Search Results
Abstract
The hydration products of hydrothermally cured OPC-quartz blends incorporating clay-brick waste were characterised using simultaneous DTA-TG, XRD and SEM. The exotherm above 800°C on the DTA curve, due to the formation of β-wollastonite (β-CS) was related to the compressive strength. The area and/or height of the exotherm increased with compressive strength and was due primarily to the presence of calcium silicate hydrate phases including 1.1 nm tobermorite. Supporting evidence is provided by XRD and SEM.
Summary The strength developments of hydrothermally cured blends comprised of ordinary Portland cement (OPC) and clay-brick waste (CB) and treated at 180°C under saturated steam for 6 h were studied. Two types of CB, of different chemical and mineralogical make-up, were used to make OPC-CB blends where 10 to 80 mass% CB replaced OPC. The nature of the hydration products formed after autoclaving were subsequently investigated by using a combination of DTA, DTG, TG, X-ray diffraction, and wet chemical techniques. The modes of the hydrothermal reactions between OPC and the two types of CB, as well as the nature of the main binder belonging to the tobermorite family, are discussed and related to the development of the strength of the autoclaved articles.
Abstract
The utilisation of fired clay-brick waste is of interest in the manufacture of building products due to both socio-economic and technological reasons. Autoclaving is an established process for manufacturing calcium silicate-based building products that affords rapid strength development. Clay-brick waste represents a source of alternative silica, which is more reactive than conventionally used silica and also contains alumina. This paper presents data on the effect of lowering the autoclaving temperature from commonly practised 180 to 170°C on OPC-quartz blends containing clay-brick fines as direct replacement of quartz at 4.3, 8.6 and 12.9 mass percentages. The hydration products of autoclaved OPC-quartz blends with clay-brick fines were characterised using simultaneous DTA-TG in combination with other methods.
Abstract
Simultaneous DTA-TG is an excellent technique for evaluating phases formed in hydrothermally treated CaO-SiO2-H2O and CaO-Al2O3-SiO2-H2O systems. Thermal analysis in combination with XRD and SEM, revealed that in the CaO-Al2O3-SiO2-H2O system the amount of hydrogarnet formed was the largest when gibbsite was used as the Al source, smallest for kaolin and intermediate for metakaolin. The endotherm peak temperature of the hydrogarnet dehydration endotherm was affected by the amount of hydrogarnet and the Si content of hydrogarnet. The thermal stability and structural order of 11 Å tobermorite were reduced with the incorporation of Al and, as a result, 11 Å tobermorite transformed into 9.3 Å tobermorite at lower temperatures while the transformation of the latter into beta-wollastonite required more energy. There exists a direct relationship between the 9.3 Å tobermorite and beta-wollastonite formation temperatures. Solid-state 29Si and 27Al MAS NMR data support these findings.
Abstract
The cement industry is one which most emits polluting gases to the environment, due to the calcium carbonate calcination, as well as to the burning of fossil fuels during the manufacturing process. Metakaolin (MK), in partial substitution to cement in its applications, is having a special worldwide growing role, for the technological increment due to its pozzolanic activity and mainly to the reduction of those emissions. In the present paper, the effect of pozzolanic activity of metakaolin was analyzed by thermal analysis in pastes and mortars of type II Portland cement in the first three days of the hydration, during which, relevant initial stages of the hydration process occur. By non-conventional differential thermal analysis (NCDTA), paste and mortar samples containing 0, 10, 20, 30 and 40% of metakaolin in cement mass substitution and using a 0.5 water/(total solids) mass ratio, were evaluated. The NCDTA curves, after normalization on cement mass basis and considering the heat capacity of each reactant, indicate that the pozzolanic activity behavior of metakaolin is different in pastes and mortars. Through the deconvolution of the normalized NCDTA curve peaks, it can be seen that ettringuite formation increases as cement substitution degree (CSD) increases, in both cases. Tobermorite formation is more enhanced in mortars than in pastes by MK, with a maximum formation at 30% of CSD. In the pastes, tobermorite formation increases as CSD increases but it is practically the same at 30 and 40% of CSD.
Abstract
Information about the sorption of radionuclides on natural materials used for cementation of liquid radioactive wastes (RAW) is important for predictions of migration rates of radionuclides in the products of fixation. Cementation process for conditioning liquid RAW uses, besides cement, materials which improve quality of products. In Czechoslovakia technology among these materials are clinoptilolite tuffit, mordenite tuff, tobermorite and fly ash. Liquid RAW issued from nuclear power plants contains the principal radionuclides60Co,134Cs and137Cs, therefore, a sorption study of these radionuclides was carried out. pH of liquid RAW was 12.5–13.7 and salt content ca. 200 g·dm–3. Results of sorption are given by distribution coefficients. The best results for cesium in those pH region and salt content had mordenite tuff DCs=100 dm3·kg–1 and the worst results had fly ash DCs=4.1 dm3·kg–1. The best results for cobalt in those solutions had tobermorite DCo=38 dm3·kg–1 and the worst results had fly ash DCo=6.9 dm3·kg–1.
Abstract
Thermogravimetry (TG) and derivative thermogravimetry (DTG) have been used by the authors as very effective tools to study hydration steps of cements used for solidification/stabilization of tanning wastes. The present paper presents a method which was applied to separate the peaks shown by DTG curves of type II Portland cement pastes, analyzed at different times during the first 4 weeks of setting. Through a specific software a more detailed study of the evolution of the cement hydration may be done, which allows the measurement of the amount of hydrated water present in tobermorite gel as well as in ettringite, which are the main phases formed from the original components of the cement. The number of moles of water present in the ettringite phase calculated by the method is in very good agreement with the values found in the literature, validating the method to calculate the same parameter in tobermorite gel. In the latter case the water content decreases significantly during the first day of hydration, then remains at a constant value over the rest of the analyzed period.
Abstract
The stabilization/solidification of tannery waste containing chromium was studied as an option for its treatment and final disposal, by using a Portland cement type II and two different commercial bentonites (sodium and organophilic) as additives. Different compositions were evaluated by compressive strength analysis, porosity measurement, leaching tests and thermal analysis. The effect on the compressive strength is directly related to the resulting effect of the components present in the original paste on the hydration degree of the cement, which can be evaluated by thermogravimetric analysis from the dehydration steps of tobermorite and ettringite phases of the pastes. The results show that this process is suitable for the treatment of the tanning waste and that the best conditions of stabilization are obtained when both additives are used.
Early stages hydration of high initial strength Portland cement
Part I. thermogravimetric analysis on calcined mass basis
decarbonation. The first two steps include the loss of free water from the pores and from the water released from any C–S–H structured phase including tobermorite (Capillary, interstitial and physically combined water) [ 14 , 15 ]. When thermal analysis
) hydration, forming tobermorite (3CaO·2SiO 2 · x H 2 O) and calcium hydroxide (Ca(OH) 2 ) [ 26 , 36 ]. This stage, called acceleration period, is characterized by an increase in the hydration rate until a maximum value of sample and reference temperature
