Investigations on Hydraulically Reactive Glasses in the system CaO-AL 2 O 3 -SiO 2

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INVESTIGATIONS ON HYDRAULICALLY REACTIVE GLASSES IN THE SYSTEM CaO-AL20s-SlOI C. HOMMERTGEN AND I. ODLER Technical University Clausthal, Institute of Nonmetallic Materials, ZehntnerstraBe 2A, D-3392 Clausthal-Zellerfeld, Federal Republic of Germany ABSTRACT A series of glasses in the system CaO-A12O3-SiO2 was synthetized and studied with respect to their hydraulic reactivity. Generally, the initial reactivity is greater but the developed maximum strength is lower in glasses yielding a hydrogarnet phase, as compared to those yielding gehlenite hydrate. Another factor that determines initial reactivity appears to be the cooling rate of the glass melt. After longer hydration times the strength of glass cement pastes starts to decline, apparently due to a recrystallization of the hydrates formed initially.

INTRODUCTION It has been reported that same glasses in the system CaO-AI203-SiO2, if ground to fine powders, exhibit hydraulic properties yielding hydrogarnet (C3ASxHe-2x) and/or gehlenite hydrate (stratlingite, C2ASHa) as hydration products [1, 2, 3, 4]. Within our work we studied systematically the hydraulicity of glasses of this type. The main aim of the work was to explore how the composition of the glass affects the kinetics of the hydration process and the structure and properties of the hardened paste formed. The obtained results are summarized in this paper.

EXPERIMENTAL Blends of the pure oxides were fired at 1630 t 10"C for two hours, quenched by pouring into a large volume of water and ground to a fineness of 500 ± 20 m2/kg (Blaine). XRD showed no crystalline phases in any of the preparations. Pastes were prepared at water solid ratios of 0.3, 0.4 and 0.5, cast in 10 x 10 x 60 mm molds and after 24 hours curing demolded and stored at 20 t 2"C in either water or at 100 % RH. Compressive and flexural strengths were determined after different hydration times. Adiabatic calorimetry was used to measure the heat of hydration on pastes (w/c = 0,7). Free water content was determined from the difference between the loss on ignition of the original paste and D-dried paste while porosity of the paste was estimated from its wet bulk density and the free water content. Pore size distribution was determined on D-dried specimens by mercury intrusion porosimetry. Quantitative X-ray diffraction was used to determine the phase composition. A gehlenite hydrate standard was prepared [5] by allowing a finely powdered glass (45 % CaO, 35 % A12O3, 20 % SiO2) to hydrate for 6 months and a hydrogarnet standard was prepared by hydration of C3A at 30"C for 30 days.

Mat. Res. Soc. Symp. Proc. Vol. 245. ©1992 Materials Research Society


RESULTS Series A In the first, preliminary, experimental series, designated series A, eight glasses of variable composition were studied. Table I shows the strength values found on pastes made witn a water/cement ratio of 0,40 and hydrated for 1, 3 and 28 days. The table also indicates the amounts of combined water found after D-drying and the -phases formed in the hydration process. In additi

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