Interesting facts about cement
Cement on its own – I mean the grey powdery stuff in the ‘heavy’ Portland Cement packet (about 50kg) is a most interesting and complex material:
There is a funny dark greyish/ brownish spherical stone called a ‘Clinker’ of varying sizes and is crushed to a fine powder to make the ‘cement’ in the bag.
Fact 2When you add clean water to the grey cement powder a number of chemical reactions begin and stone begins to form as the water reacts with the ingredients of the cement. This is called ‘hydration‘.
Strength improves with time
Possible problems with concrete and mortars
- Sulphate attack
- Cracking and spalling due to corrosion of steel reinforcement
- Alkali-silica reaction
- Efflorescence may be associated with sulphate attack, with white crystalline growth on the surface of the damaged masonry or render.
External sulphate attack
- Extensive cracking
- Loss of bond between cement paste and aggregate
- Alteration of paste composition, with monosulphate phase converting to ettringite and, in later stages, gypsum formation. The effects described so far are typical of attack by solutions of sodium sulphate or potassium sulphate. Solutions containing magnesium sulphate are generally more aggressive, for the same concentration. This is because the magnesium also takes part in the reactions, replacing calcium in the solid phases with the formation of brucite (magnesium hydroxide) and magnesium silicate hydrates. This displaces calcium precipitates mainly as gypsum.Internal sulphate attack
- Due to contamination with sufficient sulphate at time of mixing or sulphate-rich aggregate or addition of gypsum in the cement or oxidised sulphide minerals in the aggregate. Care at mixing stage should eliminate these problems.
- Delayed ettringite formation (DEF) – note ettringite is destroyed by heating above 70º C
When water flows through cracks present in concrete, water may dissolve various minerals present in the hardened cement paste or in the aggregates, if the solution is unsaturated with respect to them. Dissolved ions, such as calcium (Ca2+), are leached out and transported in solution some distance. If the physico-chemical conditions prevailing in the seeping water evolve with distance along the water path and water becomes supersaturated with respect to certain minerals, they can further precipitate, making deposits or efflorescences inside the cracks, or at the concrete outer surface. This process can cause the self-healing of fractures in particular conditions.
Calcium hydroxide leaching can be observed in a spectacular effect:
Opaque white material appears to ooze out of concrete walls or hang in a stalactite formation from concrete ceilings. In this case, water containing dissolved calcium hydroxide has leached out of the concrete and evaporated, leaving behind a layer of calcium hydroxide that reacts with carbon dioxide to form calcium carbonate (15)Ca(OH)2(s) + CO2(g) CaCO3(s) + H2O(l)(10) in a process known as efflorescence.5 Efflorescence is often a sign of water seepage problems in the concrete or cement structure.
Various types of aggregate undergo chemical reactions in concrete, leading to damaging expansive phenomena. The most common are those containing reactive silica, that can react (in the presence of water) with the alkalis in concrete (K2O and Na2O, coming principally from cement). Among the more reactive mineral components of some aggregates are opal, chalcedony, flint and strained quartz. Following the alkali-silica reaction (ASR), an expansive gel forms, that creates extensive cracks and damage on structural members. On the surface of concrete pavements the ASR can cause pop-outs, i.e. the expulsion of small cones (up to 3 cm (1 in) about in diameter) in correspondence of aggregate particles. When some aggregates containing dolomite are used, a dedolomitization reaction occurs where the magnesium carbonate compound reacts with hydroxyl ions and yields magnesium hydroxide and a carbonate ion. The resulting expansion may cause destruction of the material. Far less common are pop-outs caused by the presence of pyrite, an iron sulfide that generates expansion by forming iron oxide and ettringite. Other reactions and recrystallizations, e.g. hydration of clay minerals in some aggregates, may lead to destructive expansion as well. Typical crack pattern associated to the alkali-silica reaction affecting a concrete step barrier on a US motorway (photograph, courtesy of the Federal Highway Administration (US Department of Transportation).
Carbon dioxide from air can react with the calcium hydroxide in concrete to form calcium carbonate. This process is called carbonation, which is essentially the reversal of the chemical process of calcination of lime taking place in a cement kiln. Carbonation of concrete is a slow and continuous process progressing from the outer surface inward, but slows down with increasing diffusion depth. Carbonation has two effects: it increases mechanical strength of concrete, but it also decreases alkalinity, which is essential for corrosion prevention of the reinforcement steel. Below a pH of 10, the steel’s thin layer of surface passivation dissolves and corrosion is promoted. For the latter reason, carbonation is an unwanted process in concrete chemistry. Carbonation can be tested by applying Phenolphthalein solution, a pH indicator, over a fresh fracture surface, which indicates non-carbonated and thus alkaline areas with a violet color. Carbonation together with corrosion of reinforcement bars
Waterproofing Repair methods
A cracked and or poor or weak concrete can be repaired using a combination of two or more cement based “waterproofing” systems.
Make sure that the concrete to be treated is clean (no oil based products i.e. no bitumens of torch on membranes) also all loose and friable materials should be removed. Concrete or brick surfaces should not be painted.
Wet the concrete (no standing water).
So called ‘glue’
Use a blend of polymers in a slurry coat bulked with a fine clean graded silica sand and cement over the section to be repaired. This the ‘glue’.Improvement of the hydration process
While this is wet apply a coat of no less than 8 mm of graded Silica sands with a sulphur rich additive which will improve the hydration process. This is part of the crystalline phase being formation of Monosulphate – C4ASH12 or C3A.CaSO4.12H2O
The above system can be modified and or improved in terms of mix, materials and thickness to suite the circumstances and particular application.
Concluding Remarks The study of cement offers an opportunity to explore the chemistry of earth materials, their preparation, and resulting properties. Furthermore, examination of cement degradation comprises an extensive part of modern cement chemistry. Recent innovations in research techniques have made the study of cement preparation and degradation behaviour more accessible. Improvement of corrosion resistance in cement and concrete structures would significantly lengthen the lifetime of applications using these materials, potentially saving billions of dollars worldwide.
“Cement: Its Chemistry and Properties” by Douglas C. MacLaren and Mary Anne White
Department of Chemistry and Institute for Research in Materials, Dalhousie University, Halifax, Nova Scotia B3H 4J3,
“Products of Chemistry” edited by George B. Kauffman, California State University, Fresno, CA 93740
“Undestanding Cement” by Nick Winter