By Bryan Perrie, MD of The Concrete Institute (TCI).
The Concrete Institute’s MD, Bryan Perrie, discusses the roles that cementitious materials play in enhancing the performance and properties of the concrete mix.
The vital ingredient of concrete is portland cement (PC), the basis of all common cements and of site blends that include a cement extender. The main raw materials used in the manufacture of portland cement are limestone and shale 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.
All cement sold in SA must meet the requirements of SANS 50197 for Common Cement or SANS 50413 for Masonry Cement and the National Regulator for Compulsory Standards (NRCS) specific requirements. Bags should be clearly marked with the strength class, notation indicating composition, and a Letter of Authority (LOA) number issued by the NRCS. An LOA is issued for each cement type from any source. Masonry cement is not used in concrete.
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. Hydration is an exothermic reaction, i.e. it produces heat. The hydration of portland cement (PC) produces two main compounds: calcium silicate hydrate (CSH) and calcium hydroxide (lime). 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 a pH of about 12,5 in the pore water which helps to protect reinforcing steel against corrosion.
Portland cement extenders and fillers are materials used with portland cement, and must never be used on their own. Extenders and fillers can be incorporated in the production of cements complying with SANS 50197 or separately at concrete batch plants while fillers such as limestone can only be incorporated in cement by the cement producer. The main reasons for the widespread use of PC extenders are:
• Cost saving – extenders are generally cheaper than PC;
• Technical benefits – extenders can improve impermeability and durability of the hardened concrete; and some extenders improve the properties of concrete in the fresh state.
Portland cement extenders affect the rate of early-age strength gain, and the rate of heat develoment due to cementing reactions. Extremely fine extender particles can 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 performance of the fine-filler effect depends on the content of extremely fine particles in the extender.
Improvements to the properties of hardened concrete, brought about by the use of exptenders, can be realised only if the concrete is properly compacted and cured.
Types of extenders and filler:
When fillers are added during the cement production process, their requirements are included in the cement specification of SANS 50197. When they are added to concrete at a batch plant, they must comply the specifications noted below.
Ground granulated blast‐furnace slag (GGBS) and Ground granulated core slag (GGCS) are by‐products of the iron‐making process. GGBS and GGCS must comply with SANS 55167: parts 1 and 2. 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 PC and water are mixed, the pH of the water rapidly increases to about 12,5 which is sufficient to activate the hydration of GGBS and GGCS. Even when activated by PC, GGBS and GGCS hydrate more slowly than PC. The effect of GGBS and GGCS on the properties of concrete depends on the properties of the PC, the reactivity of the slag, its content of the cementitious material, and fineness of the slag.
Fly ash (FA) is collected from the exhaust flow of furnaces burning finely ground coal. FA must comply with SANS 50450: parts 1 and 2. The finer fractions are used as a PC 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 an industrial pozzolan. The hydration of PC produces significant amounts of calcium hydroxide, which does not contribute to the strength of the hardened cement paste. By extending PC with FA the calcium hydroxide can be used to form additional calcium silica hydrate.
Silica fume (SF) is the condensed vapour by‐product of the ferro‐silicon smelting process. SF must comply with SANS 53263: parts 1 and 2. SF reacts with calcium hydroxide in the presence of water to form cementing compounds consisting of calcium silicate hydrate. This reaction is also pozzolanic. Because the hydration of PC produces calcium hydroxide, 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.
Limestone filler is limestone, finely ground but not chemically processed. There is no separate specification for limestone as it can only be added during the production of cement. When mixed with PC and water, finely ground limestone is chemically virtually inert. 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.
