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1.
Calorimetry in the studies of cement hydration 总被引:1,自引:0,他引:1
Marek Gawlicki Wiesława Nocuń-Wczelik Łukasz Bąk 《Journal of Thermal Analysis and Calorimetry》2010,100(2):571-576
Calorimetry was applied to an investigation of the early hydration of Portland cement (PC)–calcium aluminate cement (CAC)
pastes. The heat evolution measurements were related to the strength tests on small cylindrical samples and standard mortar
bars. Different heat-evolution profiles were observed, depending on the calcium aluminate cement/Portland cement ratio. The
significant modification of Portland cement heat evolution profile within a few hours after mixing with water was observed
generally in pastes containing up to 25% CAC. On the other hand the CAC hydration acceleration effect was also obtained with
the 10% and 20% addition of Portland cement. As one could expect the compressive and flexural strength development was more
or less changed—reduced in the presence of larger amount of the second component in the mixture, presumably because of the
internal cracks generated by expansive calcium sulfoaluminate formation. 相似文献
2.
M. S. Amin S. M. A. El-Gamal F. S. Hashem 《Journal of Thermal Analysis and Calorimetry》2013,112(3):1253-1259
In this investigation the effect of addition of magnetite nanoparticles on the hydration characteristics of both ordinary Portland cement (OPC) and high slag cement (HSC) pastes was studied. The cement pastes were prepared using a water/solid (W/S) mass ratio of 0.3 with addition of 0.05, 0.1, and 0.3 % of magnetic fluid Fe3O4 nanoparticles by mass of cement. An aqueous stable magnetic fluid containing Fe3O4 nanoparticles, with a mean diameter in the range of super-paramagnetism, was prepared via co-precipitation method from ferrous and ferric solutions. The admixed magnetite-cement pastes were examined for compressive strength, chemically combined water content, X-ray diffraction analysis, and differential scanning calorimetry. The results of compressive strength revealed that the hardened pastes made from OPC and HSC admixed with different amounts of magnetic fluid have higher compressive strength values than those of the neat cement OPC and HSC cement pastes at almost all ages of hydration. The results of chemically combined water content for the admixed cement pastes showed almost the same general trend and nearly comparable values as those of the neat cement pastes. From the XRD diffractograms obtained for the neat OPC and HSC cement pastes, the main hydration products identified are calcium silicate hydrates, portlandite, and calcium sulfoaluminate hydrates. Addition of magnetic fluid nanoparticles to both of OPC and HSC did not affect the main hydration products of the neat OPC or HSC cement in addition to one main basic difference, namely, the formation of calcium iron hydroxide silicate as a new hydration product with a reasonable hydraulic character. 相似文献
3.
Portland cement reacts with water to form an amorphous paste through a chemical reaction called hydration. In concrete the formation of pastes causes the mix to harden and gain strength to form a rock-like mass. Within this process lies the key to a remarkable peculiarity of concrete: it is plastic and soft when newly mixed, strong and durable when hardened. These qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks and superhighways, houses, and dams. The character of the concrete is determined by the quality of the paste. Creep and shrinkage of concrete specimens occur during the loss and gain of water from cement paste. To better understand the role of water in mature concrete, a series of quasielastic neutron scattering (QENS) experiments were carried out on cement pastes with water/cement ratio varying between 0.32 and 0.6. The samples were cured for about 28 days in sealed containers so that the initial water content would not change. These experiments were carried out with an actual sample of Portland cement rather than with the components of cement studied by other workers. The QENS spectra differentiated between three different water interactions: water that was chemically bound into the cement paste, the physically bound or "glassy water" that interacted with the surface of the gel pores in the paste, and unbound water molecules that are confined within the larger capillary pores of cement paste. The dynamics of the "glassy" and "unboud" water in an extended time scale, from a hundred picoseconds to a few nanoseconds, could be clearly differentiated from the data. While the observed motions on the picosecond time scale are mainly stochastic reorientations of the water molecules, the dynamics observed on the nanosecond range can be attributed to long-range diffusion. Diffusive motion was characterized by diffusion constants in the range of (0.6-2) 10(-9) m(2)/s, with significant reduction compared to the rate of diffusion for bulk water. This reduction of the water diffusion is discussed in terms of the interaction of the water with the calcium silicate gel and the ions present in the pore water. 相似文献
4.
Alex Neves Junior Romildo Dias Toledo Filho Eduardo de Moraes Rego Fairbairn Jo Dweck 《Journal of Thermal Analysis and Calorimetry》2012,108(2):725-731
Thermogravimetry (TG) and derivative thermogravimetry (DTG) were used to analyze the early stages of hydration of a high-initial
strength and sulphate resistant Portland cement (HS SR PC) within the first 24 h of setting. The water/cement (W/C) mass ratios
used to prepare the pastes were 0.35, 0.45, and 0.55. The hydration behavior of the pastes was analyzed through TG and DTG
curves obtained after different hydration times on calcined cement mass basis to have a same composition basis to compare
the data. The influence of the W/C ratio on the kinetics of the hydration process was done through the quantitative analysis
of the combined water of the main hydration products formed in each case. TG and DTG curves data calculated on calcined mass
basis of all the results were converted to initial cement mass basis to have an easier way to analyze the influence of the
W/C ratio on the free and combined water of the different main hydrated phases. The gypsum content of the pastes was totally
consumed in 8 h for all cases. A significant part of the hydration process occurs within the first 14 h of setting and at
24 h the highest hydration degree, indicated by the respective content of formed calcium hydroxide, occurs in the case of
the highest initial water content of the paste. 相似文献
5.
W. Nocuń-Wczelik A. Bochenek M. Migda? 《Journal of Thermal Analysis and Calorimetry》2012,109(2):529-535
In this study, the calorimeter was applied to follow the hydration of special cement mixtures exhibiting expansion or shrinkage compensation. The shrinkage-less and expansive binders were produced by mixing of Portland cement with an expansive additive produced by sintering and composed of calcium sulfoaluminate (yeelimite), calcium sulfate (anhydrite) and lime. The studies were focused on the synthesis of this aluminate??sulfate??lime additive (temperature of burning process as a parameter controlling the relative activity of components) from the materials being the by products and subsequently on the mixture proportions to ensure the hydration process resulting in non-shrinkage or expansion effect. In the experiments the proportions of expansive mixture and cementitious materials were variable. The investigations with aim to find the relationship between the volume changes and composition of initial mixtures in cement pastes and mortars (with sand) were also carried out. The phase composition and microstructure of products were characterized. The expansive additive in the environment of hydrating cement transforms into ettringite and gives an increase of volume when the plastic material transforms to the more rigid matter but before the ultimate hardening takes place. Proper, moderate setting and hardening in strongly modified mixtures is achieved when the calorimetric curve corresponding to the heat evolution on hydration is analogous to that for the basic Portland cement. The rate of heat evolution data are well compatible with the other results related to the other methods of hydration kinetics assessment (e.g. chemical shrinkage) and discussed in terms of the phase composition of hydration products. 相似文献
6.
Calorimetry was applied to follow the hydration of special cement mixtures exhibiting expansion or shrinkage compensation.
The standard, common cements show generally less or more visible shrinkage on setting and hardening but mixed with and expansive
agent, usually of aluminate and sulfate nature, they can exhibit the increase of volume. The calcium aluminate cement CAC
40 was ground together with special sulfate–lime sinter to produce an expansive additive to Portland cement (CEM I 42.5R).
The expansive additive in the environment of hydrating cement transforms into ettringite at “right time” to give expansion
before the final setting and hardening takes place. In the experiments the proportions of components of expansive mixture
and basic cement were variable. The rate of hydration versus time for common cements is commonly known and reflects the moderate
setting and early hardening during the first days after mixing with water (two peaks and the induction period between them).
The aim of measurements presented in this study was to show the course of heat evolution curve and the heat evolved values,
equivalent to the acceleration/retardation of hydration, in case of the paste with the expansive mixture, as well as the pastes
produced from Portland cement and the components of expansive additives added in variable proportions. It was possible to
see how the calorimetric curve and consequently the hydration process itself declines from the controlled setting/hardening.
These measurements were supplied by the examples of phase composition studies by XRD. 相似文献
7.
Barbara Pacewska I. Wilińska G. Blonkowski 《Journal of Thermal Analysis and Calorimetry》2008,93(3):769-776
The paper describes an attempt of chemical activation of fly ash and claims the usefulness of combination of such investigation
methods as calorimetry and infrared absorption for investigations of early periods of cement hydration. The research samples
were cement pastes made with an addition of fly ash and admixtures of chemical activators, CaCl2, Na2SO4 and NaOH, whereas a cement paste without fly ash addition and a cement-fly ash paste (both without admixtures) were used
as reference samples. In order to investigate early periods of cement pastes hydration, the amount and rate of heat release
were registered, and IR spectrums were checked at appointed hydration moments.
As a result, it was shown that the combination of calorimetric and IR absorption methods in the investigations of early periods
of cement hydration was useful. It was confirmed that the use of chemical activators CaCl2, Na2SO4 and NaOH accelerated the hydration of cement pastes containing fly ash additive in early hours after adding water. The action
of activators on hydrating cement system is different for each of investigated compounds. 相似文献
8.
Janis Justs Mateusz Wyrzykowski Frank Winnefeld Diana Bajare Pietro Lura 《Journal of Thermal Analysis and Calorimetry》2014,115(1):425-432
Internal curing with superabsorbent polymers (SAP) is a method for promoting hydration of cement and limiting self-desiccation, shrinkage and cracking in high-performance, and ultra high-performance concrete with low water-to-binder ratio. SAP are introduced in the dry state during mixing and form water-filled inclusions by absorbing pore solution. The absorbed solution is later released to the cement paste during hydration of the cement. In this paper, cement pastes with low water-to-binder ratios incorporating superplasticizer and different dosages of SAP and corresponding additional water were prepared. Reference cement pastes without SAP but with the same amount of water and superplasticizer were also mixed. Isothermal calorimetry was used to measure hydration heat flow. Water entrainment by means of SAP increased the degree of hydration at later hydration times in a manner similar to increasing the water-to-binder ratio. Addition of SAP also delayed the main calorimetric hydration peak compared to the reference pastes, however, in a less prominent manner than the increase in water-to-cement ratio. 相似文献
9.
F. S. Hashem M. S. Amin S. M. A. El-Gamal 《Journal of Thermal Analysis and Calorimetry》2013,111(2):1391-1398
This paper represents a laboratory study on the acid resistance of hardened ordinary Portland cement (OPC) and blended OPC pastes at two different curing temperatures. The blended materials used are rice husk ash (RHA) and cement kiln dust (CKD). The blended cement pastes were prepared using a water/solid (W/S) ratio of 0.3. The effects of immersion in deionized water (pH 7) and sulfuric acid solutions (pH 1, 2 and 3) at two temperatures (20 and 50 °C) on the compressive strength and phase composition of the various hardened blended cement pastes were studied. The results of compressive strength revealed that the increase of curing temperature from 20 to 50 °C resulted in increase the reduction of compressive strength due to acid attack up 2 months, but the resistance to sulfuric acid attack increases after that time due to the formation of crystalline calcium silicate hydrates (CSH) which have higher resistance to acid attack than the amorphous CSH formed at the early ages of hydration. The presence of RHA and CKD improves the resistance to sulfuric acid attack at both curing conditions. From the results of X-ray diffraction analysis and differential scanning calorimetric technique curves, the main hydration products identified are CSH, portlandite, and calcium sulfoaluminate hydrates. 相似文献
10.
LI Weifeng MA Suhua Zhang Shengbiao Shen Xiaodong 《Journal of Thermal Analysis and Calorimetry》2014,118(1):83-91
Molasses is generally used as a grinding aid in cement and as a water reducer and retarder in concrete. In China, the output primarily consists of sugarcane molasses. In this paper, the effects of sugarcane molasses on the physical performance and hydration chemistry of conventional Portland cement were investigated. The setting times, the normal consistency of cement pastes, the compressive strengths and fluidities of the mortars were respectively determined according to Chinese Standard GB/T 1346, GB/T17671 and GB/T 2419. The effect of molasses on the hydration kinetics of cement was investigated using a calorimeter. The hydration products and pore size distribution of the cement pastes were analysed by X-ray powder diffraction, differential scanning calorimetry and a mercury injection apparatus. The results show that a small amount of sugarcane molasses retards the setting and hardening of cement paste and increases the fluidity of cement mortar, while excess molasses accelerates the setting and hardening. Molasses improves significantly the compressive strength at 3d due to the decrease of porosity. The addition of 1.0 % molasses accelerates the formation of ettringite, prevents the second hydration of aluminate phase and delays the hydration of C3S. 相似文献
11.
Oil well cementing is a vital operation to assure casing stability and zonal isolation for oil and gas exploration. However, some scenarios demand the cemented region to withstand high thermal gradients and imposed deformations, as occurs in the case of oil wells subjected to cyclic steam injection at temperatures up to 250 °C, to reduce oil viscosity and to increase well pressure to facilitate heavy oil recovery. In this paper, the hydration of ductile special cement systems using styrene-butadiene latex (SBR) and carboxylated styrene-butadiene latex (XSBR) addition was studied by conduction calorimetry. The resulting heat flow curves, presented in log–log plots, were used to analyze the influence of those copolymers on the hydration stages of three families of cement pastes of different complexity. The simpler cement systems (SCCS) contained water, oil well Portland cement class G and SBR or XSBR in its composition. In medium complexity systems silica fume was added and in the higher complexity ones (HCCS), superplasticizer as well. The primary objective of adding those copolymers into the Portland cement paste is to obtain higher ductility properties after setting, silica fume to have good thermal stability up to 300 °C, while superplasticizer was added to guarantee good workability. Rheological tests were carried out to evaluate the effect of the copolymers on the composite viscosity. Thermogravimetric analysis of selected SCCS and HCCS samples was performed to quantify the main formed phases up to 24 h of cement hydration. From the obtained results, it was noticed that SBR and XSBR addition substantially affects hydration kinetics at all early age stages. Starting from pre-induction and induction periods, the main observed effect during these stages, was related to the increased viscosity of the pastes, which was higher in XSBR containing pastes, retarding the hydration reactions of respective following stages, when compared to pastes with the same cementitious matrix without copolymer addition. 相似文献
12.
Eisa E. Hekal Salah A. Abo-El-Enein S. A. El-Korashy G. M. Megahed T. M. El-Sayed 《Journal of Thermal Analysis and Calorimetry》2013,114(2):613-619
Electric arc furnace dust (EAFD) is termed as a hazardous waste due to its contamination with heavy metals. Inertization of such very fine dust can be occurred via stabilization and solidification process within the hydrated Portland cement matrix. In this paper, the effect of the addition of various ratios of EAFD on the properties of the hardened Portland cement paste was investigated. Compressive strength, chemically combine water and free lime contents were determined. In addition, phase composition using XRD; DTA analysis; as well as microstructure of the formed hydrates for some selected samples were investigated using SEM. The obtained results showed that the paste containing 1/mass% EAFD give the highest compressive strength values at most hydration ages, specially the later ages, compared to the neat Portland cement blank paste. Whileas, the pastes containing 3 and 5/mass% EAFD showed lower values of compressive strength compared to those of the blank paste. 相似文献
13.
Wiesława Nocuń-Wczelik Paulina Golonka Grzegorz Malata 《Journal of Thermal Analysis and Calorimetry》2014,118(1):59-66
The calorimetric measurements were applied in testing the effect of some sulfates, used as Cr(VI) reducers in cement, as setting and hardening modifiers. The iron(II) sulfate is most commonly added as Cr reducer to cement on grinding. This was taken as a reference in the studies of the other potential chromium reducers, such as tin(II) and manganese(II) sulfates on cement hydration. The high percentage of admixtures was reduced steadily from very high overdosage—to find the possible effect of non-homogeneity resulted from the hygroscopic character of compounds used and to detect the possible products which can be formed—to relatively small quantity, as used in practice. The progress of cement hydration was investigated by calorimetry and chemical shrinkage measurements. The rheological properties of cement paste admixtured with iron, tin, and manganese sulfates were investigated, as well as the phase composition of hydrated pastes was studies by XRD. The compressive strength of the small paste cylinders was measured. Finally, the hydrated samples were subjected to the SEM observations. The tin sulfate showed the strongest retarding action as it was proved by calorimetry and chemical shrinkage data, as well as by strength and rheological measurements; however, at small quantities, this compound has a positive impact on setting and hardening. The detrimental effect of overdosed Mn and Fe sulfates due first of all to the formation of higher amount of ettringite at very early age was found. This can be proved additionally by the change of rheological parameters—higher yield stress and viscosity. 相似文献
14.
Barbara Pacewska Iwona Wilińska Mariola Nowacka 《Journal of Thermal Analysis and Calorimetry》2011,106(3):859-868
The influence of three mineral additives, i.e. fly ashes from pulverized combustion and from fluidized combustion of hard
coal as well as Portland cement, on early hydration (up to 28th day) of calcium aluminate cement was investigated. Cement
pastes containing 0, 5 and 25 wt% of additives were studied by the use of calorimetry, thermal analysis and infrared spectroscopy
methods. It was confirmed that hydration of calcium aluminate cement is closely dependent on the type of addition and its
amount. The influence of additives of different properties on cement hydration was discussed basing on received results and
other literature reports. 相似文献
15.
Un-hydrated Portland cement
consists of several anhydrous and reactive phases, that when mixed with water
react to form hydrates. The main hydration product of Portland cement is calcium
silicate hydrate (C–S–H). It is the main binding phase in a concrete
system, hence is important to construction chemists. The concrete engineer
measures the compressive strength of concrete after prescribed hydration periods,
typically 1, 3, 7, 28 days. It is often convenient to mimic these intervals
by stopping the hydration reaction at the same times. Several techniques can
be employed to stop this hydration reaction. One of which is solvent-based
and involves mixing a polar solvent such as acetone or isopropyl alcohol,
with the hydrated cement. This mixing should be vigorous enough to blend the
free water, in the partially hydrated cement system, with the polar solvent
without altering the cement system’s matrix. The solvent-water mixture
has a much lower boiling point and the mixture quickly evaporates out of the
system. This achieves two goals. It stops the hydration reaction at the moment
of solvent mixing, and it removes free water to prevent further hydration
from occurring. This procedure theoretically leaves behind a dry, chemically
unaltered, partially hydrated cement paste. In this way, pastes can be analyzed
after the prescribed 1, 3, 7 or 28 days of hydration. This paper uses thermogravimetric
analysis (TG) results to investigate the assumption that solvents have no
thermodynamic or chemical effect on the hydrated cement paste phases. 相似文献
16.
Shuhua Liu Lu Wang Yuxin Gao Baoying Yu Yun Bai 《Journal of Thermal Analysis and Calorimetry》2014,118(3):1483-1492
The hydration properties of slag sulfate cement (SSC), slag Portland cement (PSC), and ordinary Portland cement (POC) were compared in this study by determining the compressive strength of pastes, the hydration heat of binders within 72 h, the pore structure, the hydration products, and the hydration degree. The results indicated that main hydration products of PSC paste and POC paste are calcium hydroxide and C–S–H gel, while those of SSC paste are ettringite and C–S–H gel from the analyses of XRD, TG–DTA, and SEM. At the early curing age, the compressive strength depends on the clinker content in the cementitious system, while at the late curing age, which is related to the potential reactivity of slag. From hydration heat analysis, the cumulative hydration heat of PSC is lower than that of POC, but higher than that of SSC. Slag can limit chemical reaction and the delayed coagulation of gypsum, which also plays a role in the early hydration. So SSC shows the lowest heat release and slag can’t be simulated without a suitable alkaline solution. Based on MIP analysis, the porosity of POC paste is the smallest while the average pore size is the biggest. At the age of 90 days, the compressive strength of SSC can get higher development because of its relative smaller pore size than that of PSC and POC paste. 相似文献
17.
W. Nocuń-Wczelik B. Trybalska E. Żugaj 《Journal of Thermal Analysis and Calorimetry》2013,113(1):351-356
Calorimetry was applied to follow the hydration in the Portland cement–dolomite–limestone mixtures. In the experiments the limestone additive of various fineness (standard component of various common cements), as well as the dolomite additive (not a standard component) were used. The rate of hydration versus time for common cements reflects the proper setting and early hardening during the first days after mixing with water (two or three peaks and the induction period between them). The aim of measurements presented in this work was to show the course of heat evolution curve and the heat evolved values, equivalent to the acceleration/retardation of hydration, in case of the pastes produced from Portland cement and the carbonate additives mixed in variable proportions, as well as to verify the results by other methods. The rate of heat evolution accompanying cement paste hydration, total heat evolved, conductivity of hydrating suspension and rheological (flow) properties versus time are modified by the fine grained carbonate additives. This is due to the hypothetical nucleating effect of limestone and dolomite. 相似文献
18.
M. S. Amin S. A. Abo-El-Enein A. Abdel Rahman Khaled A. Alfalous 《Journal of Thermal Analysis and Calorimetry》2012,107(3):1105-1115
Pozzolanic cement blends were prepared by the partial substitution of ordinary Portland cement (OPC) with different percentages
of burnt clay (BC), Libyan clay fired at 700 °C, of 10, 20, and 30%. The pastes were made using an initial water/solid ratio
of 0.30 by mass of each cement blend and hydrated for 1, 3, 7, 28, and 90 days. The pozzolanic OPC–BC blend containing 30%
BC was also admixed with 2.5 and 5% silica fume (SF) to improve the physicomechanical characteristics. The hardened pozzolanic
cement pastes were subjected to compressive strength and hydration kinetics tests. The results of compressive strength indicated
slightly higher values for the paste made of OPC–BC blend containing 10% BC The results of DSC and XRD studies indicated the
formation and later the stabilization of calcium silicates hydrates (CSH) and calcium aluminosilicate hydrates (C3ASH4 and C2ASH8) as the main hydration products in addition to free calcium hydroxide (CH). Scanning electron microscopic (SEM) examination
revealed that the pozzolanic cement pastes made of OPC–BC mixes possesses a denser structure than that of the neat OPC paste.
Furthermore, the addition of SF resulted in a further densification of the microstructure of the hardened OPC–BC–SF pastes;
this was reflected on the observed improvement in the compressive strength values at all ages of hydration. 相似文献
19.
Barbara Pacewska Iwona Wilińska Maria Bukowska 《Journal of Thermal Analysis and Calorimetry》2009,97(1):61-66
The aim of this work is to compare the influence of addition of waste aluminosilicate catalyst on the initial periods of hydration
of different cements, i.e. calcium aluminate cements of different composition and Portland cement, basing on the calorimetric
studies. Cement pastes containing up to 25 mass% of additive were studied, where the water/(cement+additive) ratio was 0.5.
An attempt was undertaken to explain the mechanism of action of introduced aluminosilicate in the system of hydrating cement,
particularly in the case of calcium aluminate cement pastes.
It was found that the presence of fine-grained additive caused in all studied cases the increase of the amount of released
heat in the first period after the addition of water. In the case of aluminate cements with aluminosilicate addition, a significant
reduction of induction time and faster precipitation of hydration products were observed compared to the reference sample
(without additive). In the experimental conditions, the additive caused the acceleration of aluminate cements hydration, and
the mechanism of its action is probably complex and can encompass: nucleative action of small grains and formation of new
chemical compounds. 相似文献
20.
Effect of metakaolin pozzolanic activity in the early stages of cement type II paste and mortar hydration 总被引:1,自引:1,他引:0
A. L. Cherem da Cunha J. P. Gonçalves P. M. Büchler J. Dweck 《Journal of Thermal Analysis and Calorimetry》2008,92(1):115-119
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. 相似文献