Civil Engineering Reference
In-Depth Information
alkali salts or some amount of opC so as to increase the speed of hydration.
As a result of the combination of opC and GGBFS, firstly calcium-silicate
components such as C
3
S and C
2
S, which are structurally found in the opC
clinker, start the hydration and then C
3
S
2
H
3
, gels of C-H-S together with
Ca(oH)
2
(portlandite), which gives hydraulic binding property to cement, start
to produce CH. The reaction between the additional CH and the pozzolan
results in C-S-H. The emission of CH after the hydration of opC gives no
noticeable contribution to the resistance and is destructive in the sense of
durability. Hence, it is aimed at reducing CH after its reaction with pozzolan.
Moreover, owing to the fact that pozzolan provides economic advantages and
positive environmental contributions, in
addition
to opC, pozzolan mineral
additives such as fly ash, blast furnace slag, silica fume and natural zeolite
have started to be used. The necessary amount of CH for GGBFS to gain
hydraulic binding property is provided by CH produced by the hydration
of opC. Similarly to the hydration of opC, the reactions between GGBFS
and CH engender the formation of new gels of C-H-S which has a binding
property. The reactions of GGBFS and CH are quite similar to those of
pozzolan. under normal temperatures, the reaction between GGBFS and
opC occurs in two phases. When the hydration starts, the reaction of slag
and alkali hydroxides stands in the forefront; then, however, the reactions
of slag and CH become dominant (Erdoğan, 1995).
During the early hydration of the slag cement, the portland cement releases
calcium hydroxides (CH) with the ratio of 28% of its weight. In practice,
however, this ratio does not exceed 20% of its weight. Hardened cement paste
has a greater density and smaller pore sizes than an equivalent opC paste,
thus permeability and ionic diffusivity are reduced. Smaller pore size relates
to lower permeability, although this does not necessarily mean lower total
porosity. Slag-opC and opC mixes may result in similar total porosities,
but the slag-opC pore structures tend to be finer. Total porosity is important
to mechanical properties such as compressive strength, but is less critical to
properties that are associated with durability such as permeability. Durability
seems to be related to larger pores. However, it has been reported that a 60%
slag mortar mix not only had smaller pore sizes but also a somewhat smaller
pore volume. Higher CH contents are associated with greater permeability
and lower durability (Richardson, 2006).
The hydraulic binding property of GGBFS depends on the factors such as
chemical characteristic of the slag, the concentration of alkali in the reactive
system, the amount of vitreous structure in the slag, the fineness of the slag
and the opC used together with and the temperature during the hydration.
owing to the complexity of the factors which affect the hydraulic binding value
of the slag, in previous years, to determine this value, a simple method was
satisfactorily used. provided that the slag is in accordance with the condition
of Cao + Mgo + Al
2
o
3
/Sio
2
> 1, its hydraulic binding value is accepted
