina Cements, residual compressive strength values in a range of 30-40 N/mm2 are quite possible in spite of complete conversion, however the deteriorated bond with the reinforcement can cause a failure of the structural part. This is why in Germany Alumina Cement is not approved according to DIN 1164 and must not be applied in the constructional sector.
Fineness of grinding
Numerical representation of the specific surface of a cement. As a rule, the Blaine value is given in cm2/g or m2/kg. For a more detailed assessment of fineness of grinding, particle size distribution should be determined. The fineness of grinding generally influences the water requirement and also the reactivity of the cement.
Size classification system for aggregate materials like quartz or stone chips. Fractions are mixtures of particles which are defined by a maximum and a minimum size (sieve opening). They are given in mm without a unit, like e.g. 4/8: size fractions of 4 to 8 mm at delivery.
Method for determination of grain size distribution, e.g. in cements or aggregates like quartz sands. Coarse materials require specially adapted series of sieves of decreasing opening size. Finer materials require laser granulometry instruments, like e.g. Cilas or Sympatec instruments.
Transformation of cement grout (cement + water, pasty consistency) into cement stone or matrix (set solidified cement grout). The hardening process results from the reaction of water with the cement phases (hydration) into crystalline hydrate phases. The increasing mutual penetration of growing crystals (felting) leads to a mutual support of the crystals, leading to a development of strength in the mortar or concrete.
Hydration: (relating to cement)
Reaction of the cement with water, forming hydrate phases. This reaction is exothermic (evolution of heat during reaction), largely occurring during the first hours after contact of cement with water during setting and early hardening periods. Then the reaction slows down but does not stop completely. This is the reason for the subsequent hardening of concrete and mortars, even after years.
In Alumina Cements the main phase CA reacts with 10 H into CAH10 (note cement terminology: C=CaO, A=Al2O3, H=H2O). This phase is metastable and reacts through C2AH8 into the stable C3AH6.
A bond creating strength by the reaction of cement with water and subsequent hardening in the open air or under water, essentially by forming crystalline hydrate phases of CaO with SiO2 (in Portland Cements) or by CaO and Al2O3 (in Alumina Cements). Hydraulic bonds are, for example, applied in concretes and mortars under normal temperature conditions for bonding gravel and sand by means of cement grout (water + cement). Fireproof concretes require Alumina Cements, because concretes containing Portland Cements lose their strength when they are heated, as the hydrate phases are destroyed. Unlike Portland Cements, Alumina Cements continuously release their water and form new ceramic bonds starting at approximately 1000°C. After a temporary decrease of strength during the heating process (destruction of hydraulic bonds), the strength is gradually increased again.
Light Expanded Clay Aggregate; trade name for expanded clay material used for lightweight concretes or for fireproof compounds.
Mixtures of magnesium oxide (burnt magnesia) and concentrated magnesium chloride solution hardening like stone, forming basic chlorides (magnesia cement, sorel cement) of type MgCl2 * 3Mg(OH)2 * 8H2O, their structure being derived from the structure of magnesium hydroxide Mg(OH)2, and are, after addition of neutral fillers and colours, used for the production of artificial stones and jointless floors as well as artificial ivory (billiard-balls). (Holleman-Wiberg, Lehrbuch der anorgan.Chemie, 81st - 90th edition, p.685-585)
Mixture of cement and sand, in general with a maximum grain size of 4 mm. Mixtures with larger aggregate materials are called concrete. Standard mortars are produced with a standardized sand (standard sand) with a maximum grain size of 2 mm and a defined composition (1350 g sand, 450 g cement, 225 g water). See details in EN 196 Part 1.
Particle size distribution curve
A graphical representation of particle composition in aggregates or cements. The amount passing through sieves (in % by weight) is graphed in relation to decreasing grain size. In aggregates, a distinction is made between continuous and discontinuous curves. In the latter, individual groups of particles are omitted (discontinuous granulometry, called gap grading), whereas the granulometry in continuous curves has no gaps. The grain size distribution curve is an essential factor for the water requirement of a mortar or a concrete and its workability. A skilful selection of the grading curve can contribute to a reduction of cement requirement and thus influence properties like shrinkage.
Stability range of a chemically homogeneous substance, depending on temperature, pressure and composition. A chemically homogeneous substance like SiO2 can, for example, exist in a different state of matter (solid, liquid, gaseous), depending on the temperature, and also, as a result of a simultaneously varying pressure, in different crystal structures (christobalite, tridymite, quartz), whereas the chemical composition remains unchanged. Every one of these structures is a distinct phase. In phase diagrams, where different primary substances (e.g. CaO, Al2O3 and SiO2) are represented side by side, crystals of one modification can also have different compositions, while single atoms are placed on the wrong grid places. In this case, a phase has a stability range which depends on temperature, composition and pressure.
Quick setting binders
Quick setting hydraulic binders, produced either by addition of accelerators or by mixing Portland Cements and Alumina Cements. Used as waterproofing materials in case of water ingress, or as dowel materials and similar. The strength values of these binders, which set within few minutes, lie clearly below the final strength of their primary substances.
Additives which delay the start of setting. Citric acid is a common retarder for Alumina Cements, with small doses capable of significant retarding effect. Also, retarding can also be a side effect of methyl celluloses.
Increased viscosity of a cement grout within the early reaction stage of cement phases with water. During setting, the mortar or concrete is still pasty and workable, but its flow gradually decreases. Setting is tested according to DIN 1164 by penetration of a defined needle into a defined cement grout. As the hydration reaction proceeds, the needle will penetrate ever less into the cement grout, until the setting process turns into a hardening process, and the cement grout develops a strength resisting the weight of the needle. Initial and final set are specified by defined penetration depth of a needle into a cement grout (see DIN 1164). In case of Alumina Cements, the setting behaviour is partly also tested on a mortar, according to DIN 1164, because there are often obvious deviations in the setting times of cement in cement grout and mortar.
Decrease in volume of cement stone caused by water emission or caused by transcrystallization or chemical shrinkage. A distinction is made between early and late shrinkage. Early shrinkage occurs during setting and the first development of strength. It can be compensated by shrinkage compensators or swelling components, which must be very exactly adjusted to the reaction process of the primary mixture. It is much more difficult to control the late or long-term shrinkage, because, as a rule, the shrinkage compensators also form compounds containing water, which can shrink, too. To cope with this problem, special attention should be paid to the fact that only the smallest necessary amount of water and cement should be used, depending on strength and processing specifications.
A measure of consistency of a mortar or a concrete, determined by a slump test according to DIN 1048 (for concrete) or by means of the Hägermann table (for mortar). The tests are executed, for example, with a specified amount of fresh mortar filled into a cone-shaped mold. Subsequently this cone is pulled off in upward direction, and the diameter of the spreading material is measured. Then the table top with the material is jolted for 15 times. The material will spread out, and after 15 jolts the final slump is measured. This method gives information about the rheological behavior, the tendency of sedimentation, and possibly existing thixotropic properties of a mortar. The tests for concrete are basically similar, but are executed on a larger table which is manually lifted on one side up to a stopper.
see Magnesia binders
Stabilizers are additives as defined in concrete technology, and form a class of their own (abbreviation: ST). They are applied for control of water balance in a mixture or for adjustment of a certain consistency and workability. For example, celluloses, guar seed meal and others are applied, which are able to absorb large amounts of water and make them available again for crystallization of the cement phases, while the mortars or plasters do not lose this water by desiccation.
Additives causing great increases in flowability of mortar or concrete. Superplasticizers (FM) are a defined term of a group of additives (concrete additives) in concrete industry, used for liquefaction of concretes on site with a temporarily limited effect (as a rule, approximately 30 min). This is why they must not be added to the ready mixed concrete until it is delivered to the job site. For the liquefaction of Alumina Cements, for example, either additives based on acrylates or Na-gluconate are applied, or additives which have a liquefying side-effect (e.g. citric acid).
Increase in volume of a substance by absorption of e.g. liquids or gas.
Strength of a concrete or a mortar in bending. Tensile strength of mortars is tested by means of rectangular prism samples (similar to a beam) sized 4x4x16 cm, which are loaded in a 3-point flexural test until they break. The sample lies longitudinally on two round metal bars creating a span, and load is applied at the centre of the span. By this, compression is created on the upper side of the sample, and a tension occurs on its bottom side, as the sample tends to bend and thus elongates itself. If the applied force exceeds the strength of the mortar, the sample will break. The value determined by this test is called the tensile strength of the material, and is indicated in N/mm2.
Instrument for determination of setting behaviour of cement according to DIN EN 196 Part 3. A cylindrical needle with a diameter of 1.13 mm and a load weight of 300g is placed on a rubber ring (conical, diameter 70 respectively 80 mm) filled with cement grout, and the penetration of the needle is measured periodically. The end of setting is determined by means of a more complicated needle with an extension ring.
The water requirement of a cement is measured according to DIN EN 196 Part 3 by means of a Vicat needle. In this test, the cement is mixed with a certain amount of water to achieve a defined standard stiffness. The water requirement of a cement depends on the fineness of grinding and the mineralogical composition of the cement clinker.