Abstract
The use of crushed concrete aggregate from igneous and metamorphic rocks has increased
due to the local depletion of Quaternary deposits. In such crushed, unweathered aggregates,
the concentration and mineralogy of iron sulfides and their potential to activate internal
sulfate attack (ISA) raise concerns. Severe deterioration cases in Canada, the USA, and
Ireland have shown that even low sulfur concentrations (<1 wt.%) of pyrrhotite and associated
sulfide minerals can trigger concrete expansion, leading to cracking and disintegration
of concrete structures.
At the same time, the conservative acceptance limits for total sulfur specified in the
European standard EN 12620 for concrete aggregates have sparked discussion. The European
regulation specifies an upper limit for total sulfur of 1 wt.% (⩽1.4 wt.%) and
only 0.1 wt.% (⩽0.14 wt.%) if the non-stoichiometric monosulfide pyrrhotite (Fe1−xS;
x = 0−0.125) is present. Such low threshold values may affect many aggregate producers
who use aggregates from igneous and metamorphic rock deposits, potentially leading to
excessive rejection of otherwise high-quality aggregates for concrete production.
The analytical methods used for sulfur analysis, specified in EN 1744-1, primarily rely
on the measurement of total sulfur content (clause 11), and the determination of water and
acid-soluble sulfate content (clause 10 and 12, respectively). However, the current
regulation (EN 12620) does not state a method for determining pyrrhotite. In Norway,
the addition NA 10 to EN 12620 specifies differential thermal analysis (DTA) as the
decisive method for detecting pyrrhotite in rock samples. Differential thermal analysis can
indicate the presence of sulfide minerals, but it yields ambiguous results when multiple
sulfide phases, such as pyrite, pentlandite, and chalcopyrite, are present in the samples.
Furthermore, the DTA quantification of sulfides is insufficient, and the differentiation
between pyrrhotite superstructures (hexagonal and monoclinic) is not possible. To address
these limitations, this Ph.D. work provides a systematic review of the prevailing standard
analytical methods. Novel analytical methods for the characterization and quantification
of sulfides, with particular emphasis on pyrrhotite, in concrete aggregates were explored,
and the early oxidation sequences of pyrrhotite in concrete were studied.
The research builds on material from four Norwegian aggregates, ISA-reactive aggregates
from North America, and two natural reference pyrrhotite samples. Four research
approaches are evaluated and compared:
1. A small-scale, inter-laboratory study was conducted of the standardized total sulfur
analysis (EN 1744-1, clause 11.1 and 11.2).
2. Thermomagnetic analysis (TMA) was investigated as a potential supplement or
replacement for DTA.
3. SEM-based automated mineralogy (AM)was tested as a one-stop method to quantify
and characterize the bulk mineralogy, sulfide concentration (as low as 0.01 wt.%), total sulfur content, mineral association data, and the chemical energy-dispersive
X-ray spectroscopy (EDS) data were directly compared to wavelength dispersive
spectrometry (WDS) data acquired by electron probe microanalysis (EPMA).
4. The oxidation sequence of pyrrhotite was investigated from fresh aggregate samples,
laboratory mortar bar samples exposed to accelerated expansion testing after 12, 27,
and 52 weeks, and an ISA-deteriorated concrete field sample, containing Maskimo
aggregates from Trois-Rivières, Canada.
The thesis demonstrates that total sulfur analysis alone is insufficient for confidently assessing
the ISA reactivity of sulfide-bearing aggregates. Thermomagnetic analysis (TMA)
is confirmed as a fast and sensitive complementary technique for pyrrhotite detection.TMA
is proposed as a verification method for DTA. The in-house-built DTA instrument used
to detect pyrrhotite in the Norwegian concrete aggregate market should be validated and
compared with commercially available DTA instruments. The combined methodologies of
LECO total sulfur detection, DTA for sulfide mineral determination in bulk samples, and
TMA for distinguishing and quantifying pyrrhotite form a robust, cost- and time-effective
test protocol. Automated mineralogy allows early recognition of oxidation patterns and
reactive mineral assemblages relevant to ISA.
The research results presented in this Ph.D. work provide a foundation for improved
quality-control practices and support the current initiative to develop the European regulations
and test procedures. The findings of the laboratory investigations and test methods
presented in this dissertation should be further explored, including aggregate materials
with higher sulfide concentrations and field samples exposed to internal sulfate attack.