1156 posts
Quality control Question 5
I recently had a new driveway poured and we have encountered a "blistering/popping" problem in several areas of the concrete. An engineer from the cement delivery plant says the defects are caused by lime chips that contaminated several tanker loads of cement mix. My questions are: Should I be concerned with structural integrity of the concrete because of the defective spots caused by the lime chips? Is there any way of knowing how long the blistering/popping may continue?
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1156 posts
Re: Quality control
This problem is a bit out of the ordinary nowadays, although it seems to have concerned people a century or so ago, when cement manufacturing methods were less sophisticated and coarse particles of various materials could end up in the product. The damage is caused by the contaminant material reacting with water and forming a hydration product which is of larger volume than the starting material and therefore needs to create space in which to expand.
Finely-ground high-calcium lime would not cause any problem, as it reacts rapidly with water and expands while the concrete is still plastic. Dolomitic lime (containing significant amounts of Magnesium) is well known to give problems because it reacts slowly (weeks rather than hours), and its acceptable level is therefore limited in national standards for cement. From what you describe, it appears that you have contamination from lime which is both coarse and relatively unreactive (perhaps overburnt, dense and highly crystalline calcium lime?). You should ask the supplier to confirm that the lime is not dolomitic. If it turns out that it is dolomitic, then you can expect defects to continue to develop during the coming months and you should insist that they keep the matter open and monitor developments.
For the more likely case of high-calcium lime, there are two extreme scenarios. (a) If there is not a great deal of it in any region of the concrete and it has now more or less finished reacting, mainly affecting surface appearance, then that will be the end of the matter. You can complain to the supplier about the sub-standard appearance and demand that it is either put right or you are given a rebate. (b) On the other hand, if there is a good deal of this material in at least part of the bulk of the concrete, then its expansion may weaken the structure and shorten its working life - not that a domestic driveway is a very demanding structure in terms of its duty and of strength requirements (not like an airport runway, for example).
You should seek reassurance from the supplier that they know whether scenario (a) or (b) is correct. You can ask if they have test data or experience from which you can be assured that your driveway will be fit for normal use for as long a period of years as you had originally expected. They should be able either to replace or repair the driveway now or - because they are confident about their product - to give you a written promise that they will put it right in the future if it continues to deteriorate, perhaps with a commitment to inspect it with you in (say) a year's time. Alternatively, you might get a rebate on the price due to the substandard quality.
(If you want to know more, you are best enquiring via a concrete information service, rather than through us, as our readership is mostly concerned with making and trading in cement, not concrete.)
1156 posts
Quality control Question 6
There are two kinds of alkalis contained in clinker-soluble and insoluble alkalis. Please explain about the formation processes in the kiln and the effects of two alkalis respectively on clinker quality, especially on setting and compressive strength.
1156 posts
Re: Quality control
Alkalis in clinker will preferentially combine with chloride and sulphate, the alkali chloride and sulphate formed is volatile and leads to the build up of the alkali cycle in the kiln. Alkali sulphates are less volatile than chlorides and some pass out of the kiln in the clinker. These are the source of the soluble alkali in the clinker. They promote the hydration of the cement and lead to higher early strengths. If alkali is present in excess of the sulphates in clinker then it enters into the clinker minerals. These are the "insoluble" alkalis. They do affect the hydration behaviour of the cement as they affect the clinker mineralogy, however effects are not as clear as for the soluble alkalis.