Pore Structure and Moisture Properties of Cement-Based Systemsfrom Water Vapour Sorption Isotherms
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The results obtained were completed by those of other methods. A particular aim was to highlight similarities and differences between ordinary and V.H.P. mixes. EXPERIMENTALS Materials Three cement pastes and three concretes were tested. The six mixes were divided into two series, depending on the cement used: series 1 (with French OPC from La Frette) and series 2 (with French OPC from Le Teil). The cement from Le Teil was richer in C3S (57.28 % vs 39.29 %), and had a lower C3A content (3.03 % vs 9.73 %) making it possible to have a low watercement ratio, than the OPC from La Frette. The mix compositions are given in Table I. The cement pastes "C" and "CO" and the concretes "B" and "BO" are ordinary materials. The cement paste "CH" and the concrete "BH" are V.H.P. materials. The silica fume came from
Laudun (France) and the superplasticizer was a formaldehyde-naphtalene sulfonate copolymer with the trade name Lomar D. The dose of superplasticizer was 1.8 % by mass. At the age of 28 days, the V.H.P. concrete "BH" had a measured compressive strength of 115 MPa, which should be compared to the 49 MPa obtained for the ordinary concrete "BO". sition of mixes
TABLE I. Co
Aggregate/C
Series I 2 2
Mix C CO CH
W/C 0.45 0.34 0.19
SF/C
0.10
-
I 2 2
B BO BH
0.45 0.48 0.26
-
4.56 5.48 4.55
-
-
0.10
Water Vapour Desorption and Adsorption Experiments Water vapour desorption and adsorption experiments were performed at T = 23 'C on the hardened materials. There are not many such measurements on cement-based materials. Some of the first results on hcp were published by Feldman in 1968 [2]. The experiments consisted here in putting specimens into sealed desiccators, where the relative humidity (R.H.) was controlled by satured salt solutions, and submitting them to a first step-bystep water desorption followed by a step-by-step adsorption and possibly a second desorption. Each step lasted until hygrometrical equilibrium was reached and the corresponding water content (ratio of the mass of water contained in the specimen to the mass of the "dry" specimen) was determined by gravimetry. The specimens were discs about 3 millimeters thick with a diameter of about 10 centimeters for hardened concretes, and crushed ones (0.8 < d < 1 mm) for hcp. The slices were wet sawed from cylinders (after curing for at least five months without water exchange, with the samples wrapped in adhesive aluminium paper) and stored at 100 % R.H. before testing. Given the long duration of both cement hydration processes and water sorption experiments, the specimens were a year and a half old at the beginning of these tests, so hydration of the cement during the test can be assumed negligible (this was verified by measuring the degree of hydration of the cement in the hardened materials at different ages [1]). The experiments began with desorption to study the case of real concrete structures, which undergo drying from their virgin state. Moreover, starting sorption tests with the adsorptioncurve would have entailed first subjecting the specime
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