African Design Magazine ADM #40 May 2018 | Page 55

TECHNOLOGY
content of FA-25% reached its maximum
at the age of 28 days. This indicates that
the pozzolanic activity of ground fly ash
is higher than that of grade I fly ash and
that ground fly ash has more significant
effect on promoting the hydration of
cement.
It is known from hydration heat analysis
that grade I fly ash retards the early-age
hydration of cement, but Δ1kept positive
from three to 28 days and increased with
ages, indicating that fly ash promotes the
hydration of cement. The higher slope of
hydration heat curve of FA-25% after 24
hours compared to that of cement also
proves this point. The main reason is
that addition of grade I fly ash increased
the effective water to cement ratio so
that the retarding effect of grade I fly
ash on hydration appeared mainly in the
initial 24 hours. Δ1reached maximum
at 28 days, then generally changed to
negative, and grew negatively, indicating
Figure 6: The compressive strength
versus ages.
that the pozzolanic reaction of grade I
fly ash presented very low degree within
initial 28 days, but the reaction degree
was obviously promoted after 28 days
and the hydration rate exceeded that
of Cement, resulting in more Ca(OH)2
consumption than its formation and
negative Δ1; the value of Δ1 expressed
little difference at 90 days and 360 days,
indicating that the pozzolanic reaction of
grade I fly ash was relatively fast from 28
days to 90 days and then became slow.
Δ2 of GFA-25% was positive at three
days, meaning that ground fly ash
always promoted the hydration of
cement. After three days, Δ2 significantly
grew negatively with the increase of
ages, indicating that after three days
the pozzolanic activity of ground fly
ash began to increase greatly and the
hydration rate of GFA-25% was higher
than that of cement, even at the ages
of up to 360 days. It can be seen from
the data in Table 2 that the pozzolanic
reaction degree of ground fly ash is
slightly higher than that of grade I fly ash
at initial three days, while after three
days, the pozzolanic activity of ground fly
ash began to increase significantly and
is greatly higher than that of grade I fly
ash. With the increase of ages, the gap
of pozzolanic activity between ground
fly ash and grade I fly ash became
significant.
3.3. Compressive strength and flexural
strength
The changes of compressive strength
and flexural strength of three kinds of
Read more here
AFRICAN DESIGN MAGAZINE © | MAY 2018
55