Española de la Gross Motor Function Measure-88 (GMFM-88)

To start with, think of a Portland cement that is composed only of ground clinker and gypsum added at the cement mill. It has no interground limestone, or grinding aid that affects cement hydration. It contains no carbonate and no carbonate is allowed to penetrate from outside. (In Europe, this almost has to be a thought experiment because most CEM I contains interground limestone, unless you use white cement.) The image in Figure 11.3 below, reproduced from Figure 6.3 for convenience, is a good archetype and we'll use it for the thought experiments that follow.

Figure 11.3 Archetypal image: polished section of cement paste, age 2 years, made using ordinary Portland cement, w/c=0.40.

Key: c - unhydrated cement; C-S-H - calcium silicate hydrate; CH – calcium hydroxide; p – pore; Circle ‘A’ contains C-S-H, CH and is assumed also to contain AFm and AFt phases.

CH is just visible, AFm and AFt are not visible as they are too small.

In most of what follows, we will be thinking of the cement paste fraction of the concrete (or mortar etc.). Concrete is not just cement paste with stones in it, because the presence of aggregate introduces other important physical factors such as the water demand of the aggregate and the interface between the aggregate and the paste. However, we want to concentrate on the hydration process and this is most simply done by thinking of pastes - just cement and water.

You may first want to review the cement hydration process outlined in Chapter 6.3.4.

Thought experiment 5: effect of water/cement ratio

The next thought experiment is to visualise some cement pastes with different water/cement ratios. It should be really easy, because there are some pictures showing exactly this in Chapter 6; for convenience, here they are again (Figures 11.4-11.7).

Figure 11.4 Polished section of cement paste, age 2 years, made using ordinary Portland cement, w/c=0.33.

Figure 11.5 Archetypal image: polished section of cement paste, age 2 years, made using ordinary Portland cement, w/c=0.40.

Figure 11.6 Polished section of cement paste, age 2 years, made using ordinary Portland cement, w/c=0.50.

Figure 11.7 Polished section of cement paste, age 2 years, made using ordinary Portland cement, w/c=0.60.

In this particular experiment, don’t think about individual hydration products, just concentrate on three components: the cement hydration products as a whole, the pores and the remaining cement particles that have not hydrated. Implicitly, there is also a fourth component, water, filling the pores.

Think firstly of mixing 1 kg cement with 400 g water to give a water/cement ratio of 0.4, then curing the paste under water for a year or two, until it is mature and little further hydration will occur.

Picture the hydrating cement: as time passes, the proportion of hydration products increases, the proportion of unhydrated cement decreases and the water-filled gaps are gradually replaced by hydration product.

In principle, according to the Powers-Brownyard model, at w/c=0.4, most or all of all the cement should hydrate because we supplied additional curing water. In practice, it won’t, because of the formation of thick coatings of hydration product around the unhydrated relicts of larger cement grains that restrict further access to water.

By 28 days, perhaps about 80%-90% of the alite has hydrated, together with

Some of what remains unhydrated will probably never hydrate, although

hydration will continue at a slow, and ever slower, rate almost indefinitely. After a year or two, hydration will have mostly ceased.

The archetypal image in Figure 11.3 (and in Figure 11.5) is our archetype; it shows paste made with a water/cement ratio of 0.4 after 2 years. It is composed largely of cement hydration products, with a little unhydrated calcium silicate (top left) and some ferrite (bright, in larger relict cement grains). There are some pores visible; these are capillary pores and would have been water-filled. In the polished section, water in capillary pores has been replaced by epoxy resin.

To summarise, a mature Portland cement paste (w/c=0.4) contains hydration product, some pores, some unhydrated cement, a little water or pore fluid and maybe some entrapped air.

Next, visualise how a mature paste made with w/c=0.5 might appear. (Don't cheat and look at the images, just think about it!) There will be more water and less cement in the paste, so we would expect a higher porosity and fewer remaining unhydrated cement grains.

Now have a look at Figure 11.6, which shows exactly this; the paste

microstructure is considerably more porous than the paste made with w/c=0.4 in Figure 11.5. There is still some residual ferrite; some of this may have hydrated, we can't really tell from the images, but what is there now will probably persist almost indefinitely.

While thinking again of the paste made with w/c=0.4, now picture a mature paste made with w/c=0.6. There will be much less cement and much more fluid-filled pore space; the hydration products will be more porous, with larger

capillary pores; again, this is just what Figure 11.7 shows. There is a clear trend in the characteristics of the images in terms of porosity and residual unhydrated cement as we go from w/c=0.4 through w/c=0.5 to wc=0.6.

Finally, consider a paste made with w/c approximately 0.3. The trend continues in the opposite direction; we would expect a lot of unhydrated cement and little porosity. According to the Powers-Brownyard model, not all the cement can hydrate because there is not enough volume to accommodate all of the hydration product. That is apparent from Figure 11.4; very little porosity is visible but unhydrated cement grains are numerous. By extension, since there is little capillary porosity visible, the available water is also very limited. Assuming

hydration product to occupy 2.2 times the volume of the unhydrated cement, it is clear that most of the remaining cement cannot hydrate; there is not enough space for the hydration product, or sufficient available water.

Thought experiment 6: changing the sulfate/alumina ratio

This expands Principle 3 above. Although they represent only a small proportion of the total paste, ettringite and AFm phases are important; they affect concrete setting properties and can have an effect on long-term durability.

The balance between available alumina and sulfate in the cement paste controls the relative proportions of AFm phases and ettringite. In this thought experiment, think of three cements, all made from the same clinker as used in our cement archetype above, but containing different amounts of gypsum added when the cement was milled.

We’ll call these “low sulfate,” “medium sulfate” and “high sulfate” mixes; the actual numbers aren’t important here but think of them as, say, 1.5%, 3.0% and 4.5% SO3 if you like. The cements are used to make pastes, all at the same water/cement ratio and cured until they are mature, say for a year or more.

Assume there is sufficient sulfate to prevent a flash set at the lower sulfate contents and that any false-set that might occur is mixed through at the higher sulfate contents. Don’t worry about the exact numbers shown, they are only intended to be illustrative, and just follow the arguments for each paste.

In a mature paste, what hydration products are present?

Medium sulfate mix: suppose the cement in the archetype mix above contained a “medium” level of sulfate. In our mature paste, hydration has virtually

completed and the principal hydration product is C-S-H. Some CH is present, and there is sufficient available sulfate to combine with all the available alumina to form monosulfate phase. A small surplus of available sulfate produces a little ettringite from some of the monosulfate. This is our reference paste to compare with the following pastes as we change the amount of sulfate.

This paste contains: C-S-H; CH; AFm as monosulfate and a small amount of ettringite.

Suppose there is less sulfate:

Low sulfate mix: in this paste, the available alumina is about the same as in the medium sulfate mix; there is less available sulfate, all of which is present in AFm phase as monosulfate, but there is still an excess of available alumina.

Consequently, there is no ettringite at all, because this requires a higher ratio of sulfate to alumina. The small deficit of sulfate is accommodated by the formation of hydroxy-AFm phase, in which sulfate is partly replaced by hydroxide.

This paste contains: C-S-H; CH; AFm as monosulfate and hydroxy-AFm; no ettringite.

High sulfate mix: compared with our “medium sulfate” reference paste, this paste with a higher sulfate content has more available sulfate and there is now sufficient to convert almost all the available alumina to ettringite, with a little residual monosulfate.

This paste contains: C-S-H; CH; ettringite; a little AFm as monosulfate.

At higher still levels of sulfate, all the available alumina will have formed ettringite; there will be some residual gypsum as no other sulfate-containing phases can form.

This experiment has considered what might be present in mature pastes because it is easier to do this when the reactions have largely stopped. At early ages, the reactions are more complicated and there will be much local variability. Cement particle size and crystal microstructure, to name just two factors, affect the rate at which reactants become available. Also, not all the sulfate in the cement is ultimately present in ettringite or monosulfate; some is taken up by the C-S-H, as is some alumina. The use of the term “available” is used to allow for these uncertainties.

Thought experiment 7: add interground limestone

In recent years, an increasing proportion of Portland cement worldwide has contained a small amount of interground limestone, typically up to 5%. The next change to our archetype of a basic Portland cement should therefore be to add some interground limestone. With closed-circuit cement milling, the particle size range is less than with an open-circuit mill. There are fewer coarse particles and a much smaller proportion of very fine particles; limestone is softer than clinker, so intergrinding it with clinker in a closed-circuit mill will produce very fine limestone particles (<1 µm) and these compensate for the lack of very fine cement particles. These fine limestone particles will have both physical and chemical effects.

Imagine mixing cement containing fine limestone with water. At the instant of mixing, before the cement and water react, think of the fine limestone dispersed between the coarser cement grains. The fine limestone occupies space that would otherwise be occupied by water. The mix will be more cohesive, reducing

bleeding. The cement may also be ground coarser, giving better control over early hydration, strength growth and heat evolution and will also save energy.

The first effects of limestone intergrinding are therefore physical and are in some ways beneficial. The water demand might increase, depending on the particle size of cement and limestone; this would reduce strength and so not be beneficial.

Imagine the fine particles of limestone being engulfed by C-S-H and ettringite precipitating around them. This is Principle 5, the fine filler effect. The fine

limestone will act as nucleation sites for the formation of hydration products; this will tend to reduce setting times and increase early strengths by increasing the

rate at which the cement hydrates. As noted in Principle 5 above, it may also aid later strengths.

How will the chemistry of the cement paste change?

Although limestone is widely used as aggregate, calcium carbonate is not inert in cement pore fluid. The smaller the limestone particles are, the quicker they are likely to react. Principle 4 says that this will affect the formation of ettringite and monosulfate. Some of the interground limestone will dissolve, with the calcium contributing to the formation of C-S-H and other hydration products and the carbonate forming AFm phases.

Where monosulfate has formed, carbonate from the limestone displaces the sulfate. The sulfate forms ettringite from existing monosulfate and the carbonate forms AFm phase as hemicarbonate. Eventually, all the monosulfate disappears, replaced by hemicarbonate and ettringite.

The hemicarbonate may then react with more carbonate, if available, to produce monocarbonate. (Monocarbonate contains twice as much carbonate compared with hemicarbonate, so hemicarbonate will tend to form if carbonate is scarce in relation to the total amount of AFm phase, with monocarbonate forming as more carbonate becomes available).

Under 0.5% carbonate by mass of cement is typically enough to prevent monosulfate formation in cement paste; this is roughly 1% limestone. If 5%

limestone is present in the cement, there is a large excess of available carbonate and the principal stable AFm phase where interground limestone is present is therefore most likely to be monocarbonate.

In summary, cement containing interground limestone is likely to contain

ettringite, with monocarbonate tending to be the principal AFm phase instead of monosulfate. This should have no adverse effect on the performance of the cement; indeed it should be beneficial to strengths due to the fine filler effect and to durability by a more effective blocking of capillary pores by the platelets of monocarbonate AFm phase together with ettringite.

Intergrinding up to 5% fine limestone with the clinker is a relatively recent development (how recent depends where you are). However, limestone has been used as an aggregate since concrete was first used. If limestone is crushed for use as aggregate, it will contain limestone dust, so the presence of fine limestone in concrete is actually not all that new.

An accountant’s view of adding 5% limestone to cement might be that, since limestone is cheaper than cement, the purchaser is getting a bad deal. In practice, the performance of the cement should not be adversely affected and may be enhanced. (EN 197-1 requires that a cement containing 5% mac - which may be fine limestone - should meet the same performance criteria as if the mac were not present.) There should also be a slight saving in the energy required to produce the cement.

Thought experiment 8: add microsilica (silica fume)

Microsilica is a by-product of the silicon industry consisting of small, rounded, glass particles approximately 0.1 µm across. It is typically composed of 85% or more silica, with some potassium, aluminium, iron, magnesium, calcium and other impurities. It is highly pozzolanic with a large surface area.

Think of our archetypal Portland cement paste – use the image in Figure 6.3, reproduced yet again for convenience in Figure 11.8 below. Then imagine adding to it a small amount of microsilica, say 3% by weight of cement, and mixing to a paste with w/c=0.5. How will the microsilica affect the properties of the paste?

Figure 11.8 Polished section of cement paste, age 2 years, made using ordinary Portland cement, w/c=0.40. Key: c - unhydrated cement; C-S-H - calcium silicate hydrate; CH – calcium hydroxide; p – pore; Circle ‘A’ contains C-S-H, CH and is assumed also to contain AFm and AFt phases. CH is just visible, AFm and AFt are not visible as they are too small.

We have added very fine and reactive material composed largely of silica;

assuming the microsilica to be properly dispersed, the fineness should have the physical effect of improving the cohesiveness of the paste. In concrete, this should reduce bleeding.

How will the chemistry of the cement paste change?

Principles 5 and 6 will apply; think of the numerous small particles acting as nucleation sites for the formation of hydration products. This should improve early concrete strengths and probably also reduce setting times. As the microsilica itself then starts to react, it will contribute silica to the cement hydration products, forming additional C-S-H from the CH in the paste. Look particularly at the CH in the paste without microsilica in Figure 11.8, and then visualise the extent of CH in the paste diminishing as the C-S-H increases with microsilica addition.

The Si/Ca ratio of the C-S-H will increase from approximately 0.5 to 0.6 or more.

With increasing levels of cement replacement by microsilica, the quantity of CH will decrease further and the Si/Ca ratio of the C-S-H will increase, releasing more lime for C-S-H formation.

In summary, cement containing microsilica should have a dense paste structure with an increased proportion of C-S-H and a decreased proportion of CH.

Strength should be enhanced and permeability decreased, leading to better durability.

Assuming the microsilica to contain a negligible amount of alumina, the addition of microsilica will not affect the ratio of sulfate to alumina and so the relative proportions of monosulfate and ettringite shouldn’t be altered.

Thought experiment 9: add granulated blastfurnace slag (gbs)

Blastfurnace slag is a by-product of iron smelting. When the molten slag is cooled rapidly using water from about 1500 ºC to 800 ºC, it forms a granular, latent hydraulic, material composed of about 95%, or more, glass. The three principal oxides are those of calcium, silicon and aluminium. Compared with Portland cement, slag used in concrete contains less CaO and more SiO2 and Al2O3. Slag is not as reactive as Portland cement and ideally is ground finer.

The proportions of Portland cement and slag in a composite cement vary widely, typically from 30% slag to 70% or more. Suppose we have a mix containing 50%

of our archetypal Portland cement and 50% ggbs at w/c=0.5.

When first mixed with water, the Portland cement fraction of the mix reacts more quickly than the slag and so has a higher effective w/c ratio than if the mix were 100% Portland cement. The paste microstructure for the first few days or weeks is therefore more porous compared with a 100% Portland cement mix. As the slag hydrates, the porosity is reduced and eventually the paste should become less porous compared with a 100% Portland cement mix.

The finest of the slag particles probably provide nucleation sites on which hydration products form, especially if the slag has been ground finer than the clinker¹. Principle 4 will therefore apply to some extent. (If the Portland cement also contained interground limestone, the limestone may act to accelerate both cement and slag hydration; however, our archetypal Portland cement contains no limestone so we won’t consider this further).

The main effect of the slag is on the proportions of the hydration products and their composition, and on the paste microstructure.

In our mix of Portland cement and slag, at an early age (eg: one day) the Portland cement fraction has hydrated more than the slag component, and the hydration products are broadly those of the Portland cement archetype.

Gradually, the slag reacts.

1 Slag is harder to grind than clinker, so intergrinding slag and clinker produces cement in which the slag is coarser than the clinker. Preferably, the slag should be finer than the clinker, so ideally the two are milled separately and then blended.

How will the chemistry of the cement paste change?

Think of the composition of the slag; compared with Portland cement, the slag has a higher ratio of Si/Ca and also of Al/Ca (compare the typical analyses in Tables 3.2 and 7.1). The main effect on the hydrating system will be to increase the ratio of total available Si/Ca compared with that from the Portland cement alone and Principle 6 will apply. The proportion of CH in the paste will decrease, the proportion of C-S-H will increase and the Si/Ca ratio of the C-S-H will increase.

Principle 3 will also apply. The slag contributes more alumina than sulfate, so the

Principle 3 will also apply. The slag contributes more alumina than sulfate, so the