Page 141 - Handbook of Thermal Analysis of Construction Materials

124 Chapter 3 - Formation and Hydration

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 S H ), a precursor phase characterized by a 1.22 nm XRD spacing, was
5 9 9
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) is attended by an expansion in volume,
2
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
[6]
there was a steep increase at higher concentrations (Fig. 38). 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 S, had better strength and, hence,
3
was able to resist expansive forces. [71]
The effect of various treatments on the conversion of MgO to
Mg(OH) was also examined by Ramachandran. [71] The computed results
2
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.