The influence of wood aqueous extractives on the hydration kinetics of plaster. P. Boustingorry, P. Grosseau, R. Guyonnet, B. Guilhot. Cement and Concrete Research 35 (2005) 2081 – 2086
For several decades, many investigations are realized on the reinforcement of mineral binders (cement, concrete or plaster) in particular on the replacement of usual mineral reinforcing agents (glass or asbestos fibres) by organic agents such as sisal, Kraft pulp, or cellulose fibres.
Wood extracts were prepared with various water / wood ratios in order to observe the influence of concentration. After ground wood (ten grams) has been dipped in ultrapure water during 1h under magnetic stirring, the mixture was filtered on a Büchner funnel and refrigerated until the experiments took place. A C80 Calvet was used calorimeter with an electrical accessory to stir the mixture between plaster and wood extracts. The mixture (aqueous extracts / plaster ratio 1:1) was stirred for 2 min after putting them in contact. The experiment was performed at 25°C.
Two exothermic phenomena were observed. The first corresponds to the dissolution of plaster in the liquid immediately after contact and stirring. Then, the curve return to baseline: during a certain time no reaction is observed. Gypsum nucleation is suspected to take place at this stage of reaction.
The second exothermic phenomenon represents the hydration reaction itself: the increase of heat flow shows, at the same time, gypsum crystal growth and plaster dissolution. After the total depletion of plaster, the heat flow decreases, only further crystal growth occurs during the last part of the curve. Therefore, the results show clearly that wood have a delaying effect on the hydration of hemihydrate.
We can also note that the peak time, which corresponds to the total depletion of plaster, increases with the wood/water ratio: the concentration of extracts influences the time of hydration.