Current Publications 2000

1. Introduction

Cementitious materials for concrete are fine mineral powders. When these materials are mixed with water, they react chemically to form a strong rigid mass that binds aggregate particles together to make concrete.

The cementitious materials dealt with in this leaflet are all based on portland cement and many contain a “cement extender.”

This publication gives information on the standards that apply in South Africa to cementitious materials for concrete;
provides guidance on the selection of cementitious materials for various applications; and discusses, briefly, the manufacture and properties of cementitious materials and fillers.

The effect of cementitious materials on dimensional stability of hardened concrete is outside the scope of this publication.

Note: Masonry cements that comply with SANS 50413 are not included in this leaflet because they are not intended for use in concrete. The national foreword of this standard reads:

“This part of SANS 50413 gives the definition and composition of masonry cements as commonly used in Europe for brick- laying, blocklaying, for rendering and plastering only, and not for concrete. Users are therefore cautioned to use the cements only for their intended purpose.”

Note that it is illegal to sell common cement in South Africa without a regulatory Letter of Authority (LOA) number which indicates compliance with SANS 50197-1 or EN 197-1.

Portland cement extenders

These standards are discussed below.

2.1 SANS 50197-1

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.

2.1.1 Composition

The standard specifies composition of cements according to the proportion of constituents, ie portland cement, extenders and fillers, as shown in Table 2 (overleaf).

As can be seen from Table 2, the standard permits many different combinations of composition. In practice, however, the manufacturers are constrained by what is technically and economically feasible. The number of combinations that are currently being produced in South Africa is fewer than the number permitted by the standard.

For the performance of a particular cement users should consult the the revelant producer for these details. Helpline numbers are given in section 4.

2.1.2 Compressive strength requirements

The standard specifies strengths which are determined in accordance with SANS 50196-1 Methods of testing cement. Part 1: Determination of strength; using a water:cement ratio of 0,5. (The method is not the same as the cube test used for concrete.) Strengths are shown in Table 1. Note that strengths must clear an early-age (2 or 7 days) “hurdle;” classes 32,5 and 42,5 must fall within a “window” at 28 days.

2.1.3 Other requirements

SANS 50197-1 lists other physical and chemical requirements with which cements must comply. These are monitored by the manufacturer and compliance is confirmed by external audit control sample testing. Details can be found in SANS 50197-2.

This standard defines the specific material, states the chemical and physical requirements, and specifies packing and marking, inspection and methods of test. Physical requirements that apply are summarised in Table 3.

Note: As discussed in section 2.1.1, not all the cements shown in Table 2 are necessarily available in South Africa.

It should also be noted that generally as the extender content of a cement increases, the rate of compressive strength development at early ages is reduced. The extent of this reduction can be assessed by comparing the relevent performance curves.

Table 4 gives guidelines for selecting cement type for various applications. Unless stated otherwise, the strength class of the common cement may be 32,5N or higher.

4. Strength performance

For accurate and current details of the performance of a particular branded product, consult the technical representatives of the manufacturer.

5. Manufacture and properties

In this section, only materials available in South Africa are discussed.

5.1 Portland cement

Portland cement is the basis of all common cements covered by SANS 50197-1 (see Table 2) and of site blends that include a cement extender.

The main raw materials used in the manufacture of portland cement are limestone and shale which are blended in specific proportions and fired at high temperatures to form cement clinker. A small quantity of gypsum is added to the cooled clinker which is then ground to a fine powder – portland cement.

When portland cement is mixed with water to form a paste, a reaction called hydration takes place. As a result, the paste gradually changes from a plastic state into a strong rigid solid. The hardened cement paste acts as a binder in concrete and mortar.

CSH provides most of the strength and impermeability of the hardened cement paste. Lime does not contribute to strength but its presence helps to maintain, in the pore water, a pH of about 12,5, which helps to protect the reinforcing steel against corrosion.

5.2 Portland cement extenders and fillers

Portland cement extenders and fillers are materials used with portland cement, and must never be used on their own.

The main reasons for the widespread use of portland cement extenders are:

The portland cement extenders discussed below differ from each other but are all less reactive than portland cement. This property affects the rate of early-age strength gain, causes the “fine-filler” effect, and affects the rate of heat development due to cementing reactions.

Substituting a portland cement extender for part of the portland cement in a concrete reduces the rate of strength gain at early ages. The extent of the reduction increases with increasing substitution level.

Because extenders do not dissolve rapidly, extremely fine extender particles act as nuclei for the formation of calcium silicate hydrate which would otherwise form only on the cement grains. This fine-filler effect brings about a denser and more homogeneous microstructure of the hardened cement paste and the aggregate-paste interfacial zones, resulting in improved strength and impermeability. The extent of the fine-filler effect depends on the content of extremely fine particles in the extender. Fine particles of filler materials, eg limestone, can also exhibit the fine-filler effect.

Concrete in which part of the portland cement is replaced by an extender produces heat at a rate slower than that of a similar concrete made with portland cement only.

The slower the rate of heat development, the lower the temperature rise and therefore the smaller the likelihood of thermal cracking. The manufacture and mechanism of action of portland cement extenders and limestone filler in concrete are discussed in sections 5.2.1 to 5.2.4.

The effects of these materials on the properties of concrete are summarised in Table 5. Effects tend to increase with increased level of substitution.

Improvements to the properties of hardened concrete, brought about by the use of extenders, can be realised only if the concrete is properly cured.

5.2.1 Ground granulated blastfurnace slag

Ground granulated blast-furnace slag (GGBS) is a by-product of the iron-making process. The hot slag is rapidly chilled or quenched (causing it to become glassy) and ground to a fine powder.

When mixed with water, GGBS hydrates to form cementing compounds consisting of calcium silicate hydrate. The rate of this hydration process is however too slow for practical construction work unless activated by an alkaline (high pH) environment. When portland cement and water are mixed, the pH of the water rapidly increases to about 12,5 which is sufficient to activate the hydration of GGBS. Even when activated by PC, GGBS hydrates more slowly than PC.

The effect of GGBS on the properties of concrete depends on the properties of the portland cement, the GGBS content of the cementitious material and the fineness of the GGBS.

5.2.2 Fly ash

Fly ash (FA) is collected from the exhaust flow of furnaces burning finely ground coal. The finer fractions are used as a portland cement extender.

Ultra-fine FA is sold as a separate product. 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 (see section 5.1). The combination of FA and PC is a practical means of using FA and converting calcium hydroxide to a cementing compound.

5.2.3 Silica fume

Silica fume (SF) is the condensed vapour by-product of the ferro-silicon smelting process.

SF reacts with calcium hydroxide, in the presence of water, to form cementing compounds consisting of calcium silicate hydrate. This reaction is called pozzolanic and SF may be described as a synthetic pozzolan. Because the hydration of PC produces calcium hydroxide (see section 5.1), the combination of SF and PC is a practical means of using SF and improving the cementing efficiency of PC.

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

5.2.4 Limestone filler

This is limestone, finely ground but not chemically processed. When mixed with portland cement and water, finely ground limestone is chemically virtually inert (although there may be 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.

Note: The limestone (CaCO3) used in cements complying with SANS 50197-1 is not to be confused with:

Cementitious Materials for Concrete: Standards, selection and properties