Strätlingite in the presence of sulfate: 20–85 °C
Strätlingite–gypsum phase relation: 20–85 °C
The XRD pattern of the resulting solids (Fig. 3a) shows that strätlingite persisted in the presence of gypsum. The resulting phase assemblage was strätlingite, gypsum, and C–S–H at 20, 55 and 85 °C. The SEM image (Fig. 4a) of the strätlingite–gypsum–water mixture cured at 55 °C shows the clear monoclinic gypsum grains dispersed in strätlingite powder, suggesting coexistence of both phases, in agreement with the XRD pattern. The solid from the strätlingite–gypsum slurry at 85 °C was analysed by electron microprobe and the result show <1 % sulphur substitution over about ten random points analysed. The minute amount of sulphur detected in strätlingite may have either occurred as minor substitution or originated from the excess gypsum solid which perhaps has been physically incorporated into the grains of strätlingite.
Further investigation by the two compartment system, first used in Okoronkwo [18], also shows that strätlingite is compatible with gypsum under the range of experimental conditions. Figure 2b presents the XRD pattern of strätlingite cured in saturated gypsum solution for 70 days at 20–85 °C. The IR spectra of the strätlingite sample cured for 70 days in saturated gypsum solution is shown in Fig. 4b. The assignment of the adsorption bands is summarized in Table 1, according to [9, 10, 16, 22]. The absorption at ~1065 and ~1400 cm−1 are assigned to C–O vibrations. The minor absorption band between 1035 and 1100 cm−1, centred at ~1065 cm−1 corresponds to that reported in literature for S–O stretching vibration and C–O associated with AFm and AFt phases [2, 3, 16]. But the absence of further absorption band which correspond to sulfate and presence of multiple bands characteristic of C–O supports that the band centred at ~1065 cm−1 is due to C–O vibration from contaminants and that sulfate uptake has been negligible. No changes were noted in the powder X-ray pattern relative to pure strätlingite. Thus, the coexistence of gypsum and strätlingite is confirmed but no evidence indicating mutual solid solution was adduced. In view of the bulky nature of the aluminosilicate interlayer ion it seems unlikely that substitution of the smaller sulfate should leave d-spacings unaffected.
Table 1 Assignment of infrared spectra data from strätlingite–gypsum relation studies; spectra shown in Fig. 4b
Strätlingite–SO4-AFt phase relation: 20–85 °C
The XRD pattern of the 1:1 molar mixture of strätlingite–SO4-AFt in water, after curing for 30 days (Fig. 5a), shows that strätlingite coexists with SO4-AFt at 20–85 °C. Results of the two compartment experiment corroborate previous results suggesting the compatibility of these phases (Fig. 5b).
Strätlingite–SO4-AFm phase relation: 20–85 °C
The XRD pattern of the mechanical mixture of strätlingite and SO4
2− AFm reacted for 30 days at 20–85 °C shows no evidence of reaction (Fig. 6b). Trace amounts of ettringite occurred but these were known to have been present in the original AFm sample (Fig. 6a). Strätlingite and SO4-AFm appear to coexist for this temperature range and duration of reaction.
Strätlingite in the presence of carbonate: 20–85 °C
Strätlingite–calcite phase relation: 20–85 °C
The XRD pattern of the resulting solid after 4 weeks aging of the strätlingite–calcite slurry shows that the two phases are compatible under the conditions of the experiment; the presence of calcite, apparently does not affect the stability of strätlingite at 20 ± 2, 55 and 85 °C (Fig. 7a). The two phases coexisted at the studied temperatures and the peak positions of the X-ray reflections due to strätlingite have not been affected, suggesting, perhaps, that little or no structural substitution of CO3
2− has occurred in strätlingite as the peak position of the X-ray reflections due to strätlingite are unaffected. Further investigation by the two compartment experiment corroborated previous observation as the XRD pattern of strätlingite shows that strätlingite was not affected after curing in saturated calcite solution for 56 days at 20–85 °C (Fig. 7b). The minor XRD reflection at ~29.3°2θ in the XRD pattern of starting strätlingite reactant (Fig. 7b), due to minor calcite contaminant, has become prominent after curing for 56 days in saturated calcite solution. This apparently indicates minor precipitation of calcite into the strätlingite had occurred. IR spectra (Fig. 7c (ii)) of the strätlingite sample cured in calcite solution for 56 days at 20 °C similarly show a minor absorptions at ~1065, ~1240 and ~1400 cm−1 corresponding to C–O vibrations of carbonate, probably associated with “AFm-type strätlingite” [2, 3, 16, 22]. That indicates a minor uptake of carbonate in the form of calcite, which may have precipitated from solution and/or originating from atmospheric contamination, and like the case of sulfate has no implication for solid solution.
Strätlingite–CO3-AFt phase relation: 20–85 °C
The XRD pattern of the 1:1 molar mixtures of strätlingite–CO3-AFt cured for 30 days (Fig. 8a) show that strätlingite coexists with CO3-AFt at 20 ± 2 °C. However at 55 °C, CO3-AFt had decomposed to calcite and AFm but strätlingite persisted. At 85 °C where CO3-AFt had completely decomposed and strätlingite, being less stable at this temperature, reacted forming mainly siliceous hydrogarnet which coexisted with calcite and C–S–H. Trace amounts of strätlingite was still detected at up to 30 days at 85 °C but not thereafter.
It is known that in relevant systems such as limestone-blended Portland cement hydration, carbonate activity is initially conditioned by calcite, followed by AFt but because CO3-AFt is metastable with respect to CO3-AFm [5], CO3-AFt is not formed under these conditions, leaving the system to be buffered with respect to carbonate by calcite and CO3-AFm. Also, carbonate AFm is unstable at high temperatures and its decomposition generates high alumina activity which causes strätlingite to start reacting at above 55 °C producing the more stable siliceous hydrogarnet. See also Sect. 3.2.3.
Strätlingite–CO3-AFm phase relations: 20–85 °C
The XRD pattern of the resulting solids shows that at 20 and 55 °C, strätlingite coexisted with CO3-AFm, but hydrogarnet solid solution forms at 55 °C while at 85 °C, the CO3-AFm had decomposed to mainly hydrogarnet solid solution and calcite (Fig. 9). It can be deduced that, the decomposing of CO3-AFm at high temperature increased the alumina activity in the solution, and as calcite crystallised, shift in mass balances promotes formation of hydrogarnet solid solution at temperature of 55 °C and above.
From the results, strätlingite has shown compatibility with calcite, gypsum, sulfate and carbonate AFm and AFt at temperatures below about 55 °C. Strätlingite coexistence with AFt (ettringite), has also been predicted in previous thermodynamic models [1, 4].
Described phase compatibility is not only affected by temperature but also by ion activity and time. The early stages of cement hydration, at ~20 °C, when solid gypsum or other form of CaSO4 is present, sulfate activity is relatively high, conditioned by gypsum, but as gypsum is consumed to form AFt, the sulfate activity is instead conditioned by the composition and solubility of the AFt phase. As more calcium and alumina react, AFt is partially converted to AFm phase and the sulfate activity at this time is now buffered by the pair AFt-AFm. For most commercial cements, this state is reached within the first 24–48 h of hydration. The same principles operate for limestone-blended Portland cement hydration: carbonate activity is initially conditioned by calcite, followed by AFt but because CO3-AFt is metastable with respect to CO3-AFm [5], CO3-AFt is not formed under these conditions, leaving the system to be buffered with respect to carbonate by calcite and CO3-AFm.
Carbonate and sulfate variants of strätlingite are not well known. However the strätlingite structure can be regarded as a potential host for at least four anions common in cement systems—OH, Cl, SO4 and CO3. Chloride was not included in this study but the remaining three anions were either present or potentially present. The competition for anion content will depend on pH as well as the thermodynamic properties of the other coexisting solid phases and the aqueous activity of the relevant anion species. Assuming an alkali free system, a complete description of the anion content in strätlingite would differ for each assemblage and would be temperature dependent: the data would take the form of a series of distribution coefficients. The present data are insufficient to quantify these coefficients and their temperature dependence but a start has been made by determining the phase assemblages. However we note that many of these assemblages condition a low aqueous activities of sulfate and carbonate (the host solids containing these ions have low solubilities and the impact of solid solution on the powder patterns of strätlingite has not been quantified. But it is not surprising that in many assemblages OH strätlingite predominates. This finding is not in conflict with the observation that under other conditions especially of higher species activities of sulphate and carbonate, extensive anion substitution can occur in strätlingite. Indeed, in one experiment (Fig. 7) sufficient carbonate was recorded to influence the FTIR spectra.
In the presence of Mg, strätlingite has also been predicted to occur together with hydrotalcite-like phases and a wide range of observations supports this, for example data on systems formulated with activated calcined paper sludge, kaolinite and slag blends [6–8, 11, 12], all of which introduce Mg in various ways. However, for strätlingite to form in such blended cement systems, the compositions in terms of C–A–S ratio must lie in the phase region where strätlingite is readily stable [17, 18].
Data reported for the coexistence of strätlingite with other phases are generic: that is the amount of phase added does not affect the phase relations. However we have controlled the activity of species such as sulfate and carbonate by using those phases which are known to occur in commercial cements: if it were forced as by adding a soluble carbonate salt, we might depart form conditions in commercial cement and the stability of phases and limits of composition altered. So when we conclude that solid solution is negligible, the conclusion is conditional. That is, it applies to the conditions of the experiment, the most important condition being what other phases are present. But the result is still generic in the sense that the amount of other phases is not crucial with respect to determining reaction direction. And we have chosen conditions which are relevant to modern commercial cements including those modified by supplementary materials. For example, strätlingite is thermally destabilised in the presence of carbonate AFm where siliceous hydrogarnet forms at 55 °C and above. For similar reasons we avoid giving a single value for the upper stability limit of strätlingite: the exact temperature is conditional, depending on what other phases are present.