Whenever alkali-reactive aggregates are encountered, it has been customary to use cement of low alkali content-the limit being 0.6% Na2O eq. As elsewhere in the world, availability of low-alkali ordinary Portlant cement in India has been somewhat limited. Figure 11.18 indicates that the average level of alkalis in cements in India has increased during the last two decades26. This, to a large extent, is due to the manufacturing technologies adopted for conservation of energy and requirements of environmental protection, in that modern dry – process cement plants require hot exit gases containing the volatiles as well as the kiln dust to be recirculated in the process stream and not vented to the atmosphere. Use of blended cements like Portland pozzolana cement and Portland slag cements would also tend to increase the level of total alkalis in the cement, because pozzolanas and slags in general contain alkalis higher than in the cement clinker. In India, the proportion of blended cements is 70% of the total production and dry-process cement plants now constitute 72% of the total installed capacity.
Figure 11.18 Distribution of alkali contents in Indian cements.
A comprehensive investigation has been carried out on the role of indigenous blended cements as well as pozzolana and slags used in commercial production of such blended cements in India in alleviating ASR27. The results show that, in general, blended cements are helpful, and optimum results are obtained when the amount of substitution of pozzolana or slag is relatively high, i. e. of the order of 25-30% in the case of pozzolana and more than 50% in case of slags. Although Indian Standard specifications permit pozzolana contents in blended cements to vary between 10 and 25%, the average in practice is of the order of 11-15%. Similarly, the slag content in Portland slag cement is less than 50% whereas the maximum permitted is 65%. In the manufacture of blended cements with a prefixed quantity of pozzolana or with slags interground in the cement to meet the other requirements of specifications, the flexibility to add larger doses of cement substitutes becomes somewhat restricted.
One question that has often worried engineers is the safe limit of total alkalis in blended cements when the additives (pozzolana or slag) are not separately available for analysis. Depending upon the hydraulic activity of the slag or pozzolana, part of the alkalis contributed by them becomes available in the pore solutions28. In certain specifications, a limit of 0.9% total alkalis in the case of Portland slag cements in which the slag content is greater than 60% has been suggested10. No such limit has, however, been established for commercially produced Portland pozzolana cements. The investigation cited showed that a limit of 0.6% total alkalis in ordinary Portland cements corresponded to a limit in the case of Portland pozzolana cements of the order of 0.8-0.9% (Figure 11.19). Nevertheless, the safe values depend upon a host of factors such as the chemical composition of the cement clinker and the pozzolana, the reactivity of the pozzolana to lime-water systems and the reactivity of the aggregates, thereby making any generalisation hazardous. It is prudent, therefore, to establish a safe aggregate-cement-pozzolana (or slag) combination by prior trials. For many of the new constructions reported in 11.3, use of active pozzolana as part replacement of cement has been envisaged.
In addition to lowering the total soluble alkali content in the concrete to the extent that cement is replaced by active silicious pozzolana and hydraulic slags, they also combine with the CH liberated during the hydration of cement. It has been reported that if the CH liberated can be fully consumed by large proportions of slags, ASR would not occur29. In the context of the foregoing, modified cement compositions having no C3S phase or a lower C3S phase merit consideration12. In these cement systems, the amounts of CH liberated upon hydration are considerably lower. Use of such cements with known reactive aggregates is presently under investigation30.
Figure 11.19 Relative performance of ordinary Portland cements and Portland pozzolana cements in mortar bar expansion tests with reactive aggregates.
Instances of ASR in concrete structures in India have mainly been due to the presence of silicious aggregates such as quartzites, granites, granodiorites, granite porphyry and diorites etc containing strained quartz. The potential reactivity of these slowly reactive aggregates could not be detected by the test procedures and evaluation norms existing at the time of construction. This has led to modifications in the test methods and adoption of revised threshold values for mortar bar expansion tests, according to which aggregates proposed for many new constructions are now judged as being potentially reactive. The use of low-alkali cements, along with relatively large dosages of active pozzolana, is contemplated in such situations. Although the availability of low-alkali cements is somewhat restricted as yet, it has been possible to meet the demand through indigenous sources.
^Alkali reaction identified in many structures.
§Alkali reaction identified in small part of one 20-year-old dam (but the use of fly ash has prevented reaction in most of this structure).
to raise the Na2O eq. to 3%; then, at 24 hours they are exposed to saturated steam at 125°C (0.15 MPa pressure) for 4 hours. Based on their preliminary tests Hooton and Rogers25 modified this test method using ASTM C227 bars with the Na2O eq. raised to 4.0%.
 Chinese autoclave test32. Mortar bars are cured at 125°C in steam prior to autoclaving in a 10% KOH solution for 6 hours at 150°C. Hooton and Roberts25 had used this test with ASTM 25×25 mm mortar bars and ASTM mix proportions. None of the aggregates expanded to 0.10%, and the test was abandoned. However, they report that the test worked well with Canadian aggregates when the originally proposed proportions of bars and mix were used.
The results of this study, shown in Tables 3.2 and 3.3, led to the following conclusions. It was found that to obtain expansion > 0.10%, the ASTM C227 method needed more than 18 months to indicate potentially deleterious late – expanding alkali-silicate aggregates. Bars sealed in polyethylene bags showed
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Additionally, alkali-silicate reaction might be involved.
Copyright 1992 Blackie and Son Ltd
 Long-time storage tests show that addition of fly ash to concrete mixes has no adverse effect even if its alkali content is as high as 2.34% Na2O eq.
 Accelerated tests according to ASTM show that the addition of fly ash, even of high alkali content, to Portland cements reduces expansions due to ASR.
 Accelerated tests with unlimited supplies of alkali salt show that the addition of fly ash to Portland cement reduces, at least, the rate of
 Kristjansson, Rfkhardur (1979) Steypuskemmdir-Astandskonnun (an outline in Icelandic). The Icelandic Building Research Institute.
2. Gudmundsson, G. and Asgeirsson, H. (1975) Some Investigation on Alkali Aggregate Reaction, Cement and Concrete Research, Vol. 5. New York, pp. 211-220.
3. Gudmundsson, G. (1975) Investigation on Icelandic Pozzolans, Symposium on AAR – Preventive Measures. The Icelandic Building Research Institute.
4. Samundsson, K. (1975) Geological Prospecting for Pozzolanic Materials in Iceland, Symposium on AAR-Preventive Measures. The Icelandic Building Research Institute.
5. Gudmundsson, G. (1971) Alkali Efnabreytingar i Steinsteypu. The Icelandic Building Research Institute.
6. Thaulow, N. (1976) Undersogelse af Beton Borek^rne fra Reykjavik. Aalborg Portland.
7. Asgeirsson, H. (1986) Silica fume in cement and silane for counteracting of alkali – silica reactions in Iceland. Cement Concr. Res. 16, 423-428.
8. Olafsson, H. and Helgason, Th. (1983) Alkalivirkni Steypuefna a Islandi og Ahrif Salts og Possolana a Alkalivirkni i Steinsteypu. The Icelandic Building Research Institute.
9. Kristjansson, R., Olafsson, H., ^ordarson, B., Sveinbjornsson, S. and Gestsson, J. (1979-1987). Field Surveys of Houses. The Icelandic Building Research Institute.
 Waterproofing type of coating is not so effective since the piers in Table
10.2 and 10.3 expanded greatly after coating and cracked again.
(2) The effect of polybutadiene cannot be concluded at this stage since the
 Effects of shape and dimensions of the test specimen.
 Effects of storage conditions.
 Effects of the unit cement content and the total alkali content.
 Comparisons of the test results obtained from concrete specimens and mortar bars.
 Fine-grained, glassy to microcrystalline basalts containing more acidic glassy phases and occurring in the Deccan Plateau, west coast, Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh, Jammu and Kashmir, West Bengal and Bihar.
Denotes two generations of quartz.
Qz, quartz; Ir, iron oxide; Bio, biotite; Chl, chlorites; .Fels, feldspar; Aug, augite; Gm, groundmass; Or, orthoclase; P1, plagioclase, M, muscovite; Acc, accessory minerals.
results are shown in Figure 11.15. Such an alkali-dependent expansion of the aggregates would, prima facie, classify them as ‘alkali-reactive’.
For quantifying the effects of ‘strained quartz’, various parameters, namely grain size, proportion of quartz in the modal composition, percentage of quartz showing strain effect and UE angle, were considered. Except for very fine-grained (smaller than 0.1 mm) rocks, average grain size did not exhibit any discernible influence on the resultant expansion. The amount of expansion was more dependent upon the precentage of quartz grains showing strain effect23.