New observations on the mechanism of lithium nitrate against alkali silica reaction (ASR)

In the current study, in order to elucidate the mechanisms for the favorable effects of lithium nitrate in controlling alkali silica reaction (ASR), vycor glass disk immersion specimens and glass disk–cement paste sandwich specimens were prepared and examined by XRD, SEM and Laser Ablation Induction Coupled Plasma Mass Spectrometry (LA-ICP-MS). Results showed that when glass disk was immersed in only NaOH solution, the glass was attacked by hydroxyl ions but no solid reaction product was found, thus the presence of calcium was essential for the formation of ASR gel. In the presence of lithium, the glass surface was covered by a thick layer of Li–Si crystal. With the addition of Ca(OH)2, the glass surface was completely covered by Li–Si crystal and a lithium-bearing low Ca–Na–(K)–Si gel. These two phases either form a dense matrix with Li–Si crystal serving as the framework, and the gel filling in the void space, or the Li–Si crystal serving as the foundation to completely cover the entire reactive SiO2 surface, and the gel sitting on top of these crystal particles. Hence, the suppressive effects of LiNO3 were attributed to the formation of a layer of Li–Si crystals intimately at the reactive SiO2 particle surface and the formation of Li-bearing low-Ca ASR gel products. The Li-bearing low-Ca ASR gels may have a dense and rigid structure, thus having low capacity to absorb moisture from the surrounding paste, and exhibiting a non-swelling property.