The lime-alumina-silica-water system: implications for the long-term durability of cement-steel composites Okoronkwo M.U1,2*, Glasser F.P1 1. Department of Chemistry, University of Aberdeen, Meston Walk, AB24 3UE, Aberdeen UK 2. Department of Industrial Chemistry, Abia State University Uturu, P.M.B 2000, Uturu, Abia State Nigeria
Abstract Owing to environmental and commercial pressures the constitution of Portland cement formulations is changing towards having higher aluminosilicate contents. Mainly these changes are achieved by adding fly ash, slag, etc. to Portland cement. But as the chemistry of the paste fraction changes, the hydrate mineralogy also changes and thus affects the internal evolution of pH. Phase development in the CaO-Al2O3-SiO2-H2O system has been studied by isothermal equilibration of ~70 compositions and mixtures of relevant crystalline phases at 20 – 85 °C. The pH of coexisting aqueous phases has been monitored. Data are shown as a series of isothermal phase assemblage. The pattern of phase occurrence and compatibility may contain metastable but persistent phases. Lowering of pH at high aluminosilicate contents and the long term effect on durability of cement-steel composites are discussed. Originality Isothermal equilibration of multiple compositions generated from a variety of starting reactants including gels and crystalline phases have been employed. Data are assembled into a generic approach which should be easily accessible and enable pH evolution to be predicted. The results are used to predict the passivation of embedded steel. Keywords: Phase development; assemblage, compatibility; stability; X-ray diffraction.
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1. Introduction Much research is currently directed to reduce the environmental impact of cement production and use by reducing the cement content of concrete. Supplementary cementitious materials (SCM’s) are added such as fly ash, calcined clay and slag. These SCM’s are variable in composition but generally have less CaO and more Al2O3 and SiO2 than Portland cement. As SCM’s react and the binder chemistry changes, the hydrate mineralogy also changes but with largely unknown consequences to the evolution of internal pH. Reaction of SCM’s with cement is slow and the extent of change is not always revealed in the course of short- term tests of a few years duration. Much is known about the mineralogy of Portland cement but blended cements present a challenge on account of their complex hydration reactions and reaction kinetics (De Silva & Glasser, 1993). Most relevant studies on the properties of blended systems focus on the impact of specific SCM’s on the mechanical properties and durability. Generic knowledge about fundamental relations between bulk composition and the hydrates formed, as well as their impact on the long-term development of system properties is inadequate (Lothenbach, Scrivener, & Hooton, 2011). Equilibrium studies have proven to be a fruitful approach to modelling the properties of cementitious systems. Several thermodynamic databases for cement minerals compiled from solubility data of cement phases have been published, e.g., (Atkins et al., 1992; Bennett, Read, Atkins, & Glasser, 1992; Blanc, Bourbon, Lassin, & Gaucher, 2010a; Blanc, Bourbon, Lassin, & Gaucher, 2010b; Blanc, Bourbon, Lassin, & Gaucher, 2010b; Damidot & Glasser, 1993; B. Lothenbach & Winnefeld, 2006; Lothenbach, Matschei, Möschner, & Glasser, 2008; Matschei, Lothenbach, & Glasser, 2007; Matschei & Glasser, 2010). However, the reliability of models depends on the accuracy and completeness of databases. And, owing to the metastable persistence of some phases, like C-S-H, it is also important to include in calculations phases which are known to be metastable but persistent. Elucidation of the mineralogy of hydrated cement paste has been greatly assisted by the application of materials science which enables better characterisation of fine- grained solids. The relation between phases is assisted by depicting the data on phase diagrams (Gibbs, 1961; Gibbs, 1961; Winter, 2001). However diagrams need to be specially constructed so as to identify and focus on key variables. Properly used, these diagrams enrich our understanding of property- composition relationships and establish generic relations between chemistry, mineralogy and properties such as pH This paper present a summary of studies of the phase development in the CaO-Al2O3-SiO2-H2O system cured at 20 - 85 °C (Okoronkwo, 2014). The progresses towards elucidating the mineralogical evolution mainly by isothermal equilibration of multiple compositions from variety of reactants are described.
2. Experimental About 70 different bulk compositions were generated using a variety of starting reactants. The various starting materials employed in this study include: Lime – metakaolin – water Lime – aluminosilicate gel – water 2
Lime – calcium aluminosilicate glass – water Direct reaction on selected isotherms of phase mixtures in known ratios of pre-synthesized phases such as strätlingite, hydrogarnet solid solution and C-S-H
The compositions used are projected on the ternary diagram in Figure 1. The various mixtures were reacted at 20, 55, and 85 °C. The solid products were sampled at monthly intervals up to about 12 months, dried over silica gel and characterized by X-ray powder diffraction, XRD, complimented by SEM, FTIR and/ or TG/DTA. Mineralogical examinations of dried solids were by XRD using a Bruker D8 Advance X-Ray powder diffractometer with CuKα radiation. The angular scan was between 5 – 45 °2θ with a step size of 0.02 and count time of 1 second per step. XRD patterns were collected at ~20 °C. The morphology of selected samples was examined with a Hitachi S-520 Scanning Electron Microscope. DTA/TG data were collected with a Stanton Redcroft STA-781 simultaneous thermal analyser. Previously dried samples were heated at the rate of 10 °C/min from 20 °C to 1000 °C in a flowing N2 atmosphere. Infrared spectra of samples were either collected by the Attenuated Total Reflection (ATR) method with a Nicolet IR100 Spectrometer or PerkinElmer UATR. Both spectrometers were 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 results representing the most persistent assemblage obtained for each sample are reported.
Figure 1: Bulk compositions of the some of the reactions mixtures used in the title study projected on the CaO-Al2O3-SiO2-H2O ternary. Dark purple dots represent reaction bulk compositions 3.
Results and discussion
The details of the XRD patterns, SEM micrographs TG/DTA and IR patterns of solid products (Okoronkwo, 2014) will be published subsequently. The present state of knowledge concerning the phase development in the CASH system at 20, 55 and 85 °C are summarized 3
here. The chemographic projections of the observed phase assemblage in the lime -alumina-silica-water systems (CASH) at 20, 55 and 85 °C are shown in figure 2: (a) represents the observed phase assemblage obtained at 20 °C when AFm is stabilized by almost universal minor CO2 contamination, but in the absence of CO2 and at >8 °C, the assemblage is believed to best described by figure 2b (observe the rearrangement of the sections A,B,C,D,L,M to sections numbered 1,2,3,4,5,6); (c) and (d) represent observed phase assemblages at 55 and 85 °C respectively. Some of the interesting observations includes the role of silica and temperature: at low temperatures, high silica content favours the formation of strätlingite, a siliceous AFm-type phase, but this phase is progressively destabilized at high temperatures such that it no longer coexists with C-A-S-H at ~55 °C, (figure 2c), but only occurs in aluminosilica-rich compositions such as the region marked HI in figure 2c. A noteworthy feature is that strätlingite and portlandite, Ca(OH)2, are incompatible phases. As temperature increases further, to 85 ºC, strätlingite becomes unstable and is replaced in part by gismondine-Ca at ~85 °C (Figure 2d). In figure 2, broken and dashed lines show respectively coexisting solids and tie lines while light upward diagonal patterns show coexisting gels. The heavy black line shows the composition of siliceous hydrogarnet solid solutions starting at C3AH6 and treating the range of solid solutions as continuous (Okoronkwo & Glasser, 2015b). The system is assumed to be water-saturated and the aqueous phase contains mainly OH anions, i.e., excluding carbonate and sulfate, neither of which is particularly soluble in the coexisting pore fluid. Strätlingite corresponds to C2ASH8: it has an essentially fixed composition and at realistic concentrations does not admit sulphate into its structure although some carbonate may substitute at low temperatures, 20°C or less. . OH-AFm represents the C4AHx phase. Alternating regions of two and three solid phases are lettered and single phase gel regions are shaded. The diagram is a compromise between phase equilibrium and observation, which shows that certain metastable features need to be included to describe the observations. Nevertheless the diagram looks like a proper phase diagram and the observed phase distributions permit supplementary constructions to be made and used in the same way as if equilibrium were depicted. The coexisting phases in the regions labelled in figure 2 are summarised in Tables 1 – 3. Two gel regions had been identified as shown in figure 2a: an alumina-substituted calcium silicate hydrate based gel (C-S-H based gel, or CASH), and a calcium-substituted aluminium silicate hydrate based gel (A-S-H based gel). The proposed boundaries limiting the chemical compositions of these gels are shown in figure 2. These are summarized as follows with respect to the gel phases. A C-S-H based gel (alumina-substituted): with the compositional range 0.72 < CaO/SiO2 < 2.0 and 0 < Al2O3/SiO2 < 0.1 occurs at all temperatures. The gel is homogeneous in the range 0.72 < CaO/SiO2
