Fly ash from paper mills will replace some cement

[Stan]

At the moment, mills pay to send it to landfill but it actually has been used to replace cement in new roadway substructures.

[Oliver90owner]

Fly ash, from coal-fired power stations, has been used in concrete, allowing partial replacement of the cement for several decades.

Blast furnace slag has been used in large concrete structures, as a partial cement replacement, for even longer - it slows the curing process thus avoiding/reducing the rate heat generation of curing concrete (strength gain). This reduced the rate of heat generation (cement hydration is quite exothermic) thus avoiding/reducing cracking due to thermal expansion in those thick sections which needed to be built as a continuous pour.

Most very low strength concrete requires so little cement in the mix that it often would have too little cementitious content for long term durability, so the addition of lesser strength materials can be advantageous for some applications, such as road bases.

Fly ash from burning coal was a variable commodity, so needed more control in the concrete mixes, but one advantage was the ’lubricity’ of spherical particles in the ash allowing a much reduced water:cement ratio while maintaining a good ‘slump’ value for the concrete mix.

Most paper material wastes are burned for leccy generation if possible (Peterborough can generate up to around 8MW(?) from domestic waste (not all paper, of course). Much of the resultant ash is, I believe, exported just across the road to a thermal block manufacturer, so it likely finishes up in new home builds.

I’m not sure if paper fly ash gets hot enough to produce spheroids, but as long as the alkali metals content is sufficiently low it could be used as per the title. The cement manufacturers (not so many of them these days) would, of course prefer to blend it with cement and sell it as ‘their product’ (just as they did with slag and power station fly ash).

I remember a very well kept plan to burn straw for cement production. The plan was scrapped very late in the planning of a large(ish) scale trial when the alkali metal content of the straw was pointed out to be a very likely cause of cyclone/kiln feed pipe blockages (900+ degree Celsius powder bridges) and if the alkali metals finished up in the product (not so likely at the kiln firing temperatures) the product would make poor concrete (raised alkali metal content in concrete causes failures later in the life of the concrete).

Does the fellow mention the ash content of paper? Most is likely kaolin, calcium carbonate (chalk) and Titanium dioxide used in the process for shiny paper. Kaolin and calcium carbonate will combine to make calcium silicate materials similar to cement, of course - just not as effective as a high strength building material.

But I do agree that these wastes do need recycling rather than going to land-fill. Every little bit helps.

[Stinsy]

A lot of 1940s houses round my way are unmortgageable due to ash use in their construction. High sulphate levels in the ash reacts with the cement causing expansion.

[Oliver90owner]

A lot of 1940s houses round my way are unmortgageable due to ash use in their construction. High sulphate levels in the ash reacts with the cement causing expansion.

Presumably you are meaning ‘fly ash’?

There are several causes of long term failure of concrete.- and some short term ones, too - the main ones being too little cement, alkali-aggregate reaction and ground waters with high sulphate content sulphate.

For adequate strength surprisingly little cement is needed for most general housing requirements, but too little cement means the concrete can be porous which affects long-term durability.

High ‘alkali metals’ content within the concrete causes long term expansion of the concrete. This was a fairly late realisation of this problem where some types of aggregate caused long term expansion and eventual disruption of the concrete. Made worse if the concrete was porous - see previous para.

High sulphates in the concrete or the ground water also causes one particular compound in the cement to be dissolved. This does not expand the concrete directly but weakens it, allows more porosity which allows any steelwork within the concrete to corrode.

Iron oxide (rust) occupies more volume than the steel it is derived from, causing failures due to corroded steel reinforcement and expansion cracking the concrete (concrete is strong in compression but not in tensile strength).

Sulphates in cement are (were) limited to 4.5% by the relevant British Standard 12 of that period. Gypsum, added in the manufacturing process (to retard the setting time of the cement) is carefully controlled in the manufacturing process but sometimes further ground water attack required a ‘sulphate resisting cement’. The product chemistry greatly reduced the tri-calcium aluminate content, which is the component of the cement that is susceptible to sulphate attack, by increasing the iron content of the cement. Reduced tri-calcium aluminate allowed the concrete to resist the more demanding conditions by retaining minimal porosity to protect any embedded steel work.

Is it the groundworks or the building fabric which is failing? Is it the ground water that is challenging? Lots of buildings with precast components have since failed due to poor reinforcement cover along with poorly made components with poor construction design - obviously not understood at the time and not necessarily the fault of the cement in the concrete.

Further, use of high alumina cement as a means of rapidly increasing the strength of concrete (in the first few hours/days has also led to failures in concrete structures. Think here, particularly, of the several swimming pool building collapses/failures in the latter part of the last century. High alumina cement was inappropriate, not specified but used by component manufacturers/builders as a short cut to save time (and cost).

Also, remember that the ‘40s were war and immediate post war years of urgent building/reconstruction - where the target was the cheapest and fastest means of building replacement due to war damage. All(?) of the electricity generation was from burning coal which produced large amounts of ash - a waste product - which was basically dumped or sometimes used inappropriately (in hindsight) for building construction.

[Stinsy]

A lot of 1940s houses round my way are unmortgageable due to ash use in their construction. High sulphate levels in the ash reacts with the cement causing expansion.

Presumably you are meaning ‘fly ash’?

There are several causes of long term failure of concrete.- and some short term ones, too - the main ones being too little cement, alkali-aggregate reaction and ground waters with high sulphate content sulphate.

For adequate strength surprisingly little cement is needed for most general housing requirements, but too little cement means the concrete can be porous which affects long-term durability.

High ‘alkali metals’ content within the concrete causes long term expansion of the concrete. This was a fairly late realisation of this problem where some types of aggregate caused long term expansion and eventual disruption of the concrete. Made worse if the concrete was porous - see previous para.

High sulphates in the concrete or the ground water also causes one particular compound in the cement to be dissolved. This does not expand the concrete directly but weakens it, allows more porosity which allows any steelwork within the concrete to corrode.

Iron oxide (rust) occupies more volume than the steel it is derived from, causing failures due to corroded steel reinforcement and expansion cracking the concrete (concrete is strong in compression but not in tensile strength).

Sulphates in cement are (were) limited to 4.5% by the relevant British Standard 12 of that period. Gypsum, added in the manufacturing process (to retard the setting time of the cement) is carefully controlled in the manufacturing process but sometimes further ground water attack required a ‘sulphate resisting cement’. The product chemistry greatly reduced the tri-calcium aluminate content, which is the component of the cement that is susceptible to sulphate attack, by increasing the iron content of the cement. Reduced tri-calcium aluminate allowed the concrete to resist the more demanding conditions by retaining minimal porosity to protect any embedded steel work.

Is it the groundworks or the building fabric which is failing? Is it the ground water that is challenging? Lots of buildings with precast components have since failed due to poor reinforcement cover along with poorly made components with poor construction design - obviously not understood at the time and not necessarily the fault of the cement in the concrete.

Further, use of high alumina cement as a means of rapidly increasing the strength of concrete (in the first few hours/days has also led to failures in concrete structures. Think here, particularly, of the several swimming pool building collapses/failures in the latter part of the last century. High alumina cement was inappropriate, not specified but used by component manufacturers/builders as a short cut to save time (and cost).

Also, remember that the ‘40s were war and immediate post war years of urgent building/reconstruction - where the target was the cheapest and fastest means of building replacement due to war damage. All(?) of the electricity generation was from burning coal which produced large amounts of ash - a waste product - which was basically dumped or sometimes used inappropriately (in hindsight) for building construction.

Sulphate is the problem round here, caused by the addition of "Red Ash" in the mix. A very large quantity of very low-quality council housing was built after WWII, these have been neglected for many decades by the council and housing association that superseded. The quality of work undertaken by the people who rewired them / fitted gas central heating / fitted UPVC double glazing, on behalf of the council/housing association is hilarious.

There was a time when good money could be made buying the best of these properties (ie the ones without defective prefab reinforced concrete supporting walls in the middle) at auction gutting them completely and letting to tenants. There is no shortage of housing where I live but most of it is exceedingly low quality. However rents have stayed the same for a decade but property prices have doubled so buying to let is no longer viable.