Page 343 - Handbook of Thermal Analysis of Construction Materials

Section 5.0 - Rice Husk Ash 319

Figure 21. Conduction calorimetric curves of slag activated by some sodium salts.

The rate of reaction is increased by the addition of NaCl and NaBr
in the 2–8 hour period. By the addition of these compounds, an increase in
strength results at all ages up to 91 days.

5.0 RICE HUSK ASH

Rice husks, also called rice hulls, are the shells produced by the
dehusking operation of paddy rice. Each ton of paddy produces about 400
pounds of husk which on combustion produces about 90 pounds of rice husk
ash (RHA). Amorphous silica may be obtained by maintaining the combus-
tion temperature of RHA below 500°C under oxidizing conditions for
prolonged periods or up to 680°C, provided the total time is less than a
minute. [52]
The predominant constituent of RHA is SiO ,which exceeds 90%
2
2
of the total ash. The RHA may have a specific surface of 40 m /g and is
highly pozzolanic. Rice husk ash concrete is fabricated by utilizing high
dosages of superplasticizers. The compressive strengths of RHA concrete
are generally higher than the concrete containing no ash. Typically the 3 day
strength of mortars with 0, 30, 50, and 70% RHA is 22.7, 32.3, 26.5, and
24.3 MPa respectively, and the corresponding values at 28 days are 43, 59.5,
58.3, and 43.3 MPa. [53]