Hedvall Effect in Cement Chemistry
THE increased reactivity of a solid during and as a consequence of a crystalline transformation is commonly known as the Hedvall effect1–3. In a wider sense this effect may include increased reactivity near a transition point. The typical data quoted in support of the Hedvall effect refer to reactions in the binary systems containing silver salts with BaO, SrO and CaO yielding the reciprocal pairs4–7.
Hedvall Effect in Cement Chemistry
Ramachandran and Sereda1 have described a new method for studying the difference in reactivity of two of the major cement minerals and suggested a reason for this difference. We have found, however, that by slightly different conditioning of the starting materials we obtain very different results. The increased reactivity of a solid near a crystal transformation is known as the Hedvall effect. Hedvall2 showed that calcium oxide would react with silver nitrate at a temperature considerably lower than the “characteristic reaction temperature”. Ramachandran and Sereda have made use of the fact that tricalcium silicate (3CaO.SiO2) and β dicalcium silicate (β 2CaO.SiO2) which make up approximately 80% of cement clinker will react with silver nitrate.
Bodies Politic : Disease , Death and Doctors in Britain , 1650-1900
In spite of its intellectual, literary and comic brilliance, this book contains a dark and disturbing, but revealing, message. In some ways, my melancholy reading of Bodies Politic has inevitably been shaped by Roy's recent untimely death. Roy Porter was without doubt the finest social historian of medicine this country, or indeed the world, has produced. Expansive, humorous, intelligent, and incisive, his broad range of historical works has excited and educated several generations of students of history over the past thirty years. His death leaves an unfillable void within the global history of medicine. At the same time, however, this book itself, completed well before his death, is haunted by elements of despair and uncertainty that should make us carefully reconsider not only the purpose and direction of our professional inquiries but also our own mortality.