Tag Archives: Slag

What is Geopolymer cement?

I have been confused by the term “Geopolymer cement (concrete)” for long time. Some researchers told me Geopolymer cement is essentially alkali activated cement/material (AAMs), such as alkali activated slag and fly ash. Since alkali activated slag is indeed a kind of geopolymer cement, it looks logical to consider Geopolymer cement as alkali activated cement.

Is it correct? Recently, I read the State of the Art report on alkali activated cement (Alkali Activated Materials State-of-the-Art Report, RILEM TC 224-AAM), finding the description:

Low-calcium alkali-activated systems, predominantly dealing with alkali aluminosilicates and including those materials which are now widely known as ‘geopolymers’.

It is clear that not all alkali activated cements are geopolymers. Just those with low calcium materials activated by alkali in cement industry are regarded as geopolymer cement, in other words, geopolymers are shown here as a subset of AAMs, with the highest Al and lowest Ca concentrations.

BTW, Davidovits is the first researcher applied the name ‘geopolymer’ to these low calcium materials activated by alkali. He was working in France, patented numerous aluminosilicate-based formulations for niche applications from the early 1980s onwards.

The Influence of Slag on the Hydration of Cement

These following conclusions are summarized from the work of Kocaba’s PhD thesis.

  • alite: no influence is shown on the consumption of alite measured by XRD.
  • belite: the substitution of cement by both slags seems to result in a delay in the hydration of belite in the first days.
  • aluminate phase: there is a filler effect using inert filler at about 12 hours of hydration, which shows slag can also has filler effect in the early hydration period. Transformation of AFt to AFm causes cumulative heat shoulder at about 60 hours.

For all systems, slags did not have a strong influence on hydration of C3A phases. Taking into account the low content of C3A and the corresponding error, it was difficult to highlight any relevant difference between blended paste and corresponding pure pastes.

There was no evidence of slag itself reacting and the effect of slag on aluminate phases can be only attributed to a filler effect.

The raw calorimetry curves of pure cement system showed a peak (called IV) which was attributed to monosulfoaluminate reaction just around 60 hours of reaction. In this way, calcium hemicarboaluminate and monocarboaluminate could be some possible AFm phases corresponding to the second peak of aluminate. But there is no evidence of that and it could be some monosulfate. The corresponding XRD patterns did not show any peaks corresponding to AFm phases at early ages which indicate a very low content if they are present.

  • Ferite: The slags seem to favour the hydration of the ferrite phase.

Influence of slag on the degree of reaction of cement

From XRD-Rietveld refinement and SEM-IA, the degree of reaction of cement did not seem to be strongly affected by the slag.

Sample Preparation for Quantitative X-Ray Diffraction (XRD/Rietveld)

Theoretical background: see chapters 3 and 4 in Lieven Machiels’s PhD thesis (2010). Sample preparation method modified from Machiels et al. (2008).

Representative sampling

The mineralogy and glass content of slags depend largely on the mode of cooling of the slags, e.g. slow cooling in a slag pot can result in a large amount of crystalline phases, and fast granulation in water can result in high glass content. A mineralogical analysis will thus only represent the mineralogy of a slag system for a certain grade of cooling and strongly different results can be obtained when cooling conditions are different between the different batches.

Even in a single slag pot, mineralogy and glass content can vary strongly. Taking a sample representative for a slag pot can be done by performing the sampling after a first size reduction of the bulk material or, by mixing representative parts of the slag pot (sides of the pot, center, near cracks, in center of slags, etc.)

Crushing

A grain size of <10 micron is required for quantitative X-ray diffraction (Rietveld). To avoid amorphisation of the sample during grinding, wet milling is done in a McCrone Micronizing mill. Before grinding in the McCrone Mill, the samples are crushed and passed through a 500 micron sieve.

Take representative amounts of sample (50-100 g). Crush the sample by hand in a porcelain mortar. Use shock impact for grinding, avoid shearing. A jaw crusher can be used, but automatic milling devices which could induce shear stress of amorphisation such as ball mills should be avoided!

McCrone Micronizing mill

Micronizing of the sample

– Weigh 2.7 g of sample; add 0.3 g (10%) of ZnO internal standard. Note down the exact weights, they will be used in the calculations

– Micronize the samples in a MeCrone Micronizing mill using 5 ml of ethanol (methanol) as grinding agent and a grinding time of 5 minutes for soft material (e. g. limestones) up to 10 minutes for hard materials (eg. quartzites, slags). Since methanol tends to react with some artificial minerals, and as it is toxic, ethanol is preferred. To ensure that samples are ground up to < 10 micron, the size required for X-ray quantification, the grain size is best checked by (wet) laser diffractometry.

– After micronizing, recuperate the sample in porcelain cups. Cover the cups with plastic foil, because recovery of powder from the porcelain when dried is difficult. Wash with methanol to recuperate as much as possible of the sample.

– Dry for one—two days under a fume hood (methanol is toxic).

Preparation for X-ray diffraction

– Dried samples are gently disaggregated in an agate mortar and passed through a 250 micron sieve, to ensure good mixing of sample and ZnO standard.

– +/- 0.5 g of sample is needed for the X-ray analysis. Side-loading with frosted glass is recommended to fill sample holders, to prevent preferential orientation of fibrous zeolites and clay minerals. Sample holders are gently tapped while filling, to ensure good packing of the grains. Alternatively, back loading is used.

The content is offered by Dr. Lieven Machiels