Abstract: This paper presents long-term results and microscopic analyses for a six-year experimental study on the effectiveness of lithium nitrate in mitigating delayed ettringite formation (DEF) with or without alkali-silica reaction (ASR). In the previous publication , interesting findings were found showing that lithium nitrate is effective in controlling DEF or ASR-DEF mechanisms in concretes. In the present study, microstructural features associated with microcracking and ettringite infilling showed relatively reduced intensity of internal distress and damage when lithium nitrate was admixed in DEF expansive concrete. Microanalysis done using plots of Al/Ca and S/Ca shows that, the use of lithium nitrate leads to formation of non-expansive ettringite similar to normal early-age ettringite formed in moist-cured concretes. These observations give some insights into the mechanism responsible for mitigation of DEF by lithium nitrate.