Raw sewage sludge was characterized by XRD, FTIR, SEM, and TGA techniques and incinerated in temperature range 650-950• C for 2 h. The effect of incineration temperature on the microstructure and pozzolanic activity of the resultant ash was investigated by techniques mentioned above as well as Chapelle test. It was concluded that incineration of sewage sludge affects the microstructure and pozzolanic activity of the resultant ash. During incineration at temperatures lower than 800• C, amorphous silica captures fixed carbon resulting from incomplete combustion conditions whereas at higher temperatures crystallization of amorphous silica was enhanced. Hydration products formed from hydrothermal treatment of silica fume with lime is amorphous whereas that of sewage sludge ash is fibrous. Hence, incineration of sewage sludge ash must be optimized at 800• C to preserve the pozzolanic activity of the resultant ash.

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e aim of the present study is to investigate the hydration characteristics and the �re resistance of sewage sludge ash blended cement pastes by the determination of compressive strength, bulk density, and total porosity in addition to XRD and SEM techniques. Sewage sludge ash modi�es the hydration of cement because of its pozzolanic reaction with portlandite favoring the formation of crosslinked �brous calcium silicate of low Ca/Si ratio. Hence, it was concluded that thermal damage of cement pastes a�er the exposure to high treatment temperatures (i.e., crack formation and loss of mechanical properties) was effectively reduced with sewage sludge as content up to 20 wt% because of that the presence of crosslinked �brous calcium silicate strengthens the cement matrix.

The microstructural and compositional changes within the cement paste exposed to high temperatures were monitored by XRD, FTIR, TGA/DTA and SEM techniques to understand the nature of decomposition of C-S-H gel and the associated physicomechanical properties of thermally damaged cement paste. OPC paste (w/c ratio 0.27) was hydrated for 28 days then fired up to 750˚C for 2 hours (heating rate 10˚C/min). The relative mass percent of calcium hydrates and portlandite was estimated by calculations derived from TGA results. Beyond 450˚C, the percentage of portlandite sharply diminishes and C-S-H progressively decomposes into C 2 S and C 3 S until complete loss of calcium hydrates content occurs at 750˚C. An increase of the total porosity, severe loss in mechanical strength and propagation of harmful cracks occurs. The thermal shock as a result of cooling of the heated cement paste and the rehydration of lime enhance the propagation of harmful cracks.