The objective of the present work was to investigate the transition reaction of the calcium silicate hydrate tobermorite into xonotlite under influence of additives. Tobermorite is the main binding agent in steam hardened building materials and the appearance of xonotlite indicates the progress of hardening and an overcuring of the material. Hydrothermal experiments under addition of sucrose, calcium formate and calcium chloride dihydrate to the main components quartz and lime were done using temperatures of 220°C and a reaction time of 40.5 h. All experiments were performed with powders as well as with pressed educts. The products of all syntheses were analyzed with XRD, SEM/EDX and FTIR. The references as well as the syntheses with calcium chloride dihydrate led to the formation of 11 Å tobermorite and xonotlite. The former showed the best results and even synthesis with pressed educts and calcium chloride dihydrate revealed an accelerating effect of the additive. In contrast syntheses with sucrose had the worst reactivity and led to the formation of calcite beside the CSH-phase scawtite. The additive calcium formate was only slightly oppressing the crystallization of tobermorite and favouring the formation of xonotlite. Syntheses with pressed pellets and sucrose or calcium formate showed generally worse results.
Additives play an important role in the production of building materials. Their accelerating or retarding effects on the course of hydraulic reactions are widely used in concrete production and cement treatment to comply the requirements of the material [
In accelerators like calcium chloride the ionic potential of cations and anions contribute to the effect by influencing the ion-hydratation processes as well as the diffusion and the transport of building blocks from the solution to the growing crystals [
Compared with the comprehensive theoretical studies and experimental work connected with cement and concrete investigations on insertion of additives in the field of steam hardened building materials are much lesser numerous. Those materials were produced by autoclave curing at 160˚C - 200˚C under saturated steam pressure from quartz, burnt lime and water. Calcium-silicate-hydrate phases (CSH-phases) were formed during this reaction within a few hours [
In the present paper the influence of the additives sucrose, calcium formate and calcium chloride dihydrate on the transition reaction of tobermorite to xonotlite Ca6Si6O17(OH)2 (C6S6H) was investigated in dependence of different mass ratios of the additive beside lime and quartz. This selected reaction is of special interest in production of steam hardened building materials as the occurrence of noticeable parts xonotlite beside tobermorite indicates the start of over curing of the material.
In the present paper two synthesis series were done―series a) with powder educts and b) with pressed educts. For both types first reference syntheses without additives were prepared. As in former experiments on CSH crystallization [
Sample* | Additive | C/S |
---|---|---|
1a + b | - | 0.8 |
2a + b | Sucrose | 0.5 |
3a + b | Calcium formate | 0.8 |
4a + b | Calcium chloride dihydrate | 0.8 |
*a: synthesis with powder, b: synthesis with pellet.
as it was our aim, to insert a constant amount of 0.75 g of reactive lime CaO in each of the three experiments with additives. For series a) the powdery educts were filled into 50 ml steel autoclaves with Teflon liners and 20 ml distilled water was added. For the syntheses with pellets (series b) in each case only half of the educt masses as inserted for powders were used and first pressed under a hydraulic press with special mold at 5 t for 5’ into 4 mm thick tablets of 12 mm diameter, before placed in a special Teflon sieve inlay inside each autoclave. Again 20 ml water was filled in the liners. The autoclaves were heated in an oven at 220˚C for 40.5 h. After this reaction time the products were filtered, washed with water and dried at 80˚C for 24 h.
The synthesis products were analyzed by XRD, SEM/EDX and FTIR spectroscopy. X-ray powder data were collected on a Bruker Endeavour D4 powder diffractometer (Bragg-Brentano geometry) using Ni-filtered CuKα radiation at 40 kV and 40 mA. The measurements were performed with a step width of 0.03˚ in the range of 5˚ - 85˚ 2 Theta. 2668 steps of 1 s duration were measured. The data were analyzed using the WinXPow software of Stoe & Chi.
Beside these measurements for an overview on the qualitative course of syntheses, selected products of series (a) were analysed quantitatively by Rietveld refinement of X-ray powder data. Therefore a high quality pattern was measured in the range of 2˚ - 80˚ 2θ with 0.01˚ step wide and 10 s of measurement of each step. The Topas 4.2 software (Bruker AXS) was used for data evaluation.
The morphology and chemical composition of the synthesis products were analyzed on a Jeol JSM-6390A scanning electron microscope (SEM) at an accelerating voltage of 20 kV or 30 kV. The SEM was coupled with a Bruker QUANTAX EDX Spektrometer equipped with the XFlash M410 EDX detector. The samples therefore were sputtered with a fine layer of gold to prevent electric charging during measurement. A signal of gold (Au) therefore exists in each EDX spectrum. Infrared spectra were taken on a Bruker IFS66v FTIR spectrometer. The samples therefore were diluted by KBr (about 1 mg sample in 200 mg KBr), pressed into pellets and measured relatively to the KBr as a reference.
From the XRD powder patterns of reference syntheses products with quartz powder (
A quantitative evaluation by Rietveld method is given in
Xonotlite as main phase beside a small amount of tobermorite was detected by Rietveld analysis (
Phase | Composition* | Amount (M.-%) | Amount (without quartz) (M.-%) | |
---|---|---|---|---|
Xonotlite | C6S6H | 80.57 | 83.27 | |
11 Å Tobermorite | C5S6H5 | 16.19 | 16.73 | |
Quartz | SiO2 | 3.24 | - |
*CSH acc. [
needle-like CSH-phases of 2 - 5 µm length can be seen partly embedded in quartz powder. The EDX analysis of the surface of a needle is given in
The results of SEM investigation of the reference products from synthesis with the pellet are summarized in Figures 4-6. Already the overview image (
magnification (
The X-ray powder patterns of syntheses products obtained under addition of sucrose are summarized in
The results of the SEM investigation of the synthesis products with addition of sucrose show fine grained CSH-phases on very low scale (
The synthesis with the pellet under addition of sucrose shows very few but somewhat bigger CSH-crystals with needle-like morphology (
Also big rhombohedral crystals of calcite were observed within the product (
The X-ray powder pattern of the product from powder under addition of calcium formate (
In the XRD pattern of the product from the pellet (
SEM investigation of the synthesis product with powder and addition of calcium formate shows crystallisation of CSH-phases with needle-like morphology (
SEM analysis of the synthesis product with the pellet and addition of calcium formate shows xonotlite beside
Phase | Formula* | Amount (M.-%) | Amount, without quartz (M.-%) |
---|---|---|---|
Quartz | SiO2 | 50.35 | - |
Scawtite | C7S6H3C | 25.20 | 50.76 |
Calcite | CaCO3 | 24.45 | 49.24 |
*CSH acc. [
Phase | Formula* | Amount (M.-%) | Amount, without quartz (M.-%) |
---|---|---|---|
Xonotlite | C6S6H2 | 51.26 | 71.11 |
Quartz | SiO2 | 27.92 | - |
Calcite | CaCO3 | 20.83 | 28.89 |
*CSH acc. [
small amounts of to bermorite in band-shaped crystals up to 5 µm, which grow into the pore space (see
Formation of mainly xonotlite beside somewhat fewer amount of tobermorite and quartz can be derived from the XRD results of the synthesis with powder, given in
Phase | Formula* | Amount (M.-%) | Amount, without quartz (M.-%) |
---|---|---|---|
Xonotlite | C6S6H2 | 56.58 | 65.57 |
11 Å Tobermorite | C5Si6H5 | 29.71 | 34.43 |
Quartz | SiO2 | 13.70 | - |
*CSH acc. [
In contrast qualitative estimation of the XRD pattern of the product from pellet synthesis crystallization of somewhat more tobermorite can be found compared with xonotlite, beside the usual parts of quartz.
The EDX analysis, given in
SEM investigation of the product of synthesis with the pellet under addition of calcium chloride dihydrate, shown in
A further SEM image at somewhat higher magnification is shown in
Those absorption bands occur quite similar in all the spectra of this study and indicate silicate chain structures [
absorption bands of water occur at about 1634 cm−1 and between 2800 - 3700 cm−1 [
For the general evaluation of the influence of the different additives studied in the present work, the qualitative estimation of phase formation for all experiments is summarized in
The following aspects have to be discussed for a comparative characterization individual behavior of an additive on the reaction process:
(i) the degree of conversion during the whole reaction;
(ii) the overall amounts of CSH phase formation, independent of an individual phase;
(iii) the ratio of xonotlite/other CSH (tobermorite, scawtite); and last not least
(iv) the crystal quality according to morphology and size of the crystals.
The amount of remaining quartz after each reaction is representative for the degree of conversion (i). The quantitative values from Rietveld refinements for products under use of powdery educts show the following ranking order (starting with the best result):
reference → calcium chloride dehydrate → calcium formate → succrose
The same ranking can be estimated qualitatively from XRD (
Additive | * | (C + A)/S | A/C | Tob | Xo | Sc | Qz | Cal |
---|---|---|---|---|---|---|---|---|
Reference | 1a | 0.83 | - | ○ | ● | (∙) | ||
1b | 0.83 | - | ○ | ○ | ● | |||
Sucrose | 2a | 0.54 | 0.24 | ○ | ● | ○ | ||
2b | 0.54 | 0.24 | ○ | ○ | ○ | ● | ||
Calcium formate | 3a | 0.80 | 0.49 | ● | ○ | ○ | ||
3b | 0.80 | 0.49 | ○ | ● | ○ | |||
Calcium chloride dihydrate | 4a | 0.80 | 0.49 | ○ | ● | (∙) | ||
4b | 0.80 | 0.49 | ○ | ○ | ● |
*a = powdered, b = pressed educts; ● = high, ○ = low amount; (∙) very low amount, Tob = 11-Å tobermorite, Xo = xonotlite, Sc = scawtite, Qz = quartz, Cal = calcite.
Again an equal sequence is found for the overall amounts of CSH phase formation, independent of an individual phase (ii) and for the ratio of xonotlite: tobermorite or scawtite (iii), again for synthesis with powder as well as with pellet.
It has to be mentioned here, that the comparison of reactions with powders or pellets shows general lower reaction rates for pellets and hence more quartz and fewer CSH within these products. Here the reduced reaction space compared with crystallization within the liquid phase under use of powders and 20 ml water is responsible for the lower conversion into CSH-phases. However, a reduced reaction area introduces a formation of CSH in card-shaped structure resulting in a high stability of the building material, like, e.g. limesand stone [
According to (iv) the ranking is somewhat deviating. As the morphology is relative similar as both synthesis products from powders with calcium chloride dehydrate and calcium formate appear as needles, the crystal size differs. Were needles of 5 - 10 µm in size were found for powder syntheses with formate, much longer ones (10- 20 µm) were observed with calcium chloride dehydrate. Using pellets again more band or card shaped crystals of uniform size between 5 - 10 µm occurred.
In contrast syntheses with sucrose yield to the CSH-phase scawtite C7S6H3C instead of tobermorite and xonotlite. From the literature it is known that at low temperatures sucrose and calcium ions form a Ca-saccharate complex, whereby further reactions with SiO2 and H2O to CSH-phases could be prevented [
The results with calcium formate are surprising. Former experiments on tobermorite synthesis at 200˚C already showed the retarding effect of this additive on the formation of C5S6H5 [
From all three additives calcium chloride dihydrate yield to highest amounts of CSH in close resemblance to the results of the reference syntheses but with higher portions of tobermorite than xonotlite. Further experiments must clarify if this behavior is ruled by pH-value [