Tag Archives: XRD

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

Sample preparation by McCrone Micronizing mill before XRD/Rietveld test

In the cement science field, XRD analysis combined with Rietveld method can give quantitative results of crystalline phase of hydration products, so I try to use XRD/Rietveld method to quantify the hydration products of cementitous material. To achieve good performance of XRD test, fine powder is required and of great importance.

The question is how fine the particle sizes should be? It is showed by research that the particle size of powder has to be reduced to 10 μm or less, which is especially important for quantitative analysis (Smith, 2001).

Here comes the problem, it is very difficult to obtain these grain sizes by hand grinding, which means that mechanical grinding is required to reduce the grain size sufficiently.

There are several points should be stated. For soft minerals such as zeolites (hardness 3–4) grinding can introduce problems of size broadening and amorphous layers can be formed around grains. Prolonged grinding can even make the minerals completely X-ray amorphous (Okada et al., 1993; Kosanovic et al., 1993). Wet grinding generally lowers these effects, but in ball mills even wet grinding can still lead to problems of amorphization and line broadening (Okada et al, 1993).

Sample preparation by McCrone Micronizing mill before XRD/Rietveld test

By the help of Dr. Lieven MACHIELS from KuLeuven, it is reported in his doctor thesis that several authors have shown that wet grinding using a McCrone Micronizing mill is the most efficient method of reducing the particle size of most materials, while avoiding many of the deleterious effects that can be associated with mechanical grinding (O’Connor and Chang, 1986; Buhrke et al., 1998; Eberl, 2003).

Therefore, we applied the method to grind hydrated pastes to get particles smaller than 10 μm for XRD analysis. In the future, we will analyze and assess whether this method is good enough to obtain quantitative results.

Hydration products of slag in blended cement

Many XRD analysis results have shown the main hydration products of slag blended cement are essentially similar with that of pure Portland cement, except the amounts of CH found by this method or other are in varying degrees and less than those that should be given by the pure Portland cement constituent if the slag part did not participate the reaction.

The main hydration products of the slag-cement are C-S-H gel, Ca(OH)2, the sulpho-aluminate hydrate phases AFt and AFm and a Mg, Al-rich hydroxide phase.

TEM slag
Fig. 1. Transmission electron micrograph showing foil-like Op C-S-H in a water-activated slag paste hydrated for 3 1/2 years at 40°C (W/S = 0.4) (by I.G Richardson: The nature of C-S-H in hardened cements).

TEM cement slag
Fig. 2. A TEM micrograph that illustrates fine, dense Op C–S–H in the paste containing 75% slag. (by I.G Richardson: Composition and microstructure of 20-year-old ordinary Portland cement–ground granulated blast-furnace slag blends containing 0 to 100% slag).

In the case of C-S-H, its morphology and composition may be modified by partial accommodation of M and A within the micro- or nanostructure, its Ca/Si ratio is then lowered (e.g., 1.55) than that formed from alite and belite (e.g. 1.7). Hydrotalcite-like phase with approximate composition Mg6Al2(OH)16(CO3)·4H2O is formed from the MgO content of BFS, typically 5-9%.

As stated above, the hydration products can be classified into inner product and outer product. Inner product C-S-H from cement grains has a Ca/(Si+Al) ratio similar to outer product C-S-H, Meanwhile, Katoite (C3ASαHβ, α < 1.5) has also been suggested as a slag product, but it is less documented than the former mentioned species.