Ceramics-Silikáty 47, (3) 108 - 115 (2003)

Mortar bars with granite or chalcedonite aggregate have been tested under conventional standard testing, based on storage of specimens at at 38 ± 2°C, RH ≥ 95 %. Aggregate of granite or chalcedonite and silica sand containing opal were used in mortar bars for expansion measurement under accelerated testing, based on storage of specimens in 1 N NaOH solution at 80°C. Scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray analysis (EDX) showed that specimens that expanded in the conventional standard mortar bar test (after 30 months) contained alkali-silicate products and secondary ettringite. Specimens that expanded in the accelerated test (after 16 days) showed the same morphological and compositional alkali-silica gel only. The reaction products in the both methods appear to be the same, indicating that reaction mechanisms are the same but differ only in rates. The results of the conducted test show that the alkali content (especially potassium) in cement plays an important role in the process of expansion in the accelerated method.