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The Influence of Sulfate Availability on Rheology of Fresh Cement Paste

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The Influence of Sulfate Availability on Rheology of Fresh Cement Paste Appl. Rheol. 2020; 30:54–63 Research Article Willy Mbasha, Rainer Haldenwang*, and Irina Masalova The influence of sulfate availability on rheology of fresh cement paste https://doi.org/10.1515/arh-2020-0106 ity is regulated through preferential precipitation of sulfo- Received Jul 02, 2019; accepted Jun 20, 2020 aluminates. After the first contact between cement and water, the Abstract: Natural gypsum can degenerate into hemihy- calcium concentration in the interstitial fluid of fresh ce- drate during cement clinker grinding which changes the ment pastes does not vary much with the water-to-cement physical and chemical properties of cement hydration, af- mass ratio. In most cases, calcium ions are around sat- fecting therefore the fresh and hardened properties of ce- uration concentrations and favour the precipitation of ment based materials. Cement systems containing a con- portlandite. Conversely, the concentration of sulfate ions stant total amount of calcium sulfate (4%) with relative within cement suspensions varies depending on the water proportions of hemihydrate and natural gypsum were to cement mass ratio (w/c) along with the amount and sol- considered. Rheological measurements were executed on ubility of calcium sulfate sources in the system [1]. an Anton Paar MCR51 rheometer to evaluate the flow In practice, the amount and type of sulfate calcium properties of cement pastes. Results show that, the yield sources are generally chosen by the cement manufac- stress and the plastic viscosity of cement pastes were turers by considering the C A and alkali sulfate con- affected when the degeneration of natural gypsum ex- 3 tents present in the clinker. These can vary between gyp- ceeded 50%. Above this concentration, the yield stress sum (CaSO ·2H O), hemihydrate (CaSO ·0.5H O), anhy- remarkably increased and a variation in plastic viscosity 4 2 4 2 drite (CaSO ) or mixtures of these sulfate sources. Even of about 50% was observed. Using TG-DSC techniques, it 4 when faced with a pure gypsum-based system, through the was shown that, the amount of formed ettringite could grinding process, natural gypsum may dehydrate to hemi- not explain these rheological changes. However, centrifu- hydrate depending on the mill ambient temperature. The gational packing and SEM-SE measurements confirmed final ground cement product results therefore into two dif- that, more than the amount of ettringite precipitated, et- ferent types of sulfate bearing materials with variant solu- tringite morphology plays a major role in controlling the bility. yield stress and plastic viscosity of fresh cement pastes. This variability in sulfate sources dictates the sul- Keywords: Natural gypsum, hemihydrate, ettringite, rhe- fate concentration during the early hydration and conse- ology and cement paste quently, chemical or physical interactions within the ce- ment suspension [2]. Although the amount of hydrates formed during the first minute after the contact between 1 Introduction water and cement is almost negligible, variations in sulfate concentrations have been found able to affect the macro- scopic rheological behaviour of fresh cement pastes [3, 4]. In ordinary Portland cement manufacturing, clinker is Yamada in [5] demonstrated that the source of sulfate ions ground together with calcium sulfate as a source of sul- is of great significance for ettringite crystallisation. They fate to prevent premature and detrimental precipitation found that the amount and the morphology of formed et- of some calcium aluminate phases during the early stage tringite depend largely on the type and the quantity of cal- of cement hydration. Subsequently, the aluminate reactiv- cium sulfate used. Ettringite is often considered as nucle- ating under the form of a hexagonal rod with six large elon- gated rectangular prism faces and two six sided end faces. *Corresponding Author: Rainer Haldenwang: Department of Depending on the degree of super-saturation reached in Civil Engineering, Cape Peninsula University of Technology, PO Box the interstitial solution, which is itself driven by the sulfate 652 Cape Town 8000, South Africa; Email: [email protected] Willy Mbasha, Irina Masalova: Department of Civil Engineering, source solubility, ettringite develops different morpholo- Cape Peninsula University of Technology, PO Box 652 Cape Town 8000, South Africa Open Access. © 2020 W. Mbasha et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License The influence of sulfate availability on rheology of fresh cement paste Ë 55 gies that can vary from short hexagonal prisms to long nee- cium bearing materials can mainly be depicted from their dles [6–8]. CaO and SO3 contents. To date, the effect of ettringite morphology on cement rheological parameters during the early hydration has not been thoroughly investigated. The main challenge in such 2.1.1 Cement sample and characterisation investigations lies in the difficulty of assessing ettringite morphology at cement paste sample level. Relying solely Cement samples were obtained by adding directly to the on SEM analysis of cement samples containing only a few clinker 4% of calcium sulfate from a mixture of natural of these crystals should be considered only with care. In gypsum and hemihydrate in different proportions. The pre- this work, packing measurements are laterally used as a pared raw materials were homogenised in a ball mill bottle fast and easy tool to confirm the SEM results. The w/c ratios for a few hours as done in [9]. Cement samples were charac- of cement systems were kept constant along with the total terised by assessing their particle size distribution using a amount of calcium sulfate. The calcium sulfate availabil- laser particle analyser (Malvern Mastersizer 2000 Malvern ity was changed by varying relative proportions of hemi- Instruments, UK) under constant agitation at 2000 rpm in hydrate and natural gypsum powders as the source of cal- isopropanol as solvent. The equipment has the capacity to cium sulfate within the system. detect particles within the range of 0.02 – 2000 µm. Particle size distributions of cement mixes are given in Table 2. It can be seen that the addition of 4% calcium 2 Materials and methods sulfate bearing materials to the clinker does not have much effect on the specific surface area of the cement systems. The characteristic sizes such as d10, d50 and d90 of all the 2.1 Materials cement mixes also remain similar to the original clinker size. An ordinary Portland cement clinker was sampled during The total sulfate content of raw materials was esti- the production process of CEM I in accordance with ASTM mated using a Leco and are presented in Table 3. The C150 under stable kiln operations at a local cement plant nomenclature NG/HH means that the cement is made up of using modern technology. This clinker was then crushed NG percent natural gypsum fraction and HH percent hemi- and ground without any calcium sulfate bearing material hydrate fraction within the gypsum cement phase set at and was considered as the reference pure clinker. Two 4% total mass concentration. It can be noted that, such types of calcium sulfate materials, namely natural gypsum procedure results in an effective sulfate content decrease (NG) and hemihydrate (HH), were also considered. The of around 10% when totally substituting natural gypsum chemical composition of the cement clinker and calcium by hemihydrate within the cement system. sulfate materials as determined by XRF spectroscopy are presented in Table 1. The difference between these two cal- Table 2: Particle sizes distribution of the clinker and cement with different mass proportions of NG and HH for a 4% total mass of Table 1: Chemical and phase composition of clinker and calcium calcium sulfate within the system sulfate Sample ID/ d d d SSA Compounds Clinker Gypsum Hemihydrate 10 50 90 characteristic size [cm2/g] SiO 23.2 0.81 1.01 2 Clinker 5.96 16.4 36.1 3,840 Al O 5.96 0.03 0.10 2 3 NG 1.95 7.96 23.7 13,600 Fe O 3.61 0.00 0.00 2 3 HH 1.82 8.69 42.9 13,700 Mn O 0.38 0.08 0.08 2 3 100/0 4.86 15.8 35.2 4,220 TiO 0.27 0.01 0.02 2 70/30 5.35 16.2 36.5 3,990 CaO 62.38 42.08 40.63 60/40 5.80 16.3 36.4 3,880 MgO 1.68 0.46 0.56 50/50 5.32 16.1 36.2 3,990 P O 0.09 0.05 0.05 2 5 40/60 5.19 16.4 37.2 4,000 Cl 0.02 - - 0/100 4.85 15.9 35.1 4,230 SO3 0.81 44.71 52.43 K2O 0.45 0.03 0.03 Na2O 0.07 - - 56 Ë W. Mbasha et al. Table 3: Total SO3 concentration within the clinker with 4% calcium sulfate provided by a mixture of NG and HH in different proportions NG/HH concentration 0/100 40/60 50/50 60/40 70/30 100/0 SO3 [%] 2.89 2.56 2.97 2.46 2.69 2.38 Mole of sulfate/g of cement 0.69 0.63 0.62 0.61 0.59 0.55 It is worth noting that, in most commercial cements, The dynamic yield stress and the plastic viscosity were the total amount of SO3 is advised to be between 2.5-3% at obtained from the flow curve by extrapolating the mea- a calcium sulfate content of not more than 5% [10–12]. sured data of the down curve to zero shear rate using avail- able rheological constitutive equations. Since the famous “Yield stress Myth” paper was published by Barnes and 2.1.2 Cement paste preparation Walters in 1985 [16], much debate has raged on whether the yield stress is real or not.
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