Concrete: An Aggregate of Problems

Have you ever thought about what your house is actually made of? Before starting my Ph.D. I had never taken this question into consideration. However, for thousands of home owners across Ireland, the structure of their homes is under threat from the very stuff it is made of. Concrete. But why exactly is this commonplace material causing problems for these unlucky few? To answer this question, I will have to start by raising another: what is concrete?

Concrete is a synthetic, man-made rock composed of water, cement and aggregate. For the concrete novices out there, aggregate is the crushed rock which forms the large portion of concrete, while cement is powdered rock made of limestone and clay which is used to form the glue that binds the aggregate together. So, how does it all form? As you can see below, concrete starts off life as a pile of aggregate floating in a soup of cement and water. As time passes, the mix begins to dry and crystals of calcium-silica-hydrate begin to form and eventually surround the aggregate, binding it together and giving concrete its strength.

formation

Now that we’re up to speed on concrete, we can finally start pointing fingers as to what is causing the problem. As you’ve probably guessed from the title of this post, the problem lies in the aggregate. Aggregate is crushed rock that forms a large portion of the concrete structure, up to 75% in most cases. Aggregate is usually inert, meaning it is unreactive and ideally should be very stable. However, for some unfortunate home owners across Ireland, the aggregate in their house can look something like the picture below. This is not the aggregate you want for making concrete. It clearly contains an abundance of a gold coloured mineral called pyrite.

aggregate

Pyrite is an iron disulphide, meaning its crystal structure is made up of groups of iron atoms attached to two sulphur atoms. When this mineral comes into contact with moisture or water, oxidation tends to occur. This oxidation creates a sulphuric acid which is then free to react with the calcium that is abundant in most of the aggregate used in Ireland. The combination of calcium, sulphur and oxygen brings together all the components necessary for gypsum to form. Gypsum is a salt and like most salts it has a habit of expanding. When this expansion is confined inside of building material it can lead to a process called pyritic heave. In other words, it can lead to widespread cracking and expansion of concrete throughout the home.

pyritic-heave

So, where does my research come into all this? We want to investigate whether the different crystal structures of pyrite have a bearing on the degree of pyritic heave. For instance, is the presence of cubic pyrite worse than framboidal pyrite or does the greater surface area of the spherical framboids make them the main culprit. Similarly, we also want to investigate the elemental signature of the household pyrite and attempt to match this signature with that of the pyrite found in the quarries. Achieving this would allow us to identify the quarries that are supplying a poor standard of aggregate and make sure that this material does not end up in people’s homes.

fingerprint

By Tadhg Dornan, Ph.D. student at Trinity College Dublin conducting iCRAG funded research within the Raw Materials spoke. 

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1 Comment on "Concrete: An Aggregate of Problems"

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proff. samad alipour
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Dear Tadhg I was aware of this proble from many years ago, it isnot just for a pyrite bearing rock but it is a weak bound any way, since the pebbels are inert ant reacting with cement.. Mg is more dangerous due to hiher expansion prolem. so if your roke bear dolomiteI, it is a prolem. I sugest you make break with your mudstone, it will produce a very nice scarlet red break , your pyrite will be oxidized and gone to break coloring . you can control the olor by temperature in kiln as well . muh better building… Read more »
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