Characterisation and quantification of sulphide minerals in concrete aggregates; deterioration potential of different sulphide variants.

The research scope of this PhD project is to find reliable analytical techniques for the quantification and characterization of highly deleterious sulphide minerals in concrete aggregates and thereby enable a better utility of aggregate resources. Natural sand and aggregate deposits have traditionally been used for concrete production, which (mainly) have lower sulphur concentrations than aggregates produced from crushed rock. Due to depletion of available sources of natural sand deposits, future concrete will be entirely manufactured from fresh, crushed rocks. Recent data has shown that almost 10% of the resources for crushed aggregates will exceed present limit values unless they are blend with natural aggregates. This will increase the need for a better predictability and understanding of aggregate quality and standardized test procedures. The determination of total sulphur concentration and the identification of different sulphide variants in rocks are challenging tasks. Prevailing standard analytical techniques in use do not achieve satisfactory results regarding the strict limit values for total sulphur content of maximum 1 vol. % in samples and as little as 0,1 vol. % if the deleterious iron-sulphide pyrrhotite is detected in the aggregate (EN-12620). The described limits are close to the detection limits of standard instruments and methods used for industrial testing today. Critical values for total sulphur content are potentially intended over-careful based on a lack of understanding the mechanisms of internal sulphate attack in hardened concrete and an uncertainty of the sulphide threshold values. This knowledge gap has led to the vast disposal of otherwise high-quality concrete aggregates from crushed rock sources. It is, however, not possible yet to predict whether a strictly quantitative model considering a pre-determined limit value or a qualitative model based on specific sulphide mineral assemblages triggering expansive reactions in concrete may yield the safest, most dependable regulatory. Therefore, research on the reliable quantification and characterization of variants of iron-sulphide minerals in rock masses used for the concrete production is an essential step towards a safer and more efficient usage of stationary and locally produced resources. Instead of dumping rock masses based on somewhat ill-defined criteria, an improved classification scheme should pave way for use of onsite produced aggregates and a greater potential for increased sustainability of all scale construction projects. Local production will reduce landfill and disposal, decrease transport -costs and -emission and thereby enhance the overall viability of the project. For achieving a better utilization of ?surplus rock?, analytical techniques must not only be reliable, but yield the possibility for aggregate producers to increase sample analysis frequency in an applicable and cost?effective way.

The PhD-project characterisation and quantification of sulphide minerals; deterioration potential of different sulphide variants is part of the 2018 founded project Sulphide minerals in aggregates; impact on concrete durability (2020-2024) under the patronage of the Norwegian Road Administration (NPRA) and Bane NOR. Small amounts of sulphide minerals in concrete aggregates, particularly of the mineral pyrrhotite [Fe(1-x)S], may lead to expansion and severe deterioration of concrete structures. National awareness to the problem resurfaced during the construction of the Bane NOR Follo line project, when tunnel mass intended for use in pre-cast concrete lining of the tunnel had to be discarded because of elevated sulphur levels (> 0,1 wt.%) and the indication of pyrrhotite. What originally was intended to be a vanguard building approach, using local tunnel mass for construction with a tremendous environmental asset, caused additional expenses, due to use of external aggregates, in the range of > 100 million NOK for the project. Recent standardized analytical methods to distinguish the total amount of sulphur in aggregate samples are not precise enough and dismiss the importance of determining different sulphide variations mineralogically. Today`s standard method, the DTA-method, is after 60 years of service period still industry standard for the detection of pyrrhotite. The problem is that this equipment only is capable to indicate pyrrhotite and give a semi-quantitative results for the total S-content in the sample. Effective limit values are a combination of low analytical resolution and little knowledge about deteriourating mechanisms of sulphide minerals in concrete. This discreet value lead to that large amounts of otherwise high- quality resources are discarded for the use of concrete production rising the need for further Research.