Evaluation and comparison of various methods used for aggregates investigations

Abstract

Natural aggregates from sedimentary rock, like limestone and dolomite, are of a great use in various practical applications. To evaluate their quality, among others, the test of methylene blue adsorption (MB value) using a filter paper is recommended. However, one can consider it as a rough test. In this paper we wished to evaluate its quality by comparison with a more precise spectrophotometric method, i.e., to perform adsorption isotherms of methylene blue from aqueous solutions, as well as determine other parameters characterizing the aggregates. For this purpose, methylene blue adsorption on samples of limestone and dolomite natural aggregates having various grain sizes were studied to assess quality (fine particles content) of the manufactured aggregates. To determine the amount of adsorbed dye two methods were used: the methylene blue stain test and the dye adsorption from its solutions at various concentrations under static conditions. From the linear form of Langmuir adsorption isotherms of methylene blue, the monolayer capacity was determined, and then the specific surface areas of all fractions of aggregates. The structural (N2 adsorption/desorption), textural (SEM/EDS) and crystallographic structure of the aggregates were studied. It was determined that the MB values for 0–2 and MBF for 0–0.125 mm aggregates fractions fulfill the criteria set out in the specifications required for pavement construction. A very good repeatability of the adsorbed amount of methylene blue on the dolomite and limestone aggregates were obtained by these two different methods. These results confirm the reliability of the method blue test used typically in industrial conditions. The measured specific surface areas of limestone and dolomite using N2 adsorption (SBET) are smaller than SMB determined by methylene blue adsorption from aqueous solutions. This is because in aggregates, apart from calcite and dolomite, there is a small admixture of quartz and clay minerals. During N2 adsorption in dry condition, the external surface of the grains is determined, while in the aqueous solution of methylene blue, both the external and inner surfaces of clay minerals are determined.