A new-design conduction microcalorimeter is described, which has been used to measure the heat of cement hydration evolved
in the initial period of hydration. The calorimeter is 30 cm3 in volume; the heat loss coefficient is 27.2700.015 W V–1, the time constant is 300 s.
Investigations of physico-chemical properties
of three kinds of fly ash and their influence on cement hydration were performed
in this work. Thermal analysis, microcalorimetry, infrared absorption and
others were used. It was confirmed that the kind of coal and combustion conditions
essentially influence physico-chemical properties of fly ash and in consequence
influence cement hydration. Investigated fly ashes show in cement system so-called
pozzolanic activity. Fly ash from combustion of brown coal in fluidized furnace
revealed better activity compared to other investigated ones. This work is
an introduction to more extensive investigation of fly ash activation.
This paper presents a method to study cement hydration at ambient temperatures by using a micro processed non-conventional
differential thermal analysis (DTA) system, which was used to evaluate the solidification/stabilization process of tannery
wastes produced in the leather industry. The DTA curves of pastes composed by slag cement, Wyoming bentonite and waste are
obtained in real time and used to analyze the heat effects of the reactions during the first 24 h of hydration. By applying
a deconvolution method to separate the overlapped DTA peaks, the energy released in the several hydration stages may be estimated
and consequently, the effects of each component on the solidification process. The highest separated DTA peak occurs during
the several early stages of cement hydration and is due mainly to tricalcium silicate hydration. Very good correlation shows
that the greater is the waste content in the paste composition, the higher is its effect on the rates of reactions occurring
during the induction (dormant) period of cement hydration. The presence of bentonite used as a solidification additive in
the stabilization process has a similar but less dramatic effect on the dormant period.
technique seeks the immobilization of hazardous wastes, using the properties of cementhydration [ 5 ]. The mechanism used for S/E and the evaluation of its effectiveness has been widely studied and discussed [ 6 , 7 ].
The cementhydration process
The low temperature
of decomposition of some calcium carbonates and the bending of the TG curves
of hydrated cement between 500 and 800°C suggested the presence of some
complex compound(s), which needed complementary investigation (XRD, TG). Stepwise
transformation of portlandite (and/or lime) into calcium carbonate, with intermediate
steps of calcium carbonate hydroxide hydrates (CCH-1 to CCH-5), was indicated
by the previous study of two OPC.
This was checked here on four
cements ground for tg=15,
20, 25 and 30 min and hydrated either in water vapour, successively at RH=1.0,
0.95 and 0.5 for 2 weeks each (WR1, WR2 and WR3, respectively) or as mortars
in liquid water (1m), followed by WR as above. The d spacing of portlandite
was confirmed to vary: here between the lowest and the highest standard values.
The diffractograms of n=32 different samples
were analyzed for presence of standard CCH peaks, generally slightly displaced.
These were: CCH-1 [Ca3(CO3)2(OH)2]: N=11 peaks, of three different d[hkl] spacings, CCH-2 [Ca6(CO2.65)2(OH657)7(H2O)2]: N=10 for two d[hkl], CCH-3 [Ca3(CO3)2(OH)2·1.5H2O]: N=14 for five d[hkl], CCH-4,
ikaite [CaCO3(H2O)6]: N=13 for six d[hkl], CCH-5[CaCO3(H2O)]: N=15 for five d[hkl]. Thus the most probable is the presence of the
last three. The stepwise transformation of Ca(OH)2
into CaCO3 was confirmed:
portlandite (varying d)→CCH-1→CCH-2→CCH-3→CCH-4→CCH-5→CaCO3
The content of CCH was the highest at tgr=15
min, decreasing down to tgr=25
min and increasing slightly at 30 min, as inferred from the number of the
peaks observed. After cement powder hydration at RH=1.0 (WR1) peak number
increased gradually from CCH-1 to CCH-5, whereas in the hydrated mortar (1m)
the peak number decreased from CCH-1 to CCH-5, indicating the respective progress
of the carbonation reaction.
In the present work, a Portland cement blended with calcium carbonate is being used to study the solidification/stabilization
(S/S) of a Brazilian tanning waste arising from leather production. Chromium is the element of greatest concern in this waste,
but the waste also contains a residual organic material. Using thermogravimetry (TG) and derivative thermogravimetry (DTG)
to identify and quantify the main hydrated phases present in the pastes, this paper presents a comparative study between the
effects of Wyoming and Organophilic bentonites (B and OB) on cement hydration. Samples containing combinations of cement, B, OB and waste have been subjected to thermal analysis after different setting times during the first 28 days of the waste S/S
process. Both bentonites affect the cement hydration, with no significant differences in hydration degree after 1 week. This
work shows further examples of the great utility of thermal analysis techniques in the study of very complex systems containing
both crystalline and amorphous mineral materials as well as organics.
used in fluidized catalytic cracking (FCC) units of refineries after several
recovery cycles in regeneration units, reduces its activity and it is partially
substituted by new catalyst in the process. As it has a high silicon and aluminum
oxides content, the pozzolanic properties of a Brazilian FCC spent residual
catalyst, used in different substitution degrees to cement, were evaluated
by three thermal analysis techniques during the early stages of hydration
of a type II Portland cement. NCDTA curves show in real time that the residual
catalyst, accelerates the stages of cement hydration. TG and DSC curves of
respective pastes after 24 h of hydration evidence the pozzolanic activity
of the waste, respectively, by the lower water mass loss during the dehydroxylation
of the residual calcium hydroxide and by the lower dehydroxylation endothermal
effect. Within the analyzed period, the higher is the cement substitution
degree, the higher is the pozzolanic activity of the residual catalyst.
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.
Influence of the additives and admixtures on portland cementhydration
The main components of OPC are tricalciumsilicate (3CaO·SiO 2 , C 3 S, alite), dicalciumsilicate (2CaO·SiO 2 , C 2 S belite), tricalcium