ALKALI RELEASE BY VOLCANIC AGGREGATES IN CONCRETE

Selective leaching techniques that dissolve hardened cement paste in c oncrete while leaving aggregate particles intact, reveal mass transfer from the aggregate to the hydrated phases during concrete hardening. Leaching techniques based on selective cement dissolution with alkalin e EDTA-TEA were applied to experimental mortars. Results are presented for hive solvents whose only difference lies in the choice of cation: MET, the Li+, NH4+ version, and TET, the tetramethyl-ammonium (TMA)-v ersion. TET, which features large cations providing limited exchange c apability, attacks aluminosilicates the least while dissolving clinker and paste faster. In contrast to dacite and phonolite, which reduce t he level of alkali, some basalts release large amounts of alkali into the pore solution of concrete, especially when the aggregate considera ble amounts of fine material and the water-cement ratio (w/c) is high. The alkali release during the first 6 months ranges from about 1%Na2O equivalent for a rapidly cooled fresh basalt with poorly crystallised feldspars to about 0.1% for a well crystallised basalt. It follows th at, despite the use of low-alkali cement, sufficient alkali con be rel eased from some aggregates to initiate alkali-aggregate reaction.