# Methods for Quantitative Determination of Iron Sulfides in Rocks

## TL;DR

This paper introduces a new method to accurately measure iron sulfides in rocks used for concrete, improving durability and reducing harmful effects.

## Contribution

A novel on-site quantification procedure for pyrite and pyrrhotite in aggregates using selective chemical dissolution is proposed.

## Key findings

- The pyrrhotite quantification method achieved a relative standard deviation of 3.60% and a mean relative error of 3.19%.
- The pyrite quantification method showed a relative standard deviation of 3.11% and a mean relative error of 4.70%.
- Quantitative results of pyrite were verified by XRD with an error less than 0.6%.

## Abstract

When iron sulfides are used as aggregate in concrete production, it easily oxidizes to form harmful substances such as sulfates. This results in acid corrosion and internal sulfate attack (ISA), significantly reducing concrete durability. To date, the quantification methods for iron sulfides in aggregates remain inaccurate, often neglecting pyrrhotite (a type of iron sulfide). No standardized methods or threshold values for the sulfide content in aggregates have been established, nor have technical guidelines for the application of sulfide-containing aggregates, limiting their use. This study proposes an on-site quantification procedure for determining the pyrite and pyrrhotite content in tailings using a selective chemical dissolution process. An orthogonal experiment was designed to determine the optimal dissolution conditions by considering four factors: particle size, reaction temperature, acid concentration, and reaction time. The pyrrhotite quantification method showed a relative standard deviation (RSD) of 3.60% (<5%) and a mean relative error of 3.19% (<5%), while the pyrite quantification method showed an RSD of 3.11% (<5%) with a mean relative error of 4.70% (<5%). The results were further optimized under engineering conditions to reduce costs and enable on-site quantification without relying on complex precision instruments. The quantitative results of pyrite in mineral samples were verified by the XRD internal standard method, and the error was less than 0.6%. This approach ensures the effective monitoring and management of sulfide content in concrete aggregates, promoting the practical application of sulfur-bearing aggregates.