J. Rivera

Influence of the microsilica state on pozzolanic reaction rate

Silica fume is generally supplied as an ultrafine powder in its original state, densified or in suspension. The difference in states should not influence the pozzolanic behaviour of this by-product in a cementitious matrix. In the present work, experimental results relating to pozzolanic reaction have been obtained by different methods: a chemical method, in which lime fixed by pozzolanic material through time is quantified, and a physicochemical method based on the Langavant calorimeter. By the second method, information regarding silica fume is obtained from the value of heat of hydration released by a mortar containing this by-product. These results show that the silica fume in its densified state considerably decreases its chemical reactivity with calcium hydroxide (pozzolanic activity).

Microstructure of Concrete Made with Blends of Fly Ash Cement and Waste Clay by Backscattered Electron Images

Industrial waste clay materials, which are good pozzolans, can be used in place of cement in place of cement in the manufacture of mortars and concretes. During the manufacturing process, which involves dehydration followed by firing at controlled temperatures ranging from 700 degrees Celsius to 1000 degrees Celsius , the clay minerals found in high proportions in the natural materials used to make bricks and similar products acquire pozzolanic properties. This study examines the microstructure and morphology of the industrial concrete made with standard Portland cement containing fly ash partially substituted with waste clay material, by backscattered electron images (BSE) and mercury porosimetry (MP) techniques. The microstructure of concrete test samples is not altered by the inclusion of clay discards and the pore size did not vary too, the only difference found was a slight decline in the proportion of larger diameter pores.

The Effect of Temperature on the Formation of Hydrogarnet in Matrices Made with Metakaolin/Lime Mixtures

The effect of temperature on the acceleration of the pozzolanic reaction rate is well known. In the case of the pozzolanic reaction between metakaolin (MK) and calcium hydroxide, the researchers have been focused on the hydrated phases and mainly on their stability, taking into account the possible transformation from metastable phases to cubic phase with the corresponding risk on durability. This paper reports a study to evaluate the influence of temperature on the formation of hydrogarnet in metakaolin/lime matrices subjected to 60 degrees C curing temperature. The MK and lime were mixed in a ratio of 1:1 by weight, stored and cured at 60 degrees C and up to 123 days of hydration time. The kinetics of the pozzolanic reaction, sequence of appearance of different reaction products, quantities and crystallinity of the hydrogarnet as well as other hydrated phases, has been investigated using thermal (TG and DTA) and diffraction (XRD) analysis. The results showed that under the conditions tested here the hydrogarnet is formed very quickly in the first hours of curing (after 30 hours of hydration). This phase coexists at the same time with gehlenite and C4AH13. The formation and amount of the hydrogarnet was closely related to the calcium hydroxide content in mixtures. There was not any evidence that the hydrogarnet was formed from a transformation reaction, but it was a result of the reaction between MK and lime.