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CEMENT

There are cements formulated primarily for use in concrete, although some may be suitable for sand-cement mixes. Common cements consist of portland cement only, or a blend of portland cement and extender or filler.

From July 1996, when European standard was adopted, the South African standard for common cements became SABS ENV 197-1 “Cement - composition, specifications and conformity criteria. Part 1: Common cements”. The standard specifies a number of properties and performance criteria. Composition and strength are required to be displayed by the manufacturer on the packaging of each cement produced.

The standard specifies composition of cements according to the proportions of constituents, ie portland cement, extenders and fillers.

The standard specifies strengths which are determined in accordance with SABS EN 196-1 “Methods of testing cement. Part 1: Determination of strength”; using a water:cement ratio of 0.5.

The standard permits many different combinations of composition and strength class. In practice, however, the manufacturers will be constrained by what is technically and economically feasible. The number of combinations that are likely to be produced in South Africa will therefore be considerably fewer than the number permitted by the standard.

Masonry cements are formulated primarily to impart good workability to mixes for rendering, plastering and masonry work. Masonry cements are normally a blend of portland cement and finely ground limestone or hydrated lime; some masonry cements include an air-entraining agent.

From July 1996, the stand for masonry cements is SABS ENV 413-1 “Masonry cement. Part 1: Specification’. The standard defines masonry cement as ‘a factory made finely powdered hydraulic binder which relies essentially upon the presence of portland cement clinker to develop strength. When mixed with sand and water only and without the addition of further materials it produces a workable mortar suitable for use in rendering, plastering and masonry work.

The standard specifies composition, strength performance, fineness, setting times, soundness and the properties of fresh mortar.

Fly ash (FA) is collected from the exhaust flow of plant buring finely-ground coal. The finer fractions are used as a cement extender. FA reacts with calcium hydroxide, in the presence of water, to form cementing compounds consisting of calcium silicate hydrate. This reaction is called pozzolanic and FA may be described as a synthetic pozzolan.

The hydration of Portland cement produces significant amounts of calcium hydroxide which does not contribute to the strength of the hardened cement paste. The combination of FA and PC is a practical means of using FA and converting calcium hydroxide to a cementing compound.

FA should not be used on its own as the binder for concrete. The effect of FA on the properties of concrete depends on the FA concrete of the binder. General trends are as follow:

Fresh concrete:
Improves the workability of fresh concrete, ie FA tends to reduce water requirement for a given slump.
Slightly retards the setting of fresh concrete.
Hardened Concrete:
Reduces the rate of hardening and strength gain particularly at low temperatures
Reduces the rate at which heat is generated by the reactions of PC and FA
Improves the sulphate resistance of concrete with adequate FA content. Specialist advice is recommended
Reduces the rate of chloride diffusion through concrete
Can prevent or retard the reaction between alkalis and alkali-reactive aggregates in concrete if used in sufficient quantities, ie > 20%
Results in a finer pore structure and lower permeability if well cured. To achieve good durability all concrete should be well cured.
Condensed silica fume (CSF) is the condensed vapour by-product of the ferro-silicon smelting process. CSF reacts with calcium hydroxide, in the presence of water to form cementing compounds consisting of calcium silicate hydrate. This reaction, as mentioned before, is called pozzolanic and CSF may be described as a synthetic pozzolan. Because the hydration of PC produces calcium hydroxide, the combination of CSF and PC is a practical means of using CSF and improving the cementing efficiency of PC.

In addition to the chemical role of CSF, it is also a ‘fine filler’. The extremely small CSF particles in the mixing water act as nuclei for the formation of calcium silicate hydrate which would otherwise form only on the cement grains. CSF will also change the microstructure of the interfacial zone. The result is a more homogenous microstructure that has greater strength and lower permeability. (To ensure thorough dispersion and effective use of the CSF, the use of plasticising admixtures is recommended).

CSF affects the properties of concrete as follows:

Fresh concrete:
Reduces the workability of fresh concrete, ie CSF tends to increase the water requirement for a given slump.
Increases cohesion
Significantly reduces the bleeding of fresh concrete
Hardened concrete
Marginally retards strength development at one day
Reduces permeability of concrete
Reduces the rate of chloride diffusion through concrete
Increases the strength of concrete.
Finely ground limestone is chemically virtually inert when mixed with portland cement and water (although there are some minor reactions). Depending on its fineness, limestone may however act as a ‘fine filler’ in fresh paste. Limestone may be used as a filler in common cement or as a workability improver in masonry cement. The effect of limestone on the properties of concrete or mortar depends on the specific limestone, whether a grinding aid is used in production, and the fineness of the limestone. General trends are as follow:

Fresh concrete or mortar:
Has no significant effect on water requirement
Prolongs the bleeding period but reduces the amount of bleed water
Limestone may improve the workability of mortar


  

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