Effect of hydration temperature on the solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pastes
Introduction
Temperature is a key variable affecting the curing of cement-based materials because it influences both the early hydration kinetics and the properties of the hardened cement paste or concrete. Although concrete initially gains strength more rapidly when cured at elevated temperatures, the final strength is lower and the permeability is higher. Elevated temperatures also reduce the tendency for irreversible creep and shrinkage. Most of these effects can be related to the increased rate of silicate polymerization at elevated temperatures [1], [2], [3], which densifies and stiffens the C-S-H as it forms. SEM studies [4], [5], [6] have shown that at elevated temperatures the outer-product C-S-H gel is denser and does not fill the capillary pore space as effectively, and thus the microstructure is more heterogeneous. Elevated temperatures may also alter the equilibrium assemblage of solid phases in cement paste. A particularly important example is the relative stability of ettringite and calcium monsulfoaluminate (monsulfate). During the early hydration period when sulfur concentrations in the pore fluid are high, ettringite is more stable than monosulfate at ambient temperatures. As the temperature increases, monosulfate eventually becomes the favored phase due to its smaller enthalpy of reaction. Lack of formation of ettringite during initial curing can lead to an expansive and damaging condition known as delayed ettringite formation (DEF) [7].
This study reports measurements of the concentration of major elements in pore solutions from two different types of cement paste cured at constant temperatures ranging from 5 to 50 °C over the first 28 days of curing. Of particular interest were any changes in the solubility behavior of C-S-H gel with temperature, and the changes in the calculated relative stability of ettringite and monosulfate as a function of temperature.
The hydration of cement produces solid phases in intimate contact with a pore system, and the composition of the pore solution can thus provide information about the hydration products. Comparing the ion activity product for a solid phase with its equilibrium solubility product gives the degree of over or undersaturation, and tracking this saturation level over time, or comparing the levels in different types of paste, can be quite useful. Dilute suspensions of cement powder are not representative of normal cement paste and concrete, and thus hardened cement pastes should be used. Pore fluid can be extracted from cement paste after set by applying pressure to crushed paste with a steel die [8], [9]. A number of studies of this type have been conducted (e.g., [10], [11], [12], [13], [14], [15]), and the basic solubility behavior in the Na, K, Ca, S, and hydroxyl system has been established. Solutions become modestly supersaturated with respect to portlandite a few hours after mixing, and then saturation is approached slowly over a period of several days. Gypsum, which is present in the starting cement, is saturated during the first few hours of hydration and then undersaturated once it is consumed through reaction with C3A to form less soluble calcium sulfoaluminate phases. The details of this process depend on the presence of free lime, the time to set, the presence of mineral admixtures, and other factors.
Few studies extending past the first few hours have reported concentrations of aluminum and silicon, which occur at such low concentrations that they are difficult to measure. Their neglect, while it does not significantly affect calculations for portlandite and calcium sulfate, prevents saturation levels of the calcium sulfoaluminate phases and C-S-H from being calculated. Exceptions include studies by Lawrence [13], Xue et al. [15], and recent work by the present authors [16], all conducted near room temperature. The latter study [16] reported elemental concentrations of Ca, S, Na, K, Al, and Si during hydration of two types of Portland cement at 20 °C, along with the saturation levels with respect to ettringite (AFt) and calcium monosulfate (AFm), as well as a few different ion activity products for C-S-H. The present study extends this work to different temperatures. In addition, data from the three studies noted above are reanalyzed to try to obtain a more complete picture of the solubility behavior of C-S-H gel in hydrating cement paste.
Section snippets
Experimental
Type I ordinary Portland cement (OPC) pastes were mixed at a water-to-cement ratio (w/c) of 0.4 by weight and hydrated under sealed isothermal conditions at 20, 30, 40 and 50 °C. White Portland cement (WPC) pastes were mixed at w/c=0.5 and hydrated at 5, 20, 37, and 50 °C. The composition of the cements are listed in Table 1. The WPC has significantly lower alkali content than the OPC, and this, along with the higher w/c used to hydrate the WPC, led to significantly lower alkali contents and
Thermodynamic analysis
Calculating saturation levels requires measuring the elemental concentrations in the pore fluid and then performing a thermodynamic analysis to obtain ionic activities. In electrolytes such as cement pore solutions, each element exists as more than one ionic species. These species can be simple charged ions such as Ca+2, or ion pairs such as CaOH+ and CaSO4°. Calculating the equilibrium ionic activities from the elemental concentrations is an iterative process that is facilitated by the use of
Elemental concentrations
Fig. 1 shows the total alkali (Na+K) concentrations in the OPC and WPC pastes at each temperature. The alkali concentrations in the OPC paste are significantly higher than in the WPC paste, as expected. In both pastes, alkali concentrations increase with time as the hydration reactions consume liquid water and concentrate the pore solutions. This process occurs more quickly at higher temperatures, leading to differences in the alkali concentration at earlier times. At later times the alkali
Summary
The elemental concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of normal w/c OPC and WPC pastes hydrated at temperatures in the range of 5–50 °C were reported for the first time. Changes in the saturation levels during the early period of hydration occurred more rapidly at higher curing temperatures, as expected. At all temperatures, portlandite exhibited small and decreasing levels of supersaturation, and gypsum was saturated during the first few hours and greatly
Acknowledgements
This work was supported by the National Science Foundation under contract CMS-007-0922.
References (30)
- et al.
Silicate polymerization during the hydration of alite
Cem. Concr. Res.
(1983) - et al.
Backscattered electron imaging of cement pastes hydrated at different temperatures
Cem. Concr. Res.
(1990) The effect of heat treatment on inner product C-S-H
Cem. Concr. Res.
(1992)- et al.
Microstructural features of concrete in relation to initial temperature—SEM and ESEM characterization
Cem. Concr. Res.
(1999) - et al.
Expression and analysis of pore fluids from hardened cement pastes and mortars
Cem. Concr. Res.
(1981) - et al.
Evaluation of the validity of the pore solution expression method from hardened cement pastes and mortars
Cem. Concr. Res.
(1994) - et al.
The chemistry of the aqueous phase of Portland cement
Cem. Concr. Res.
(1982) Effects of two Danish flyashes on alkali contents of pore solutions of cement–flyash pastes
Cem. Concr. Res.
(1981)- et al.
Solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pore solutions as a function of hydration time
Cem. Concr. Res.
(2002) - et al.
Solubility of ettringite (Ca6[Al(OH)6]2(SO4)3 26 H2O) at 5–75 °C
Geochim. Cosmochim. Acta
(1999)
Solubility properties of ternary and quaternary compounds in the CaO-Al2O3-SO3-H2O system
Cem. Concr. Res.
Thermodynamic investigation of the CaO-Al2O3-CaSO4-H2O system at 50 °C and 85 °C
Cem. Concr. Res.
The solubility of ettringite at 25 °C
Cem. Concr. Res.
Ettringite solubility and geochemistry of the Ca(OH)2-Al2(SO4)3-H2O system at 1 atm pressure and 298 K
Chem. Geol.
Thermodynamic investigation of the CaO-Al2O3-CaSO4-H2O system at 25 °C and the influence of Na2O
Cem. Concr. Res.
Cited by (111)
Roles of ultra-fine waste glass powder in early hydration of Portland cement: Hydration kinetics, mechanical performance, and microstructure
2024, Construction and Building MaterialsThe effect of varying cement replacement level on alkali metal distribution in cement pastes
2024, Cement and Concrete CompositesMorphology-structural change of C-A-S-H gel in blended cements
2023, Cement and Concrete ResearchCovalently bonded AMPS-based copolymer–C–S–H hybrid as a fluid loss additive for oilwell saline cement slurry in UHT environment
2023, Construction and Building MaterialsPhase development of hydrated cement pastes with SCMs under delayed heating conditions below 100 °C
2023, Construction and Building MaterialsNovel agricultural waste based hardening accelerator for early strength development of fly ash-based concrete
2023, Materials Today: Proceedings