Duncan Herfort

A thermodynamic model for predicting the stability of thaumasite

Abstract A model based on the phase rule has been used to predict the hydrate phase mineralogy and phase proportions from the chemical composition of hydrated Portland cement altered by sulfate attack. The eight-component system on which the model is based consists of CaO, SiO2, Al2O3, Fe2O3, MgO, CaSO4, CaCO3 and H2O. The phases included in the model are C–S–H, portlandite, ettringite, hydroxy-AFm, monosulfate, monocarbonate, calcite, gypsum, thaumasite, brucite and the pore solution. The model predicts, among other things, that thaumasite, which forms at low temperature, is unstable in the presence of AFm phases, and can only form in systems that would otherwise form gypsum at higher temperatures. The model has been tested experimentally on cement pastes containing 15 and 30 wt.% limestone dust stored at 5 °C, and which were either mixed with different amounts of gypsum and stored in water, or stored in solutions of different MgSO4 concentrations. The fully hydrated pastes have been analysed by XRD and 29Si CP/MAS NMR, whilst the remaining solution was analysed by ICP. Thaumasite is only found in regions where it has been predicted to form as a stable phase.

29Si cross-polarization magic-angle spinning NMR spectroscopy––an efficient tool for quantification of thaumasite in cement-based materials

Abstract 29Si{1H} cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy is a powerful and reliable tool for the quantification of thaumasite in cement-based materials. The most efficient method for quantifying thaumasite from 29Si{1H} CP/MAS NMR spectra is described and it is shown that the method allows detection of thaumasite contents below approximately 10 wt.% with a relative precision of 15% and contents above 10 wt.% with a relative precision of 10%. The applicability of 29Si{1H} CP/MAS NMR for quantification of thaumasite is demonstrated for different Portland cement pastes and shows that thaumasite contents as low as 0.2–0.5 wt.% can be detected in cementitious systems with low concentrations of paramagnetic impurities. For a Portland cement containing various amounts of limestone dust and stored at 5 °C in a MgSO4 solution, large quantities of thaumasite have been detected. Furthermore, the quantity of thaumasite is found to be less sensitive to the amount of added limestone dust. For samples of a Portland cement with a fixed content of limestone dust but different quantities of added gypsum, the increased contents of gypsum are observed to result in larger quantities of thaumasite after prolonged hydration.

Durability of Portland Cement Blends Including Calcined Clay and Limestone: Interactions with Sulfate, Chloride and Carbonate Ions

The durability has been investigated for mortars made from a pure Portland cement (CEM I) and five Portland cement – SCM blends, using a cement replacement level of 35 wt% and the following SCM’s: (i) pure limestone, (ii) pure metakaolin, (iii) metakaolin and limestone (3:1 w/w), (iv) metakaolin and silica fume, and (v) metakaolin, silica fume and limestone. The blends with metakaolin and silica fume employ a fixed ratio for these components which mimics the alumina-silicate composition of a 2:1 clay (i.e., montmorillonite). All mortars were demoulded after hydration for one day and cured saturated in water at 20 °C for 90 days prior to exposure. Expansions induced by sulfate attack, chloride profiles, and carbonation depths were measured to investigate the durability performances of the mortars. Porosity and pore connectivity were analysed before exposure by mercury intrusion porosimetry. The results show that mortars incorporating metakaolin, independent of additional silica fume or limestone, all exhibit very high resistance towards sulfate attack and chloride ingress, but are vulnerable to carbonation. The binary Portland cement – limestone blend is most susceptible to all types of studied chemical attacks, as expected. The pure Portland cement exhibits poor resistance to sulfate attack and chloride ingress, but high resistance to carbonation. The observed performances for the different blends can be explained based on their microstructure and phase assemblages. For example, the presence of metakaolin increases the chloride-ion binding capacity and enhances chloride resistance by the low pore connectivity present in the hydrated blends with metakaolin.

Phase Assemblages in Hydrated Portland Cement, Calcined Clay and Limestone Blends From Solid-State 27 Al and 29 Si MAS NMR, XRD, and Thermodynamic Modeling

The use of calcined clays as SCM’s provides a valuable contribution to the reduction in CO2 emission associated with cement production since they can be produced at significantly lower temperatures and do not involve a calcination reaction. Moreover, it is well known that binary blends of Portland cement and small amounts of limestone powder can increase the strength of the resulting concrete. In this study we have investigated the substitution of cement by metakaolin and limestone as well as blends of cement, metakaolin, silica fume and limestone, in all cases with a 35 wt% replacement level of Portland cement. The phase assemblages of the hydrated blends are characterized by XRD, 27Al and 29Si MAS NMR spectroscopy, and thermodynamic modelling. The X-ray diffractograms show that larger amounts of the AFm phases are formed for the samples containing both limestone and calcined clay whereas 29Si MAS NMR studies of the hydrated pastes provide information about the uptake of Al in the C–S–H phase and structural details about the resulting C–A–S–H phases. 27Al MAS NMR is used to follow the formation of the hydrated aluminate species, in particular the formation of stratlingite. The results from XRD and NMR after prolonged hydration are compared with the phase assemblages predicted from thermodynamic modelling. Overall, good agreements are observed between the experiments and the modeled predictions.

Performance-Based Design Procedure Applied to the Selection of Low-CO 2 Binder Systems Including Calcined Clay

The reduction of the clinker content in concrete by replacing cement with supplementary cementitious materials (SCM) is widely used to lower the environmental impact of concrete. However, for some SCMs such as calcined clay this often leads to higher water requirements for the same concrete consistence. This severely limits the widespread implementation of green concrete technologies, both in practical terms, and in meeting the requirements of relevant concrete standards such as the national annexes to EN 206. In order to overcome this, a shift in mind-set from prescriptive to performance-based design criteria would be beneficial. Thus, we have proposed a performance-based design approach for the selection of binder systems, which we have successfully applied it to several combinations of SCMs. In this paper the procedure is described for a binder system consisting of calcined clay, limestone filler and fly ash. A parametric study was carried out on mortars, which included rheological properties, compressive strength and chloride migration. The requirements for each parameter was chosen based on the intended concrete application (highway bridge) and values obtained for a reference mix typically used for the application, i.e. a concrete that fulfils current Danish regulations. The proposed alternative binder system features a CO2 footprint reduction of 30% compared to the reference concrete mix. However, the alternative concrete mix does not conform with current Danish regulation on the minimum w/c ratio.

A discussion of the paper “Role of delayed release of sulphates from clinker in DEF” by Weislaw Kurdowski ☆

Note: Times Cited: 0Export Date: 1 June 2011Source: Scopus Reference EPFL-ARTICLE-166443doi:10.1016/S0008-8846(02)00980-8View record in Scopus URL: ://WOS:000180956500022 Record created on 2011-06-06, modified on 2017-05-10