The effect of alkali hydroxides and equilibration time on Al uptake by calcium silicate hydrates (C-S-H)

Cement production accounts for approximately 8% of man-made CO2 emissions. Lowering these CO2 emissions is currently one of the most important and urgent research topics within the cement community. To reduce these emissions, the Portland cement (PC) is partially replaced by supplementary cementitious materials (SCM) such as blast furnace slag, fly ash or silica fumes. Reaction of these SCM with PC during hydration leads to the formation of additional calcium silicate hydrates (C-S-H), which is the single most important phase in cements based on silica-rich SCM. The high Al2O3 and SiO2 content of the SCM results in C-S-H compositions with more Si and Al than in PC, which affects the stability and durability of such cements. Therefore, it is crucial to determine the role of Al on C-S-H properties to predict the formed hydrate phase assemblages and their effect on durability. Al sorption isotherms including very low Al concentrations have been determined for C-S-H with Ca/Si ratios from 0.6 to 1.4. The solubility, structure and composition of calcium silicate hydrates incorporating aluminum âC-A-S-Hâ as a function of different parameters such as Ca/Si ratio, equilibration time, Al and alkali contents were investigated. Elemental measurements were performed with ICP MS and ICP-OES. The presence of secondary phases was investigated by using TGA and XRD and the structure of C A S H was investigated by FTIR. High alkali hydroxide concentrations led to an increased Al(OH)4- formation in solution, which reduced the Al uptake in C-S-H. Increasing the pH values and decreasing the Ca/Si in C-S-H increased not only the Al concentrations but also in parallel the Si concentrations in solution. This comparable behavior of Al and Si towards changes in pH, pointed towards the uptake of Al within the silica chain both at low and high Ca/Si ratios. A higher Al uptake in C-S-H was observed at higher Ca/Si ratios, which indicated a stabilizing effect of Ca in the interlayer on Al uptake. The effect of equilibration time on Al uptake in C-S-H was investigated using equilibration times from 7 days up to 3 years. Lower Al concentrations were measured in the solution after longer equilibration times. In addition, a decrease in the content of secondary phases was observed by TGA indicating a higher uptake of Al in C-S-H. Little further decrease in Al concentrations was observed after 2 years and longer at low Ca/Si ratios. At high Ca/Si ratios no significant change in solution concentrations was observed after more than 3 months, while the destabilization of secondary phases continued up to 1 year, indicating that a (meta)stable equilibrium was reached faster at higher Ca/Si ratios. In addition to the C-A-S-H phase, the formation of secondary phases such as strÃ€tlingite, katoite and Al(OH)3 was observed at Al/Si â¥ 0.03, which limited the Al uptake in C-S-H. More secondary phases were observed at higher Al concentrations and/or lower pH values. At low Al/Si ratios, a more significant decrease of Al concentrations with time was observed indicating a slower equilibration than at higher Al concentrations. The Al sorption isotherms showed a linear trend between the Al in solution and Al in C-A-S-H from Al/Si of 0.001 up to 0.2. The linear trend pointed towards an Al uptake on one or several types of sorption sites, with a high sorption capacity, which would be consistent with an Al uptake in the bridging position of the silica chains suggested based on NMR studies

Effect of alkali, aluminium and equilibration time on calcium-silicate-hydrates

Yu Yan

Calcium silicate hydrate (C-S-H) is the main hydration product in Portland and blended cements, and greatly affects durability and mechanical properties of the hydrated cement. In the presence of Al-rich supplementary cementitious materials (SCMs), C-(A-)S-H can contain more Al than C-S-H in plain Portland cements. The use of SCMs in cementitious system not only lowers CO2 emission, but may also enhance the mechanical properties in the long term, its durability and can help to suppress alkali silica reaction. However, important questions regarding the effect of Al, alkali and equilibration time on C-A-S-H structure and solubility are still not fully understood. This thesis focuses on the interplay between aluminium, alkali hydroxides in solution, the structure of C-A-S-H and aluminium, alkali uptake by C-A-S-H and kinetics of C-A-S-H formation. For aluminium free C-S-H, KOH and NaOH have a similar effect, they increase pH values and silicon concentrations, and decrease calcium concentrations. At higher alkali hydroxide concentrations, more portlandite precipitates, while amorphous silica dissolves. This increases the Ca/SiC-S-H at low Ca/Sitarget but lowers the maximum Ca/SiC-S-H from 1.5 to 1.2 in 1 M KOH/NaOH. The amount of alkalis bound in C-S-H increases with increasing alkali hydroxide concentrations and is higher at low Ca/SiC-S-H. KOH/NaOH lead to a structural rearrangement in C-S-H, increasing the interlayer distance, number of layers stacked in c direction and shortening the silica chains. Comparison with the independently developed CASH+ thermodynamic model showed a good agreement between the observed and modelled changes, including the shortening of the MCL.For C-A-S-H with Ca/Si=1, the presence of Al led to higher amounts of secondary phases, larger interlayer distance, longer silicate chain, higher dissolved Al and Si, and lower Ca concentrations. The amount of secondary phases is reduced at higher pH; strätlingite and Al(OH)3 dominate at lower pH, and katoite at higher pH values. High pH is also associated with shortened silicate chain length, increased Al and Si concentrations, maximum Al/SiC-S-H and lower Ca concentrations. The Al uptake in C-A-S-H increased with aluminum concentration in solution, indicating the predominance of a common Al sorption mechanism independent of dissolved Al concentration or pH values. XANES data showed the presence of both Al(IV) and Al(VI) with different chemical environments in C-S-H.The effect of equilibration time on C-A-S-H was investigated from 6 hours to 3 years. Rapid changes within the first day were observed in both the solid phase and the aqueous phase. Portlandite was formed in all samples with different Ca/Si and its amount decreased rapidly within the first day and remained stable after more than 7 days. Gismondine-P1 precipitated between 1 and 3 years and destabilized the C-A-S-H phases at target Ca/Si 0.6. In the solid phase, the Al migration from secondary phases to C-A-S-H and an increasing fraction of Al(V) and Al(VI) in C-S-H were observed with longer equilibration time. The Ca/Si ratio of C-S-H played an important role on the extent of changes in aqueous concentration: an increasing of Al concentrations with time was observed in C-A-S-H with Ca/Sitarget=0.6, while a decrease was observed for C-A-S-H with Ca/Sitarget=1.0. The uptake of Al and Na in C-A-S-H was observed to decrease after 4 days and 1 day separately, which indicates a rearrangement of C-A-S-H with time.

EPFL2022

Adsorption de polycarboxylates et de lignosulfonates sur poudre modèle et ciments

François Perche

Placing concrete requires much more water than the cement needs for its hydration. This results in an important porosity in the hardened concrete, which accentuates the degradation of this material. By adding small amounts of polymeric admixtures, called superplasticizers, to the fresh concrete one can significantly reduce the amount of water required to obtain the suitable workability. The plasticizing effect of superplasticizers has been studied for many years, and remains today, an ongoing field of research. This research led to a better understanding of the forces that act during the deflocculation and dispersion of cement grains induced by superplasticizers. This research work was performed within the framework of the "Superplast" European project. Its main objective has been to determine which parameters (molecular structure, induced charge, adsorption mode, molar mass) influence the plasticizing effects of superplasticizers. The results of this study will allow synthesis new polymeric admixtures with better performances than those currently available. For this study, two types of superplasticizers were selected: lignosulfonates and polycarboxylates. For each one, different samples were synthesised and characterised. For each sample, adsorption, rheological and interaction forces measurements were performed by our partners while we have performed adsorption and electroacoustic measurements on model powder suspensions. Different cement model powders were investigated and a magnesium oxide was selected. The particle size distribution and reactivity were carefully characterized. The inert model system (MgO) allowed us to study adsorption mechanisms without the complexity linked to cement hydration reactions that modify surface and solution of the suspension. Particle surface charge and pH are the two main parameters that influence the polymer adsorption and the polymer conformation. Magnesium oxide which has a high isoelectric point (around at pH 12.4) allows a surface charge similar to cement suspensions at high pH. First, we have measured the adsorption isotherms of all superplasticizers on model suspensions in NaOH (0.01M). This study allowed us to evaluate the affinity of each polymer for MgO and its adsorption plateau. These measurements showed that the lignosulfonates have a higher affinity than the polycarboxylates. They also showed that lignosulfonate adsorption is mainly influenced by their molar mass and their carboxylic group content (for similar sulfonate group content) while polycarboxylate adsorption is mainly driven by the backbone length, the side chain length and the carboxylic group content. Adsorption plateaux allowed us to calculate the surface coverage ratio and to estimate the superplasticizer conformation on the surface. Adsorption isotherms were finally measured on a Portland cement. The polymer adsorption is influenced by the same parameters as on MgO. The model system MgO is representative for the polymer adsorption on cement. In a second step, different electrolytes were added to model suspensions. This practice allowed us to study separately the effect of the main ions present in cement suspensions (Na+, Ca2+, SO42-, OH-) and to mimic the ionic composition of the aqueous phase of the cement suspension. Lignosulfonate adsorption is neither influenced by the studied ions or the pH. Only an increase of ionic strength increases the adsorbed polymer mass. Polycarboxylate adsorption is influenced by calcium and sulfate ions and by the particle surface charge. Finally, superplasticizer adsorption was studied on different cements by our partners and on a fly ash and a silica fume by ourselves. Lignosulfonate adsorption isotherms on cements showed that affinity and adsorbed polymer mass increases with the C3A content and decreases with the alkalis content. The isotherms measured on industrial by-products showed that superplasticizers adsorb on fly ash, but not on silica fume.

EPFL2004

Aluminium and alkali uptake in calcium silicate hydrates (C-S-H)

Calcium silicate hydrate (C-S-H) is the main hydrate phase in Portland cements. Its composition varies depending on the Ca/Si ratio and on the presence of other ions such as aluminium (called CA- S-H) or alkalis. This thesis aimed to study the composition and the structure of C-S-H. An experimental data base was realised on the effect of different Ca/Si and Al/Si ratios, alkali concentrations (potassium and sodium hydroxide) and of different temperatures on C-S-H after equilibration times up to 1 year. The composition of the solid phases was analysed by TGA, XRD/Rietveld analysis and 29Si and 27Al MAS NMR. The solutions were analysed by ionic chromatography and pH measurements. In the absence of alkali, only C-A-S-H is formed at Al/Si ≤ 0.1. At higher Al/Si ratios, katoite, stratlingite and/or AH3 precipitate in addition to C-A-S-H, such that a part of the aluminium is consumed and limits the Al/Si ratio in C-S-H to 0.15±0.05 regardless of the Ca/Si ratios. A strong correlation between the aqueous aluminium concentration and the aluminium uptake in the solid phase is observed. Aluminium in C-S-H is observed mainly as tetrahedrally, Al(IV) or octahedrally coordinated, Al(VI) aluminium. At high Ca/Si ratios, Al(VI) is favoured and the aluminium uptake in C-S-H increases with the aqueous aluminium concentration. At low Ca/Si ratio (≤ 0.8), aluminium is observed mainly as Al(IV) and the low aqueous aluminium concentrations (below detection limit) indicate a higher affinity of aluminium towards C-S-H. The uptake of aluminium increases slightly the mean basal spacing slightly at low Ca/Si ratio and up to ≈2Å at higher Ca/Si ratios due to incorporation of aluminium and calcium in the interlayer. The presence of alkali hydroxide increases the pH leading (i) to a destabilisation of stratlingite and (ii) to higher aqueous silicon and aluminium concentrations and to lower calcium concentrations. As the aluminium uptake in C-S-H is strongly related to the aqueous aluminium concentrations, the aluminium uptake in the C-S-H increases significantly in the presence of alkali hydroxides, as aluminium is more soluble at higher pH. The increase of the temperature from 7 to 80 °C increases the phase-purity and long-range order of the C-S-H. The mean chain length remains stable up to 50°C, while at 80°C chain length increases and crosslinking is observed in the presence of aluminium. At all temperatures studied, the presence of aluminium has little effect on the calculated solubility of C(-A)-S-H. The incorporation of alkali ions in C-S-H increases with the initial concentration of alkali and with the decrease of Ca/Si ratio. The aqueous calcium concentration play a dominant role in alkali uptake, as calcium competes with the alkali ions to charge balance the negatively charged silanol groups of the C-S-H. A low concentration of calcium favours the alkali uptake in C-S-H. The uptake of alkali ions shortens the silica chain length as calcium is redistributed from the interlayer to the main sheet. The alkali uptake is not significantly influenced by the presence of aluminium in C-S-H, the hydration time or the type of cation (sodium or potassium).

EPFL2014