Cement Chemistry & Durability
At the recent Concrete Society Conference in Taupo, Phil Bamforth was the keynote speaker.
In his presentation, “Concrete Durability by Design: Limitations of the Current Prescriptive Approach and Alternative Methods”,he made reference to changes in cement chemistry over time.
I have been able to locate a copy of his referenced paper(*).It refers to a survey of cements from 15 sources found in literature. It was found that in 12 of these cases the effective diffusion coefficient reduced only slightly with time.
However, in 3 cases of concrete from the 1930’s, this was not so. One concrete was produced in the UK, while the others were from Japan. The principal difference was given as a higher C2S/C3S ratio in the older cements. This gave rise to lower early strengths but higher long term (post 28 day) strengths. Further, these concretes had higher cement contents and lower w/c ratios.
As you will be aware, in 1997, Golden Bay Cement introduced a cement improver, CBA 1104, to its cement milling process. This acts to enhance 28 day strengths, in both mortar and concrete. As a result of this, C3S content has decreased and the total silicate content has remained largely static, while maintaining strength performance. Hence, the C2S/C3S ratio has moved higher.
This should improve the durability of the concrete, given equal mix design parameters.
The following graphs show the change in C3S and C2S/C3S ratio over the past 5 years. In both cases, the trendline represents the running average of 200 data points.
I hope that this has been helpful in comparing the New Zealand situation to that of the UK.
While the chemistry of modern GBC cement has undoubtedly differed from that of the past, the more recent changes we have been able to achieve through the use of cement technology have served to improve the performance of the cement.
Please do not hesitate to contact me if I can be of any further assistance.
Raki Harding
Technical Manager
Golden Bay Cement
PO Box 1143, Whangarei, New Zealand
Ph: +64 9 432 9705
Fax: +64 9 432 9730
Mob: +64 27 473 6084
raki.harding@goldenbay.co.nz