Internal Sulphate Attack in Slag Blended Systems

Abstract

With rapid industrialisation, the infrastructure sector has seen exponential growth in prefabricated
concrete elements due to their speedy construction and efficient usage of material. Precast concrete
elements have thus observed some deterioration due to increased internal temperature as
a result of rapid curing, and also through deliberate heat curing techniques. This has led to researchers
in the past to study the effect of such curing conditions on the durability aspect of the
binders, especially their impact on Delayed Ettringite Formation. Precast elements such as railway
sleepers, exposed to in the humid environment were thus prone to internal sulphate attack and
needs to be investigated.

Use of Ground Granulated Blast Furnace Slag (GGBFS) as a substitution for binder content
in conventional portland cement in the Netherlands has been prevalent since the early 1900s
primarily because of its abundant resource from iron industry. The benefits of slag have been
then exploited as it was one of the supplementary cementitious systems which along with being
a sustainable solution, provides good resistance to environmental degradation such as chloride
penetration resistance. However, their advantages surrounding extreme curing conditions have to
be studied, unless used at optimised quantities.

The research focused on the potential of blast furnace slag systems to undergo internal sulphate
attack due to high internal temperatures. The simulation of high internal temperature was
done through heat curing inside an oven following which continuous storage under lime solution
was carried out in order to saturate the system. Slag systems at low and high substitution levels
(20% & 50%) were used along with a combination of coarser and finer surface areas, to investigate
their subsequent influence was chosen for the study. Also, since infrastructure industry adopts
CEM II & CEM III-A cement type, where the former was low slag concentration with moderate
fineness and the latter with higher substitution level of slag in combination with high overall
fineness, their potentials for DEF have also been studied.

For all the mixes, the influence of high curing temperature and exposure to moisture was
studied through microstructural changes, pore size variations and mineralogical composition effect
along with fineness on paste specimens. All studies were compared to reference systems of CEM
I, which was observed to be the most detrimental due to DEF.
Test results indicate that at lower substitution levels of slag secondary ettringite forms in significant
quantities in neat systems along with traces of carbo-aluminate phases in the case of slag
systems. Also, higher substitution levels does not appear to completely suppress the formation
of ettringite after exposure. Its formation in both the cases showed more or less no influence of
fineness of slag added, except in the case of pore size distribution. The significant presence of
carbo-aluminates was observed in the case of all slag systems that could prove to be beneficial as
they do not translate to deteriorative expansion.