The reaction between alkalis in cement and aggregates composed of reactive forms of silica is detrimental to concrete. Therefore, it is important to identify the factors that affect the gravel reactivity as well as how to minimize the effects of the reaction. Lithium compounds, especially lithium nitrate, which are neutral and soluble, have been found to be effective in reducing the detrimental effects of the alkali-aggregate reaction. The lithium salt does not increase the pH of the solution in the concrete pores, but incorrect amounts of lithium ions may lead to increased expansion which is harmful to concrete.
Polish researchers, Justyna Zapala Slaweta and Zdzisława Owsiak at Kielce University of Technology set out to investigate the role of the lithium ions in mitigating the alkali-aggregate reaction. Their work is now published in Construction and building Materials.
In their study gravel aggregates containing large amounts of quartz, calcite, plagioclase, and lower amounts of clay minerals were sampled. Portland cement containing 0.66% equivalent alkali content was used. Both the accelerated and the long-term methods were used in evaluating the potential reactivity of the aggregates. From the results of silica leachability, the gravel aggregates were classified as potentially reactive. The aggregate grain types were subjected to a petrographic analysis which showed that chalcedony, opal and strained quartz played a major role in the reactivity of the aggregates.
Preparation of grain composition was done and bar samples were made, with and without the lithium nitrate. The bars were then immersed in a solution containing lithium and alkali ions at a molar ratio of 0.74. Data was collected for fourteen days in the accelerated method and three hundred and sixty days for the long-term method. A microanalysis of the reaction was then studied.
For mortars without lithium nitrate, the authors observed that the expansion surpasses the reactivity threshold of the aggregate for both the short-term and long-term methods which confirmed the reactivity of the aggregates used.
For mortars containing lithium nitrate in the short-term method, there was a markedly reduced expansion by about ninety three percent which was limited to a safe level of 0.1% and half the value up to ninety days. In the long-term method, the expansion was reduced by about seventy three percent and was limited to a safe level of 0.1% after ninety days. Percentage expansion differences for the two methods is due to the changes in the immersion solution composition.
Microanalysis of samples not having lithium nitrate exhibited substantial amounts of microcracks. For samples with lithium nitrate the number of cracks was lower. It was also observed that the products of the reaction that filled the microcracks had little amounts of potassium and sodium.
From the research, the researchers found that compounds of lithium especially lithium nitrate are effective in controlling the expansion of mortars composed of reactive aggregates. Further research is important in order to properly understand the effects of lithium compounds because current analysis methods fall short in analyzing the distribution and content of lithium as a result of its light weight.
J. Zapala- Slaweta, Z. Owsiak. The role of lithium compounds in mitigating alkali-gravel aggregate reaction. Construction and Building Materials 115(2016) 299-303Go To Construction and Building Materials