Portland cement consists mainly of compounds of calcium silicate and calcium aluminate, the calcium silicates are predominate being between 55% and 85%. There is also tricalcium aluminate, 7% to 12% and ferrites 6% to 10%.
It is made by burning at high temperature a mixture of chalk and clay in a rotary kiln. The clinker is ground, and gypsum is added to control the set. BS 12 limits the amount of sulphur (expressed as SO3) to 3.5%. The fact that Portland cement contains sulphate is important when investigating the possibility of sulphate attack on the concrete or mortar.
The hydration of the cement (the addition of water), results in a complex chemical reaction accompanied by the evolution of heat.
Revised British Standards for cement were published in 1991 and included BS 12: Portland cement and BS 4027: Sulphate-resisting Portland cement.
The new designations for Portland cements likely to be used for repair are as follows:
• Portland cement-class 42.5; to BS 12:1991 (CEM 1)
• Portland cement-class 52.5; to BS 12:1991
• Portland cement-class 42.5R; to BS 12:1991
• Sulphate-resisting Portland cement-class 42.5; to BS 4027:1991
• Masonry Cement: BS 5224:1995—ENV 413–1
The letter R denotes high early strength.
The revisions were mainly concerned with methods of test and terminology and were intended to agree with the European Standard for cement (ENV 197–1). Minor changes in composition were also introduced.
If a cement equivalent to ‘ordinary Portland is required, then this should be ordered as ‘Portland cement—class 42.5, to BS12:1991”. If a rapid hardening Portland cement is required, equivalent to ‘rapid hardening Portland cement’, then a Portland cement-class 52.5 to BS 12:1991, or
Portland cement-class 42.5R to BS 12:1991 should be ordered. The above listed cements are the ones used almost exclusively for repair work.
In 1990 and 1991, a completely revised edition of BS 5328:1991: Concrete, was issued in four Parts.
Blended cements consisting of mixtures of Portland cement and pulverized fuel ash (pfa) and Portland cement and ground granulated blast furnace slag (ggbs) are used in concrete for special purposes, but I have not come across their use in repair mortars and normal repair concrete.
The principal characteristics of Portland cement are as follows.
1. A very fine powder, particle size 1–50 microns.
2. The paste (cement and water) is highly alkaline, having a pH of about 13.5. This high alkalinity is relevant to the occurrence of alkali aggregate reaction.
3. The setting time (initial and final) is in the range of 45 minutes to 10
hours.
4. Both setting, and hardening (rate of gain of strength) are affected by temperature; an increase in temperature speeds up the chemical reaction between the cement and the mixing water.
5. Portland cement provides a comparatively high compressive strength to concrete and mortar. Tensile strength is only about 10% of the compressive strength.
6. The compounds which are responsible for the cementing action of the cement paste are mainly the calcium silicates (the C2S and the C3S).
7. It is the hydration products of the cement which, other things being equal, determine the strength of the concrete/mortar. The hydration products are very complex chemical compounds, the principal
compounds are calcium silicate gel, calcium hydroxide (about 20%) and tricalcium aluminate hydrate. Calcium hydroxide (Ca(OH)2) is liberated by the hydrolysis of the calcium silicates. The various hydration products hydrate at different rates, but the hydration is rapid to start with and then slows down.
8. The two major factors which influence the rate of gain of strength are its chemical composition and its fineness. With modern cements the increase in strength after the first 28 days is likely to be very small and should generally be ignored.
9. The amount of water in the mix (usually referred to as the water/ cement ratio) is a vital factor in determining the strength, permeability and absorption of the concrete/mortar. For higher strength and durability the w/c should not exceed 0.50, and for special purposes, 0.40–0.45; this is the free w/c which means the aggregates are saturated but surface dry.
The action of acids on Portland cement
The cement is very vulnerable to attack by acids. The reaction between the acid and the cement takes place immediately the two (acid and hydrated cement) are in contact. While the severity of attack is influenced significantly by the pH of the acid, the chemical composition of the acid is also important.
Generally, mineral acids, such as nitric, sulphuric, hydrochloric etc. are more aggressive than organic acids in equal concentrations.
Solutions of sulphates and their effect on Portland cement
Reference has been made to the reaction between Portland cement and solutions of sulphates of various bases. Calcium sulphate is only moderately soluble (a saturated solution is formed at about 1100pm). The formation of gypsum by the reaction between calcium hydroxide and sulphate solutions, more than doubles the volume (Lea, 1970).1 Lea also stresses that the combination hydrated calcium aluminate and gypsum in solution forms the compound calcium sulphoaluminate (ettringite) and this also doubles the solid volume. These chemical reactions lead to the expansion and disruption of concrete and mortar.
Magnesium sulphate is much more soluble than calcium sulphate, and has a more destructive action than other sulphates except ammonium sulphate which is probably the most destructive of all sulphates.
Chlorides of calcium, sodium and potassium in normally found concentrations do not attack Portland cement, but they cause corrosion of ferrous metals. However, chlorides react with the tricalcium aluminate hydrate (C3A) in Portland cement to form a compound which tends to inhibit the chlorides from attacking ferrous metals, e.g. steel reinforcement,