Civil Engineering Reference
In-Depth Information
at temperatures greater than 905°C. Poor durability dolomite showed very
little weight loss until 700°C.
DTA, TG, IR, and X-ray microanalysis techniques were applied to
identify the materials that formed around the rim of sandstone or silt stone
aggregate in a thirty-year old concrete.
[17]
An alkali-substituted okenite
(C
5
S
9
H
9
), a precursor phase characterized by a 1.22 nm XRD spacing, was
identified.
9.2
Magnesium Oxide
Magnesium oxide in cement exists mainly in a free state and its
content does not exceed 6%. At the clinkering temperature of 1400-
1500°C, free MgO is in a dead-burnt state in the form of periclase. Under
normal conditions of exposure it may take years for periclase to hydrate.
The conversion of MgO to Mg(OH)
2
is attended by an expansion in volume,
hence there is a possibility of concrete to crack. Most specifications place
a limit on the amount of MgO in cements and also a limit on the volume
expansion of cements exposed to an autoclave treatment. Ramachandran
[6][71]
applied thermal techniques to explain the effect of curing conditions, viz,
50°C, boiling in water and autoclaving, on expansion and degree of
hydration in cements containing different amounts of dead-burnt MgO. It
was found that at MgO contents < 2% the expansion was lower and then
there was a steep increase at higher concentrations (Fig. 38).
[6]
The amount
of expansion depended on the strength of the matrix. At any particular
concentration of MgO, cement B showed higher expansion than cement A.
Cement A contained higher amounts of C
3
S, had better strength and, hence,
was able to resist expansive forces.
[71]
The effect of various treatments on the conversion of MgO to
Mg(OH)
2
was also examined by Ramachandran.
[71]
The computed results
from DTA are shown in Table 6.
[6][71]
Curing at 50°C was found to be
ineffective for hydrating MgO unless curing is extended to several days.
Expansion is lower in pastes cured under non-autoclaved conditions. At 5%
MgO addition, autoclaving (3 hours), boiling (2 days), steam curing (1 day)
and curing at 50°C (2 days) produced length changes of 1.3, 0.3, 0.09, and
0.3, respectively in cement A. The corresponding values for cement B
were 0.52, 0.3, -0.13, and -1.10%, respectively. These results demonstrate
how the curing conditions and the type of cement influence the length
changes.
