Compatibility studies between N-A-S-H and C-A-S-H gels. Study in the ternary diagram Na2O–CaO–Al2O3–SiO2–H2O

doi:10.1016/j.cemconres.2011.05.006

Cement and Concrete Research

Volume 41, Issue 9, September 2011, Pages 923-931

https://doi.org/10.1016/j.cemconres.2011.05.006 Get rights and content

Abstract

Sodium aluminosilicate hydrate (N-A-S-H) gel, the main reaction product of the alkali-activated aluminosilicates, differs of the aluminium-modified calcium silicate hydrate (C-A-S-H) gel of PC pastes. Increasing the level of SCM to reduce PC content of binders are being considered to address reduction in CO₂ emissions, activation of the additional SCM content by alkali activation represents a possible environmentally sustainable solution. Therefore, mixtures of C-A-S-H and N-A-S-H gels might be anticipated and the present study assesses the compatibility relationships between them.

Compositional diagrams are provided to indicate phase compositional ranges and the phase assemblages obtained under equilibrium conditions. In calcium-rich formulations (pH in excess of 12), C-A-S-H and C₂ASH₈ form as stable phases. However, in the lime poor part of the diagram an amorphous gel (N,C)-A-S-H precipitates but its stability is dependent on system pH and available Ca. (N,C)-A-S-H gels are de-stabilised by Ca to give C-A-S-H gels in suitable systems.

Introduction

In the context of addressing environmental issues, specifically reducing CO₂ emissions, the possibility of diluting Portland cement with high volumes of SCMs (Supplementary Cementitious Materials) is currently being considered. However, by diluting the PC (Portland cement) content, the ability to activate the SCM is reduced and the use of additional alkaline activator offers a possible solution.

The study of alkali activation of aluminosilicates is a relatively new field when compared with traditional Portland cement-containing systems [1], [2], [3], [4]. Alkali-activated aluminosilicates are differentiated from hydrated Portland cements by their higher initial alkalinity and the absence of lime. This is already sufficient to define quite different hydration products from the different systems so that predictions of the properties of alkali-activated aluminosilicates made based on Portland cement chemistry are inappropriate. Whereas the main binding phase of hydrated Portland cement is an aluminate-substituted calcium silicate hydrate (C-(A)-S-H) gel, the main product in alkali activated systems is sodium aluminosilicate hydrate gel designated N-A-S-H.

The compatibility of the two cementitious gels (C-(A)-S-H and N-A-S-H) has important implications for hybrid Portland cement – alkali activated aluminosilicate systems in which both products might be expected [5], [6], [7], [8], [9]. Previous studies using synthetic gels explored the effects of the constituents of each gel on the other, i.e. high pH conditions and the presence of aqueous aluminate strongly influence C-S-H composition and structure [7], [8] and aqueous Ca modifies N-A-S-H gels leading to a partial replacement of sodium with calcium to form (N,C)-A-S-H gels [9]. Despite these observations, the conditions required for such modifications have not been fully defined and with the possibility of constructional cements having both gels co-existing, a systematic study of N-A-S-H – C-A-S-H compatibility seems appropriate.

Section snippets

Experimental procedure

In assessing gel properties and compatibilities, two approaches have been adopted: (i) solids for characterisation have been precipitated from solutions of soluble salts (precipitation/dissolution approach), and ii) pre-synthesised gels have been mixed directly prior to ageing.

Precipitation/dissolution approach

Fig. 3(a) and (b) shows XRD patterns from precipitates from the two extremes of bulk composition, CS32 and CS024 respectively, corresponding to the highest and lowest calcium environments during initial mixing (i.e. mix identifier indicates relevant initial CaO:SiO₂ ratios—see Table 1). Data for the intermediate calcium loaded sample are not shown because these results were similar to the results obtained for the high calcium-loaded (CS32) samples.

The XRD diffraction pattern for the CS32 sample

Conclusions

Based on a series of solution/precipitation studies, a tentative phase diagram has been proposed for the Na₂O-CaO–Al₂O₃–SiO₂ system at 25 °C. At cement relevant pH, the dominating phase is a solid solution designated C-A-S-H having the compositional range 0.72 < CaO/SiO₂ < 1.94 and 0 < Al₂O₃/SiO₂ < 0.1; there is negligible Na₂O content in this and in related CaO-rich phases. Note that in the present study, all phases are assumed to be water saturated.

C-A-S-H is observed to be compatible with Ca(OH)₂,

Acknowledgement

The support of NANOCEM, a European industrial/academic partnership for fundamental research on cementitious materials, is acknowledged. Valuable discussions with Duncan Herfort, Thomas Matschei, Ellis Gartner and Karen Scrivener are also gratefully acknowledged.

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Compatibility studies between N-A-S-H and C-A-S-H gels. Study in the ternary diagram Na2O–CaO–Al2O3–SiO2–H2O

Abstract

Introduction

Section snippets

Experimental procedure

Precipitation/dissolution approach

Conclusions

Acknowledgement

Cem. Concr. Res.

Cem Concr. Res.

Cem. Concr. Res.

Cem. Concr. Res.

Micropor. Mesopor. Mater.

Cem. Concr. Res.

Geopolymer technology: the current state of the art

J. Mater. Sci.

Compatibility studies between N-A-S-H and C-A-S-H gels. Study in the ternary diagram Na₂O–CaO–Al₂O₃–SiO₂–H₂O