|
Hydraulic limes, "Natural"
cements and "Artificial" cements all relied upon their
Belite content for strength development. Belite develops strength slowly.
They contained no Alite (which is responsible for early strength in
modern cements) because they were burned at temperatures below
1250 °C.
Thus these suffered
from two major disadvantages:
1. Fast-setting : this gave
insufficient time for placement
2. Low
early strengths: this required a delay of many weeks before formwork
could be removed.
The first cement to
consistently contain Alite was that made by Joseph Aspdin's son William Aspdin
in the early 1840s. This was what we call today "Modern
Portland Cement".
Because of the air of
mystery with which William Aspdin surrounded his product, others (e.g. Vicat
and I C Johnson) have claimed precedence in this invention, but recent
analysis of both his concrete and raw cement have shown that William Aspdin's
product made at Northfleet, Kent was a true Alite-based cement.
However, Aspdin's
methods were "rule-of-thumb": Vicat is responsible for establishing
the chemical basis of these cements, and Johnson established the importance
of sintering the mix in the kiln.
However manufacturing
cost of William Aspdin's product was considerably high & as such, his
product was not received very well by the cement manufacturers of that time.
But the product set reasonably slowly and developed strength quickly.
Slowly a market opened
up for its use in concrete. The use of concrete in construction grew rapidly
from 1850 onwards, and was soon the dominant use for cements. Thus Portland
cement began its predominant role.
|
