Alkali Reaction of Aggregates | Building Materials | Concrete Technology

Till 1940, aggregates were considered as inert materials, but after 1940, it has been clearly established that aggregates are not fully inert. Some of the aggregates contain reactive silica which reacts with the alkalis (Sodium oxide Na₂O and potassium oxide K₂O) present in cement.

For the first time in U.S.A. it was observed that many failures of concrete structures as sea walls, piers and pavements occurred due to the alkali-aggregate reaction. On the basis of systematic study since these failures now it has been established beyond doubt that certain type of reactive aggregates are responsible for promoting alkali-aggregate reaction.

The rocks which contain reactive constituents are siliceous lime stones, trap, andesite and certain types of sand stones. The reactive constituents may be in the form of volcanic glass, zeolites, opals, charts etc. The reaction starts with the attack on the reactive siliceous minerals in the aggregate by the alkaline hydroxide formed by the alkaline hydroxide (K₂O and Na₂O) present in the cement. As a result of this reaction the alkali silicate gel of unlimited swelling property is formed and the alteration of boarders of the aggregates takes place.

This gel swells if favourable conditions for its swelling are met. The gel swells by absorbing water. As this gel is confined by the surrounding cement paste, internal pressure increases resulting disruption of concrete by expansion, and cracking of concrete and eventually failure of concrete structures takes place. The rate of deterioration may be fast or slow depending upon the conditions.

The expansion may be due to hydraulic pressure generated by the process of osmosis but it can also be caused by the swelling pressure of the still solid products of the alkali silica reaction. It is believed that the swelling of the hard aggregate particles is most harmful to the concrete.

Some of the relatively soft gel latter is leached out by water and deposited in the cracks already formed by the swelling of the aggregate. The size of siliceous particles controls the speed with which reaction takes place. Fine particles of 20 to 30 microns lead to expansion within a month or two, while larger particles cause expansion only after some years.

Factors Affecting the Reactivity of Aggregates:

Following factors affect the reactivity of aggregates:

(a) Particle size and its porosity. These factors influence the surface area over which the reaction takes place.

(b) Quantity of cement. The quantity of alkalies depends on the quantity of cement. The minimum quantity of alkali in cement at which expansive reaction may take place is 0.6 percent of the soda equivalent (Na₂O + 0.658 K₂O). The quantity (Na₂O + 0.658 K₂O) is known as soda equivalent.

(c) Availability of non-evaporable water in the paste.

(d) Permeability of paste.

(e) Type of aggregate.

(f) Optimum temperature conditions.

The reaction is accelerated under conditions of alternate wetting any drying. Moisture is necessary for the reaction to take place. The necessity of the presence of Ca(OH)₂ also has been suggested. The ideal condition of temperature for the promotion of alkali aggregate reaction is the range of 10 to 38°C. The temperature above or below this range is not ideal for this reaction.

Thus various physical and chemical factors make the problems of alkali aggregate reaction complex. In particular the gel can change its constitution by absorption and thus exerts a considerable pressure, while at other times diffusion of the gel out of the confined area takes place. It may be noted that as the hydration of cement progresses, much of the alkali concentrates in the aqueous phase raising PH value and all silica minerals become soluble.

Though it can be predicted that with the given materials an alkali aggregate reaction will take place, but generally it is not possible to estimate the extent of deleterious effects from the knowledge of the quantities of the reactive materials alone.

The potential reactivity of aggregate can be determined as suggested by I.S. 2386 Part VII-1963.

The aggregate is pulverized and treated for 24 hours with a normal solution of sodium hydroxide (NaOH) at 80°C. The degree of reactivity is determined from the amount of silica dissolved by the solution and reduction in alkalinity of the solution. The aggregate is pulverized and sieved to pass a 300 micron sieve and retained on a 150 micron sieve. The finer material than 50 micron is removed and the sample washed and dried at 100°C to 105°C for 20 ± 4 hours. This sample is now cooled and sieved on 150 micron sieve again.

Now weigh three samples of 25 grams each from the prepared sample as described above. Each of the three samples is put into three separate reaction containers and 25 c.c. solution of 1 N (normality) of sodium hydroxide (NaOH) is added to each container. In a fourth container only sodium hydroxide solution of the same consistency is taken.

All the four containers are sealed and gently shaken to liberate trapped air. After sealing these containers they are put in liquid bath at 80 ± 1 °C for 24 hours. After 24 ± ¼ hours the contai­ners are removed from the water bath and cooled for 15 ± 2 minutes under flowing water till the tempera­ture falls below 30°C.

After cooling, the containers are opened and their solutions filtered and the filtrate is collected in flasks. After the completion of filtration, the filtrate is stirred. Now take about 10 c.c. of filtrate and dilute it with water to 200 c.c. in a volumetric flask. This dilute is used for the determination of dissolved silica and reduction in alkalinity.

Determination of Dissolved Silica:

The quantity of dissolved silica can be determined either by gravimetric method or by the photo­metric method. Here only gravimetric method has been discussed. In this method 100 c.c. dilute filtrate as prepared above is transferred to an evaporating dish and 5 to 10 c.c. of hydrochloric acid (HCl of Sp. gr. 1.19) is added and the dilute is evaporated to dryness on a steam bath.

Now without further heating of the residue, it is treated with 5 to 10 c.c. of HCl (Sp. gr. 1.19), and again evaporated to dryness on the steam bath. Now the 10 c.c. to 20 c.c. of HCl (1:1) is poured on this residue, covered and left for 10 minutes on a hot-plate. Now the solution is diluted with hot water and filtered. The separated silica is washed with hot water and the residue is kept reserved.

Again this filtrate is evaporated to dryness by heating in an oven for one hour at 105°C to 110°C. The residue is treated with 10 c.c. to 15 c.c. HCl (1:1) and heated on a hot plate. The solution is diluted with hot water and filtered to separate any silica particles left. The filter paper along with the silica particle is put in a platinum crucible and ignited. When the filter paper is completely burnt, then it is heated upto 1100°C to 1200°C till the weight becomes constant.

The results obtained are plotted as shown in Fig. 4.10. The degree of alkali reactivity is indicated if the plotted data falls to the right of the boundary line shown in the figure below. If the test results fall on right side of the curve of Fig.4.10 then the aggregate is reactive and not suitable for concrete. If the results fall on left of the curve, the aggregates may be used in the concrete.

Mortar Bar Expansion Test:

As suggested by STANTON this test has proved to be very reliable for determining the reactivity of the aggregate. The test is carried out as per IS 2386-1963 Part VII. A specimen of size 25 mm x 25 mm and 250 mm length is cast, cured and stored as per IS 2386 part-VII. The length of the specimen is measured periodically at the age of 1, 2, 3, 5, 9 and 12 months.

The difference in length of the specimen is observed nearest to 0.001% and the expansion of the specimen is noted. If the expansion after 3 months is found more than 0.05% and after 6 months more than 0.1 %, then the aggregate tested is harmful and should be rejected.

High Alkali Content in Cement:

High alkali content in cement are the most important factors contributing to the alkali aggregate reaction. As per standard specification the contents of alkali in the cement should be less than 0.6%. Cement containing total alkali (Na₂O + 0.658 K₂O) known as soda equivalent as 0.6% is called low alkali cement. Generally Indian cements do not contain high alkalies as that of U.K. and U.S.A cements.

The results of investigations on Indian cements for alkali contents are shown in the table 4.15 below. From the table it will be seen that out of 26 samples only 11 showed more than 0.6 alkali content. These results are based on cements manufactured before 1975. The present day modern cements have even lower alkali contents.

Reduction in Alkalinity:

20 c.c. out of 200 c.c. of dilute prepared for the test in a flask is taken and 2 or 3 drops of Phenolphthalein, solution is added to it. Now this solution is titrated with 0.1 N HCl to the end point of the Phenophihalein, then reduction in alkalinity is given-

Re = (20N/V₁) (V₃ – V₂) x 1000

where,

Re = Reduction in alkalinity in milimoles per litre.

N = Normality of HCl used for titration.

V₁ = Vol. in ml (c.c.) of dilute solution used.

V₂ = Vol. of HCl used to attain the end point of phenolphthalein in the test.

V₃ = Vol. of HCl used to attain the end point of phenolphthalein with the blank.

Control of Alkali Aggregate Reaction:

The alkali aggregate reaction can be controlled by the following methods:

1. By the selection of non-reactive aggregates.

2. By the use of low alkali cement 0.6 to 0.4 alkali content cement.

3. By the use of admixtures such as pozzolana.

4. By controlling void space in concrete.

5. By controlling moisture and temperature.

Presence of water or moisture is necessary for the development of alkali-aggregate reaction. It has been observed experimentally that lack of water greatly reduces this reaction. This deterioration will not occur in the interior of mass concrete, but will be more on the surface. The deterioration of surface may be controlled by the application of some water proofing agents on the surface of the concrete which will prevent the ingress of water into the structure.

Temp Conditions:

As stated above the ideal range of temperature for the promotion of alkali-aggregate reaction is 10°C to 38°C. If the temperature condition is less or more than this, it will not provide an ideal condition for the promotion of alkali-aggregate reaction.