Shadowing Practice: Introduction 2, Why "Portland" cement - Learn English Speaking with Video
Creating lesson...
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Okay, so welcome back.
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In this second unit of this introductory module, I've called it Why Portland Cement?
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And what we're going to look at in this module is why the composition of cement is as it is.
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Why Portland cement?
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99.99% of cement made today is based on what we call Portland cement or more precisely Portland cement clinker.
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So clinker is these nodules you see in the picture here and that's the material that comes out of the cement kiln.
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And the name Portland, this is purely a marketing name,
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this was chosen by Joseph Aspin who was the person who patented the term Portland cement in 1824
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and what he wanted to do, he he wanted to highlight the appearance of his new material to Portland stone
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which was then regarded as the best building material in England.
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So don't worry about this word Portland, it's purely a marketing term.
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So the dominance of this type of cement is really not by chance.
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It's a direct consequence of the raw materials which we have available on earth.
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So here on the left, we see the composition of the Earth's crust.
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And it's really remarkable to see that we have just eight elements, that's to say oxygen, silicon, aluminium, iron, calcium, sodium, potassium, magnesium.
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These together make up more than 98% of the Earth's crust.
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This is quite remarkable, but those of you who may have done some astronomy
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will realise that it's a direct consequence of the formation of elements in stars.
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and it also means that this is not just the composition of the Earth
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but will be the similar composition for all the rocky planets we can imagine in the universe.
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What this means is that even elements we regard as quite common,
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like for example copper, are very very much less abundant than these eight rock forming elements,
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like for example silicon.
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And this relative abundance of 1 to 1000 will translate in terms of the availability and the cost of these materials.
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So having seen in the last module how the very large amount of cement is produced,
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clearly we can't think about making cement out of all the other elements that's in that remaining 2%.
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So now we can understand that the composition of the earth limits the practical chemistries we can have for cementitious materials.
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But looking on the bright side, it also means that we can explore all possible options.
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It means that there's no kind of breakthrough out there that we haven't discovered yet.
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And what we need to really know in a bit more detail is how cement works
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to understand how these different elements can be used.
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So this is what I've illustrated in the next few slides.
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So this schematic here really tries to explain this.
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When you mix grey cement powder with water, what you have is grains just floating about,
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and that is very convenient because it means you have a flowable material, you can cast into moulds, you can make into different shapes.
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And then we have a chemical process whereby these grey cement grains are dissolving
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and then the ions are reacting with the water to give us this new solid which are called the hydrates.
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So in this image here the red areas are the cement hydrates and these have a higher volume of solid
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and therefore it holds the cement grains together, creating a rigid solid.
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So let's go back and look at this composition of the earth.
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Most of those elements are associated with oxygen as oxides,
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and look at whether these different oxides can react to give us a cementitious material.
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Now if we take the two alkali oxides, sodium and potassium, the problem here is these produce very, very soluble salts.
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So maybe in your chemistry lessons you've used sodium hydroxide
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and you'll know that you can have very strong sodium hydroxide because a very high amount can go into solution without precipitating.
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So clearly this is no good for producing solid hydrates which hold the structure together.
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On the other hand, if we look at iron oxide and magnesium oxide, the problem here is that these oxides have fairly low mobility in alkaline solutions,
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which is the solutions we have in cementitious materials.
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So what we can see in this image here, this is just a piece of concrete taken from a normal concrete wall.
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And in this image we have some very bright areas.
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And these very bright areas are where all the iron oxide is located.
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And in fact, those bright areas are exactly the same as in the original cement grains.
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During that whole 30 years, that iron in the cement grains has just stayed in the same place.
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So it's not doing any harm, but basically it means it's not going out into solution,
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it's not precipitating new solids and therefore making very little contribution to the bonding and the development of strength.
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It's a similar story for magnesium.
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Here in this picture now, we see a blended cement.
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We're going to talk about that in a few modules time.
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And in this blended cement, the darker grains, such as in the center of the picture here,
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you see are now surrounded by a region of dark products.
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And these darker products are because all the magnesium that was in that slag grain is concentrated in that area.
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And same situation, this magnesium has not moved into the space between the grains.
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it's not really contributed to bridging the space between them.
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So this means we end up with just three oxides, which are really the most useful, and are the essential component for the Portland cement we have today.
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So this is really in no way a coincidence that we've ended up with this composition.
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Because if we now look at that ternary system between calcium oxide,
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silicon dioxide and alumina, we see the composition of Portland cement.
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And in fact, Portland cement, as we see it here, this is composed of calcium silicates.
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There's really only one other region in this diagram where we have minerals
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that will react with water in the way we previously saw and give us cementitious materials.
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And that's this region here, which is the region where we find calcium illuminate cement and calcium sulphur illuminate cement.
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Small amounts of sulphur is the sort of extra dimension which is not shown in this diagram here.
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So these other compositions, calcium aluminate,
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calcium sulphur aluminate, have a lot of advantage because they have less calcium in.
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You see, they're further away from that calcium oxide corner, means they have less calcium oxide,
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and therefore there's less CO2 produced chemically during the production of these phases.
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So that sounds very good, but the problem is that it's not so easy to make these
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because the raw materials we need are not so widely available.
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And we have to ask the question, what sources of minerals are there which contain much higher amounts of aluminium to silicon?
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Most minerals have about the same ratio of silicon to aluminium as in Portland cement, about two to one.
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To get a composition down here in this alumina rich part of the diagram, we have to go to materials like bauxite.
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Now bauxite is much more localized than the materials used for making Portland cement.
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It's estimated that only 10 countries contain about 90% of the reserves of bauxite.
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And of course bauxite is the first raw material for aluminium production.
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So the consequence of all this means that it's rather expensive
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and it means that you cannot produce calcium aluminate cement
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or calcium sulphur aluminate cement for the same cost as producing Portland cement.
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We can look at this effect of chemical CO2 and how it's related to composition in more detail on this slide here.
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So at the top we see the two calcium silicates which are present in Portland cement.
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These are known as tricalcium silicate or C3S, dicalcium silicate or C2S.
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And many people have proposed that what we should be doing is making Portland cement with more C2S as opposed to C3S.
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But the problem here is that the reduction in chemical CO2 is really very marginal.
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It's only around 10%.
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And that's more than offset by the much slower kinetics of C2S.
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So the strength development is much slower.
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And because the strength development is much slower, this would mean that in practical situations where people need a given strength at a given time,
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then they will use more cement.
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And that use of more cement will completely cancel out this 10% saving.
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So belight-rich clinkers really don't contribute at all to lowering CO2 emissions.
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Down here, we can see the figures for yellow mite.
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That's this complex phase called C4A3S bar.
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and as I said on the previous slide, this has a good reduction potential in terms of lower chemical CO2.
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But we have the problem of the cost of the raw materials.
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And finally on this slide I want to point out this line here
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because many people nowadays are proposing that we should use magnesium-based cements.
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There can be context in which this might be possible,
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not now, but if somebody can invent a way of producing magnesium-based cements from magnesium silicate rocks,
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it might be interesting.
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But what's happening at the minute, most people are using magnesium carbonate.
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And what you can see here very clearly is
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that the chemical CO2 emissions from a breakdown of magnesium carbonate
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are way higher than they are from the calcium silicate phases we have in Portland cement.
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So here we come back to that composition of the earth we've discussed in detail,
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and we compare it with the composition of a typical Portland cement.
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And you can see the elements present are similar.
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The big difference between the Portland cement and the composition of the earth is the dominance of this sector for calcium.
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And this is, as we explained in the last lecture, where the chemical CO2 is coming from.
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But from a practical point of view, it's very easy to make things with this composition because limestone, which is usually very pure calcium carbonate,
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is very widely distributed across the globe.
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And it means that you can make these porthens cements out of limestone.
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The other elements are generally coming from something like clay, but you can make this almost everywhere in the world.
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And therefore, transportation of the final product is minimized.
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So the other important aspect is the pattern of reaction of the Portland cement.
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And this is shown in this diagram here, which plots the heat evolution, which is a signature of the rate of reaction.
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And what we can see is at the beginning of the reaction, we have a burst of reaction,
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and then we have this period here where the reaction remains slow for several hours.
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From a practical point of view this is extremely important as it gives us time to mix the cement
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and transport it to the building site.
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And then after about three hours the reaction takes off again.
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Now this pattern of reaction means that really it's very very simple to use cementitious materials.
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Here we see a picture from India where people are making concrete under really quite rustic conditions.
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But nevertheless, because it's such a robust chemical reaction, it still works, it still gives very good quality building materials.
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Really, what we're going to see is that for these reasons I've explained to you here,
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cements based on Portland clinker will be the most important materials for the foreseeable future.
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They can be made from widely available raw materials, there's an incredible economy of scale, leading to very low cost,
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and they're very easy to use even by unskilled workers.
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Given the composition of the earth, this composition, Portland cement clinker, really is the best option.
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But in the next lecture, what we're going to see, how by replacing part of this Portland cement, we can lower environmental impact.
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So, thank you very much.
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Vocabulary and speaking notes for this lesson
This C1 speaking lesson is built on the video “Introduction 2, Why "Portland" cement”. The speaker keeps coming back to these words: cement, Portland, material, composition, calcium. This video has 139 sentences and 2024 words to shadow. The speech runs for 14:22. The speaker talks at a steady 141 words per minute, a comfortable pace for shadowing. Only 80% of the words are among the 3,000 most common in English, so the vocabulary is demanding.
Key vocabulary in this video
The 15 most advanced words in the video, with pronunciation and meaning:
| Word | Pronunciation | Meaning |
|---|---|---|
| cement noun | /səˈmɛnt/ | A powdered substance produced by firing (calcining) calcium carbonate (limestone) and clay that develops strong cohesive properties when mixed with water. The… |
| calcium noun | /ˈkælsi.əm/ | The chemical element (symbol Ca) with atomic number 20: a soft, silvery-white alkaline earth metal which occurs naturally as carbonate in limestone and as… |
| oxide noun | /ˈɑksaɪd/ | A binary chemical compound of oxygen with another chemical element. |
| magnesium noun | /ˌmæɡˈnizi.əm/ | The chemical element with an atomic number of 12. It is a light, easily flammable, silvery-white alkaline earth metal. |
| silicate noun | /ˈsɪlɪkət/ | Any salt of silica or of one of the silicic acids; any mineral composed of silicates. |
| cementitious adjective | /ˌsɛmɛnˈtɪʃəs/ | Resembling or having some properties of cement; containing cement. |
| clinker noun | /ˈklɪŋkɚ/ | A very hard brick used for paving customarily made in the Netherlands. |
| silicon noun | /ˈsɪləˌkɑn/ | A nonmetallic element (symbol Si) with an atomic number of 14 and atomic weight of 28.0855. |
| aluminate noun | A compound, containing aluminium and oxygen with more electropositive elements, that is a salt of the hypothetical aluminic acid | |
| bauxite noun | /ˈbɔːksaɪt/ | The principal ore of aluminium; a clay-like mineral, being a mixture of hydrated oxides and hydroxides. |
| sulfur noun | /ˈsʌl.fə/ | A chemical element with atomic number 16, having a bright yellow color and characteristic smell, used commercially in a variety of products such as… |
| aluminium noun | /ˌæl.(j)uˈmɪn.i.əm/ | A light, silvery metal extracted from bauxite, and a chemical element (symbol Al) with an atomic number of 13. |
| diagram noun | /ˈdaɪ.ə.ɡɹæm/ | A plan, drawing, sketch or outline to show the function or operation of something, or to show the relationships between the parts of a whole. |
| sodium noun | /ˈsəʊ.di.əm/ | The chemical element (symbol Na) with an atomic number of 11 and atomic weight of 22.990. It is a soft, waxy, silvery, reactive alkali metal that is never… |
| module noun | /ˈmɑd͡ʒul/ | A self-contained component of a system, often interchangeable, which has a well-defined interface to the other components. |
Phrasal verbs you will hear
| Word | Pronunciation | Meaning |
|---|---|---|
| go back verb | To return to a place or state after having been there at a previous time. | |
| make up verb | /ˌmeɪk ˈʌp/ | To constitute, to compose. |
| point out verb | To identify among a group of similar subjects, or in a scene where the subject might not be readily seen or noticed, with a gesture of the body. | |
| take off verb | To remove. |
Grammar in this video
The structures the speaker uses most, with the exact words from the video:
| Structure | In the video |
|---|---|
| Passive voice be + past participle — the focus is on what happens, not who does it | is based · was chosen · is produced |
| Present perfect have/has + past participle — a past action that still matters now | I've called · haven't discovered · I've illustrated |
| Relative clauses who / which + clause — extra information about a person or thing | person who patented · solid which are · oxides, which are |
Pronunciation to watch
The speaker uses 17 contractions and reduced forms, such as we're, I've, don't. Say them the short way, as you hear them.
- The “sh” and “zh” sounds: cementitious /ˌsɛmɛnˈtɪʃəs/, emission /ɪˈmɪʃ.ən/, dimension /daɪˈmɛn.ʃən/
- Long words — get the stress right: magnesium /ˌmæɡˈnizi.əm/, cementitious /ˌsɛmɛnˈtɪʃəs/, aluminium /ˌæl.(j)uˈmɪn.i.əm/, alumina /əˈl(j)uːmɪnə/, illuminate /ɪˈl(j)umɪneɪt/
How to practise with this video
- Listen to the whole video once without speaking and note the words you do not know.
- Shadow it sentence by sentence at normal speed, repeating each one until your rhythm matches the speaker.
- Record yourself and compare with the original, paying attention to words like cement, calcium, oxide.
What is the Shadowing Technique?
Shadowing is a science-backed language learning technique originally developed for professional interpreter training and popularized by polyglot Dr. Alexander Arguelles. The method is simple but powerful: you listen to native English audio and immediately repeat it out loud — like a shadow following the speaker with just a 1–2 second delay. Unlike passive listening or grammar drills, shadowing forces your brain and mouth muscles to simultaneously process and reproduce real speech patterns. Research shows it significantly improves pronunciation accuracy, intonation, rhythm, connected speech, listening comprehension, and speaking fluency — making it one of the most effective methods for IELTS Speaking preparation and real-world English communication.