structure which tends to surround the cement particle and hinders the access of water for further reaction. After a short time, each particle thus consists of a central core of untreated cement, surrounded by layers of partially reacted material, which will have reacted more and more com- pletely the further they are from the centre. Thus, in spite of the fact that in ordinary concrete there is always present much more water than is sufficient to react completely with the cement, it is not always able to do so efficiently because of the difficulties I have mentioned.
Normally, a water-cement ratio of about 0.25 represents that theore tically required by the cement for complete reaction, but a higher value than this is always necessary in the interest of workability. The water represented by the excess over the 0.25 ratio is free water and merely dries out with the concrete, leaving microscopic voids in the latter which give rise to porosity, shrinkage and some loss of compressive strength.
In the greater part of the concrete made today, the difficulties associ- ated with efficient hydration of the cement are accepted as being un- avoidable. But this is by no means the case. Taking as an example the first difficulty-the clumping of the particles together-it will be readily appreciated that any means of separating the cement particles so that water can pass freely between them would result in a more efficient contact between water and cement and more complete hydration.
One of the ways in which this can be achieved is by the use of a surface-active agent, namely, a sub- stance, the molecules of which have the property of collecting at the interface between a liquid and gas or a liquid and a solid. One of the effects of such behaviour is a lower- ing of the surface or inter-facial tension of the liquid concerned so that it wets solids more readily and is thus assisted in penetrating be tween any closely packed solid particles. In other words, it tends to keep the particles away from each other and prevents them from joining together in unwetted clumps.
This has been demonstrated by making concrete with and without the addition of a suitable surface- active agent and grinding it to powder after it has set hard. On re-moistening the powder with water, it was found that the un- treated concrete hardened again,
indicating that it contained an ap preciable proportion of unused cement. On the other hand, the concrete containing the surface- active agent did not set again, showing that in this case all the cement had reacted with the water present in the first mixing.
Apart from the obvious advantage of bringing the cement and water into more intimate contact, the use of surface-active agents of the correct type may result in important prac tical advantages. Among these is the increased workability resulting the increased workability resulting from the dispersing effect referred to above, which renders mixes more fluid and more readily compacted without the necessity of adding excessive water for this purpose. This is a very important feature, as will be appreciated from the remarks already made about workability and the need for keeping the water- cement ratio as low as possible. It means, in practice, that mixes, which would otherwise be too dry to com- pact properly or which could be worked round reinforcement only with difficulty, may be used satisfac- torily and advantage taken of the higher strength obtainable with the drier mix. This increase in fluidity is particularly advantageous when concrete pumping is under sideration, since mixes with relative- ly low water-cement ratios may be pumped with ease in the presence of a suitable additive.
One of the first requisites of a surface-active agent for concrete is, of course, that it must be chemically stable in the presence of the concrete mix; in particular, it must not react with the soluble calcium salts, which are always present in normal con- crete and which may, in fact, be added deliberately (as calcium chlo- ride) when rapid setting is required. Secondly, it should be free from sulphates, which tend to have disin- tegrating effects on concrete and to reduce its reduce its durability. These re- quirements are stringent ones, and in the past many agents, which have been offered for use in concrete, have been proved unsatisfactory in prac tice because they have failed to meet these conditions. However, surface- active agents are now available which, unlike those previously marketed, are extremely stable chemically and are incapable of reacting with calcium salts or any of the components of concrete. They are also free of sulphates and cannot attack metal reinforcement. This
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high degree of stability is due to the fact that they are not ionized in solution and are thus referred to generally as non-ionic surface-active agents. Typical of these agents is the product marketed under the name of "Lissapol N", the use of which as a means of improving concrete is now well established.
It is important that these non-ionic surface-active agents should not be confused with aerating agents, which improve the workability of concrete by entraining small air-bubbles in it; this method certainly does increase workability but only at the expense of an appreciable loss in strength caused by the inclusion of the air- bubbles.
Greatly improved workability can be obtained by the use of exceedingly small amounts of agents, quantities of the order of 4-oz. per cwt. of cement present in the mix being sufficient in many cases to produce a marked improvement. The manu- facturers' instructions should, of course, always be followed closely to obtain the best results.
In regard to improving workabili- ty, I have only discussed the simple addition of a non-ionic agent to a concrete mix without making any alteration in the water-cement ratio. If, however, normal workable mix be considered, it will be found possible in many cases to reduce the water-cement ratio appreciably, and, by adding a non-ionic surface-active agent, restore the workability lost by using a drier mix. In this case, a concrete of improved strength, due to the lower water-cement ratio will be obtained, but the workability will be similar to that of a much wetter mix. Furthermore, due to the lower water content of the mix, shrinkage on drying will be greatly reduced.
or
Mixes containing these agents of- ten appear to be too dry by ordinary standards (visual appearance slump test) but behave quite satis- factorily on working or vibrating. For this reason the workability of a treated mix should not be assessed from its appearance but from actual consolidation.
The use of non-ionic surface-active agents, such as Lissapol N, is not a means of turning bad concrete into good, but they can contribute very considerably towards the improve- ment of harsh mixes and the making of good concrete even better.
By W. J. Wilson, B. Sc., Ph D. A.R.I.C., M. Inst P., of Imperial Chemical Industries, England.
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