Pasty z cementů hrubozrnných (do 63 µm) vykazují pozvolnější hydratační proces než z cementů jemnozrnných. Tento jev je způsoben menším reakčním povrchem cementu a menším počtem kontaktních ploch mezi zrny. Výsledkem pozvolnější hydratace jsou vyšší mikropórovitost a nižší pevnosti v ohybu a tlaku. Vlastnosti cementů jemnozrnných (pod 20 µm) jsou poznamenány
potřebou vysokých vodních součinitelů pro zpracování na stejnou, normovou, konzistenci vzhledem k cementům hrubozrnnějším. Vysoké vodní součinitele způsobují podstatné zhoršení vlastností mikrostrukturálních i fyzikálně mechanických. Nejpříznivější výsledky v oblasti mikrostruktury i fyzikálně mechanických vlastností poskytují cementy střední granulometrie (do 40 µm), které mají dostatečně velký reakční povrch a počet kontaktních ploch mezi zrny, ale dosud nevyžadují vysokých hodnot vodního součinitele.
The study is aimed at investigating the properties of cement pastes prepared from cements of identical mineralogical composition but with different granulometries. Two series of samples were tested. Series A was derived from Malomerice clinker with an addition of 5 wt. % gypsum, series B from Portland cement PC 400 Malometice. Both the cement and the clinker were classified into samples of set grain size composition, namely 0-10, 0-20, 0-30, 0-40 and 0-63 μm. The granulometry of the samples is given in Table I, and the specific surface areas are listed in Table II. Pastes of standard consistency at various cement to water ratios were prepared from the samples of both series (Table III). Test specimens 20X20X100 mm in size were made from the pastes and placed in a medium of 95-100 % relative humidity. The cement pastes were subjected to microstructural and physicomechanical tests after 1, 3, 7 and 28 days of hydration.
With respect to phase composition, all the test specimens of both series exhibited the same mineralogical composition: C-S-H gel, residual clinker minerals C₃S and β-C₂S, portlandite, ettringite, C₃A.CaSO₄.12H₂O monosulphate and the C₃A.CaCO₃.11H₂O carbonate complex compound. The granulometry affected the quantitative representation of the minerals. The coarser the initial cements, the slower was the hydration process.
The microporosity was investigated over the interval of 15 to 0.015 μm. The maximum amount of micropores was found in pastes of the 0-10 μm cements. The lowest specific volume of micropores was exhibited by pastes of 0-40 μm cements in both series (Table IV, Figs. 1, 2). The prevailingly most favourable bending strength values were provided by specimens of the 0-30 and 0-40 μm cements (Table V). Lower were the bending strengths of specimens of fine-grained cements (0-10 and 0-20 μm), as well as those of coarse-grained cement (0-63 μm).
The highest compressive strengths were those of the 0-30 and 0-40 μm cement specimens (Table VI); the compressive strengths of the 0-10, 0-20 and 0-63 μm specimens were lower. The lower compressive strength of fine-grained cement specimens can be attributed to a high water to cement ratio, the lower compressive strenghts of the coarse-grained cement ones to slower hydration resulting from a smaller reaction area of the cement particles and a smaller number of points of contact between the grains. The properties of cement stone can be improved by changing the granulometry of cement in favour of fine particles. However, this dependence holds up to a certain boundary of cement fineness, 0-30 μm according to the results obtained. More finely ground cements appear to be ineffective as their application is influenced by some subsidiary phenomena (larger amounts of mix water, use of plasticizers, vibration, etc.) which are either demanding with respect to energy consumption or may even impair the properties of hardened cement stone. |
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