Strätlingite: compatibility with sulfate and carbonate cement phases
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The silicate AFm, strätlingite, has been shown to be stable in high aluminosilicate cement systems but its stability with respect to the anion content of hydrated Portland cement paste is unknown. The stability of strätlingite in the presence of sulfate and carbonate phases relevant to cement systems are reported. Results show that strätlingite persists at the sulfate activity conditioned by gypsum, ettringite and at carbonate activity conditioned by the presence of calcite, carbonate AFm, or carbonate AFt. Structural incorporation of anions such as carbonate or sulfate in strätlingite was not observed in the temperature range 20–85 °C.
KeywordsSträtlingite Sulfate Carbonate Stability Compatibility
Calcium silicate hydrate
Ettringite (SO4-AFt, C6As3H32)
It is a common practice to add gypsum to Portland cement . Also, most novel cements such as the calcium aluminosulfate cements contain substantial amounts of sulfate. Cement could also be blended with limestone, mainly CaCO3, and atmospheric CO2 is readily uptaken by wet cements pastes cured in the open. In this way, sulfate and carbonate phases form normally in the hydration process of cement paste. We know that AFm and AFt are common sulfate and carbonate bearing phase product of cement hydration. High saturation in sulfate –rich service environments and elevated temperature may also lead to the formation of gypsum and calcite. The impact of these sulphur and carbon bearing phases on the stability of the crystalline phases occurring in the CaO–Al2O3–SiO2–H2O system is important as reactive supplementary materials are often added with the result that the composition of the reactive fraction of concrete varies over wide limits of composition.
Thermodynamic data has shown that the constitution of minor phases, AFm and AFt is very sensitive to temperature and the activity of anions, especially CO3 2−, SO4 2− and OH−, and that the resulting distribution of anions is temperature dependent over short temperature ranges, examples are given in the range 0–40 °C by Matschei and others [4, 13, 14].
2.1 Analytical methods
Mineralogical examinations of dried solids were by X-ray Powder Diffraction (XRD) using a Bruker D8 Advance X-Ray Powder Diffractometer with CuKα radiation. The angular scan was between 5 and 45°2θ with a step size of 0.02 and count time of 1 s per step. XRD patterns were collected at laboratory temperature of ~20 °C. Infrared spectra of samples were collected by the Attenuated Total Reflection, ATR method with PerkinElmer UATR Spectrometer equipped with a diamond cell. Measurements were collected in the mid-infrared region 400–4000 cm−1 at a resolution of 4 cm−1. The morphology of selected samples was examined with a Hitachi S-520 Scanning Electron Microscope (SEM). A voltage of 20 kV was applied across the tungsten filament electron gun. Selected samples where ground to fine powder and a thin layer was collected on the brass sample holder and gold coated with an Emscope SC-500A sputter coater to prevent charging or overheating.
2.2 Sample preparation and reactions
Ettringite (SO4-AFm), Ca6Al2(SO3)3(OH)12·26H2O: Synthesized according to Matschei et al.  and Matschei . A slurry of stoichiometric amounts of NaAlO2, Na2SO4 and CaO (obtained from CaCO3 heated at 1000 °C for ~12 h) in 10 w/v sucrose solution, liquid/solid ~10, was stirred for 3 days and left to age at room temperature 22 °C in HDPE bottle for 4 weeks before filtration. The sample was flushed several times with degassed deionized water. Alternatively, ettringite was prepared from slurry of 1:3 molar ratios of C3A and CaSO4·2H2O in degassed deionized water. The w/s ratio was adjusted to ~20 and the mixture was stirred for 2 weeks at room temperature (20 ± 2 °C).
CO3-AFt, Ca6Al(CO3)3(OH)12·26H2O, was synthesized from a stoichiometric mixture of CaO, NaAlO2 and Na2CO3 in a 10 % w/v sucrose solution according to the modified Carlson and Berman method used elsewhere [13, 15]. Previously prepared slurries of sodium aluminate and sodium carbonate were mixed with 10 % w/v sucrose—CaO mixture, stirred for 3 days and then aged with periodic agitation at room temperature 20 ± 2 °C for 2 weeks before filtration and washing. Matschei  has shown that when well washed, the product is sucrose-free.
SO4-AFm, Ca4Al2SO4(OH)12·6H2O, was prepared according to previous protocol [13, 15], by mixing C3A and CaSO4 in a 1:1 molar ratio, slurried in boiling ultra pure water and thereafter cured at 85 °C for two weeks in PTFE bottles prior to filtration.
CO3-AFm, Ca4Al2CO3(OH)12·5H2O, was prepared according to prvious protocol [13, 15], by mixing previously prepared C3A and CaCO3 in a 1:1 molar ratio with previously degassed ultra pure water (w/s ∼10) at 25 °C and stored with agitation in HDPE-bottles for two weeks prior to filtration.
2.2.1 Strätlingite–gypsum phase relations
Single compartment experiment
Mixtures of strätlingite and gypsum CaSO4·2H2O were slurried in degassed deionized water and reacted at 20, 55 and 85 °C for 4 weeks in PTFE bottles with periodic agitation. The mixture was prepared with a target of achieving a 10 % substitution for (OH) i.e. sufficient sulfate to form Ca2Al2SiO2(SO4)0.5(OH)9·2.25H2O. Equivalents of ~2 g/L excess of gypsum were added to the mixtures respectively. After 4 weeks of ageing, the samples were filtered, dried and characterized by XRD, FTIR and SEM.
Two compartment experiment
The solution was stirred with the aid of a magnetic stirrer for the set-ups at 5 °C and 20 ± 2 °C and by shaking at the rate of 60 strokes/min in a water bath for the set up at 55 °C.
2.2.2 Strätlingite–calcite phase relations
Similar experiments as described in Sect. 2.2.1 were repeated with calcite in place of gypsum.
2.2.3 Strätlingite: AFt phase relations
A 1:1 molar mixture of strätlingite and SO4-AFt was slurried in degassed deionized water and aged at 20, 55 and 85 °C in HDPE/PTFE bottles for 4 weeks with periodic agitation. Thereafter, the sample were filtered and characterised by XRD.
Similarly, 1:1 molar mixture of strätlingite and CO3-AFt was slurried in degassed deionised water and aged at 20, 55 and 85 °C in HDPE/PTFE bottles for 4 weeks with periodic agitation. Thereafter, the sample were filtered and characterised by XRD.
2.2.4 Strätlingite: AFm phase relations
A 1:1 molar mixture of strätlingite and SO4-AFm (Ca4Al2SO4(OH)12·6H2O) was slurried in degassed deionised water and aged at 20, 55 and 85 °C with periodic agitation for 30 days in HDPE. Filtered and dried product solid was characterised by XRD, FTIR and SEM.
Similarly 1:1 molar mixture of strätlingite and CO3-AFm Ca4Al2CO3(OH)12. 5H2O was slurried in degassed deionized water and aged at 20, 55 and 85 °C as in the previous section.
3 Results and discussion
3.1 Strätlingite in the presence of sulfate: 20–85 °C
3.1.1 Strätlingite–gypsum phase relation: 20–85 °C
Assignment of infrared spectra data from strätlingite–gypsum relation studies; spectra shown in Fig. 4b
IR absorption band (cm−1)
530; 590; 965
710; 860; 1065;1240; 1410
Assignments of vibration
3.1.2 Strätlingite–SO4-AFt phase relation: 20–85 °C
3.1.3 Strätlingite–SO4-AFm phase relation: 20–85 °C
3.2 Strätlingite in the presence of carbonate: 20–85 °C
3.2.1 Strätlingite–calcite phase relation: 20–85 °C
3.2.2 Strätlingite–CO3-AFt phase relation: 20–85 °C
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 , 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.
3.2.3 Strätlingite–CO3-AFm phase relations: 20–85 °C
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 , 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, 7, 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.
At the level of soluble anion concentrations found in Portland based cements, a few tens of ppm in pore fluid, sulfate and carbonate do not substitute significantly for OH in the strätlingite structure. Thus, while strätlingite is structurally an AFm type phase, it is unlike silica—free AFm members: strätlingite binds insignificant sulfate and carbonate at pH ~12. Strätlingite shows compatibility with gypsum, calcite, sulfate and carbonate AFm and AFt at temperatures below about 55 °C. The resistant of its structure to attack by anions is attributable to the stability of the double tetrahedral aluminosilicate interlayer. Results can safely be used to predict conditions under which strätlingite will form and persist in Portland and modified Portland cements.
The TETFund Nigeria and Abia State University Uturu are acknowledged for financial support.
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