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.
Applicability of the Hedvall effect to study the reactivity of calcium silicate hydrates
The Hedvall effect was used to investigate the reactivity of calcium silicate hydrates (C–S–H) with variable C/S ratios. The solid state reaction between silver nitrate and various C–S–H preparations occurring during the crystalline transition of silver nitrate was evaluated. Thermal methods as well as microscopic and X-ray diffraction analyses were employed to study the nature of the reaction products and estimate the chemical reactivity of the C–S–H systems. The overall reactivity included cationic and anionic species; namely both calcium and silica entered into the reaction. Silver silicate was identified as one of the reaction products.
Stereoselective interaction between the R enantiomer of warfarin and cimetidine.
The stereoselectivity of the pharmacokinetic interaction between warfarin and cimetidine was investigated in eight healthy volunteers. The warfarin enantiomers were given separately as single doses (15 mg) alone and during chronic administration of cimetidine (1 g day-1). Cimetidine did not interact with S warfarin but there was an interaction with the R enantiomer of warfarin. Cimetidine caused a significant increase in the mean plasma half-life of R warfarin (from 47.8 h to 57.8 h) and a significant decrease in its mean plasma clearance (from 2.3 to 1.7 ml h-1 kg-1) (P less than 0.02). Administration of a pharmacological dose of vitamin K1 together with the enantiomers of warfarin was necessary clinically and resulted in elevation of vitamin K1 2,3-epoxide concentrations, which were similar in each case.