AGGREGATES - Materials for Civil and Construction Engineers - page 190

T ABLE 5.10 Main Deleterious Substances and Their Affects on Portland Cement

Concrete

Substance Harmful Effect

Organic impurities Delay settling and hardening, may reduce

strength gain, may cause deterioration

Minus 0.075 mm (No. 200) Weaken bond, may increase water

materials requirements

Coal, lignite or other low-density Reduce durability, may cause popouts or

materials stains

Clay lumps and friable particles Popouts, reduce durability and wear

resistance

Soft particles Reduce durability and wear resistance,

popouts

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5.5.10 Alkali-Aggregate Reactivity

Some aggregates react with portland cement, harming the concrete structure.
The most common reaction, particularly in humid and warm climates, is be-
tween the active silica constituents of an aggregate and the alkalis in cement
(sodium oxide, and potassium oxide, ). The alkali-silica reaction
results in excessive expansion, cracking, or popouts in concrete as shown in
Figure 5.15. Other constituents in the aggregate, such as carbonates, can also
react with the alkali in the cement; however, their reaction is less harmful.
The alkali-aggregate reactivity is affected by the amount, type, and particle
size of the reactive material, as well as by the soluble alkali and water con-
tent of the concrete.
The best way to evaluate the potential for alkali-aggregate reactivity is by
reviewing the field service history. For aggregates without field service his-
tory, several laboratory tests are available to check the potential alkali-aggregate
reactivity. The ASTM C227 test can be used to determine the potentially ex-
pansive alkali-aggregate reactivity of cement-aggregate combinations. In this
test, a mortar bar is stored under a prescribed temperature and moisture con-
ditions and its expansion is determined. The quick chemical test (ASTM
C289) can be used to identify potentially reactive siliceous aggregates. ASTM
C586 is used to determine potentially expansive carbonate rock aggregates
(alkali-carbonate reactivity).
If alkali-reactive aggregate must be used, the reactivity can be minimized
by limiting the alkali content of the cement. The reactivity can also be re-
duced by keeping the concrete structure as dry as possible. Fly ash, ground
granulated blast furnace slag, silica fume, or natural pozzolans can be used
to control the alkali-silica reactivity. Lithium-based admixtures have also
been used for the same purpose. Finally, replacing about 30% of a reactive

Na 2 O, K 2 O