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Concrete Analysis and Alcubierre Drive
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Concrete Analysis by Electron Microscopy

From holding up the roof above your head, to getting you between A and B, concrete is used in a myriad of ways that make our lives what they are.
 
Concrete, a mix of tri-calcium silicates (otherwise known as cement), sand, aggregates (as well as few other plasticity modifiers, setting retardants, etc.) and most importantly, water, is a compound that has been studied by people (not unlike us at Microanalysis) since before the Romans learned how to take a bath.
 
Once wetted, the hydration products start to form and concrete begins to set - hey-presto, your ornate mosaic tiles are hopefully forever fixed to your steam room floor! What you actually end up with is hopefully a solid mixture of fine air voids, capillary pores, hydration products, unreacted cement and sand/aggregates. But, get the mix wrong and you’ll have more than Caesar’s face sticking to your feet.
 
Non-compliance these days can see investigations into water:cement and sand:cement ratios as well as aggregate types (alkali-silicate reaction) and non-heterogeneity of mix. Traditional methods employ lengthy physio-chemical techniques such as BS1881:124:1998 but have been suspected of large inaccuracies for many years (A.M.Neville (2003), “How closely can we determine the water-cement ratio of hardened concrete”, Mat. Struct. 36, 311-318.).
 
Now the clever people at Imperial College London have published a paper (http://www3.imperial.ac.uk/concretedurability/researchprojects/determiningwatercementratio) outlining a relatively simple method using scanning electron microscopy for concrete petrography.
 
Apart from obtaining colourful images, the method allows you to determine the volume fraction of reacted /unreacted cement, capillary pores and hydration products. So, a quick thin-section, carbon coat and 15 minutes worth of x-ray mapping and the images below can be interpreted to report both the water:cement and sand:cement ratios as well as an in-depth interpretation of swelling aggregates (ASR), ettringite formation (sulphate presence), carbonation, chloride penetration and calcium dissolution/leaching.

Figure 1: Concrete petrographic image                         Figure 2: Percentage of features in figure 1

Not Quite an Alcubierre Drive


Many of you probably don’t know (or care) about the Alcubierre Drive – with good reason. Those who have heard of it probably know of it as the ‘Warp Drive’ from Star Trek which originally inspired the theoretical drive. The Alcubierre drive is a theoretical propulsion method which would allow a spacecraft to effectively move from one point to another faster than it would take light to travel the same distance, but without technically exceeding the speed of light. The theoretical drive instead distorts space-time around the spacecraft, creating a positive energy-density area behind the spacecraft and a negative energy-density area in front of the spacecraft, propelling the space containing the spacecraft forward.
This kind of drive has always been considered theoretical and a bit far-fetched, although entertaining to speculate. So entertaining that one NASA group – the Advanced Propulsion Team – is dedicated to conceptualising and developing advanced propulsion techniques including practical experimentation into the existence of Alcubierre drives. 
The team performed an experiment using their White-Juday warp-field interferometer with an EmDrive (invented by Roger Shawyer) which resulted in the detection of variations of the path-time of laser light around the drive. Although temperature anomalies were also detected, it has been theorized that these variations in path-time may indicate the warping of space-time within the chamber. The results, which were deemed to be significantly above system noise, were replicated four additional times with this system.
While those that are over-excited are claiming that this is a technology that will take us to Mars with a travel time of less than 3 months, even skeptics admit that if this proves not to be anomalous interference or miscalculation then it could be an interesting new step towards new technologies. Definitely something to keep an eye out for in the future!
 

Image Of The Month
This month’s image is of a growth on the surface of a baby Slater.

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Image taken by Sandy Lam

 
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