Jelica Zelić

The role of silica fume in the kinetics and mechanisms during the early stage of cement hydration

Abstract The role of silica fume in the reaction kinetics and mechanisms of the early stage hydration of Portland-slag cement–silica fume pastes (W/S=0.5 at 20°C) has been studied. Using differential scanning calorimetry (DSC), the rate of calcium hydroxide (CH) formed during hydration has been determined quantitatively and the percentage of reaction of hydration has been calculated. The kinetic analysis was used to obtain parameters, which were employed to find out the influence of the silica fume content on the reaction rate constant value. The reaction stages have been analysed and explained by kinetic terms. The delay in CH formed during hydration between 8 and 10 h from the beginning of hydration has been noticed and a mechanism has also been discussed. The investigations have shown that the addition of silica fume of more than 8 mass% reduces the duration of the phase boundary interaction as a rate-determined process resulting in the fast diffusion rate-determining process. The results of this study have also revealed evidence of the accelerator effect of silica fume during the first 8 h of hydration when it still exists as chemically inert filler. The pozzolanic reaction between silica fume and CH formed during hydration is occurring after three days of hydration.

Durability of the hydrated limestone-silica fume Portland cement mortars under sulphate attack

Abstract The effect of silica fume, emerging as a by-product in production of ferrosilicon, on corrosion resistance to sulphate attack of Na 2 SO 4 and MgSO 4 solutions has been studied in Portland cement mortars containing limestone and mortars containing no limestone. Expansion and changes in the elasticity modulus of mortars as a function of silica fume content have been investigated. The phases formed and the microstructural changes in the mortar exposed to sulphate corrosion have been determined by differential thermal analysis (DTA), X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The presence of monocarbonate and the absence of monosulphate were detected in the mortars containing limestone. The addition of silica fume results in less CH formed by the hydration process and consequently less gypsum and ettringite during the sulphate immersion of mortars. Mortars containing more than 5 mass% of SiO 2 , or simultaneously limestone and more than 2 mass% of SiO 2 , are characterised by a good sulphate resistance and show lower expansion than the control, the sulphate-resisting mortars independent of the type of the sulphate solution.

The properties of Portland cement-limestone-silica fume mortars

Abstract This work has studied the influence of the combined action of silica fume and limestone on strength development, porosity, pore structure and morphological features in the system where 15 wt% of cement was substituted by finely ground limestone. Silica fume was added in amounts of 0, 2, 5, 8, 11 and 15 wt% on a cement basis, respectively. It has been established that limestone addition considerably increases the total porosity of mortars. However, if introduced together with silica fume up to 8 wt% of silica, porosity decreases. More than 8 wt% of silica increases the porosity again. The cement mortar containing 8 wt% of silica fume shows the highest compressive strength, the minimum value of the total porosity, and its pore size distribution curve shows a discontinuous pore structure. Limestone is taken up to the system and reacts with aluminate and ferrite phases from cement. Approximately 5 wt% is available for reaction after 120 days hydration of mortars containing no silica fume. The quantity of limestone incorporated is affected by the silica fume content. The replacement of Portland cement by 15 wt% of silica fume causes reduction both in the amount of cement and in the free CH content available for limestone chemical activity, and in this condition limestone acts only as a filler addition.

Kinetic analysis of thermal decomposition of Ca(OH)2 formed during hydration of commercial portland cement by DSC

AbstractIn this paper, evaluation of kinetic parameters (the activation energy – E,the pre-exponential factor – A and the reaction order – n) with simultaneous determination of the possible reaction mechanism of thermal decomposition of calcium hydroxide (portlandite), Ca(OH)2 formed during hydration of commercial Portland-slag cement, by means of differential scanning calorimetry (DSC) in non-isothermal conditions with a single heating–rate plot has been studied and discussed. The kinetic parameters and a mechanism function were calculated by fitting the experimental data to the integral, differential and rate equation methods.To determine the most probable mechanism, 30 forms of the solid-state mechanism functions, f(αc) have been tried. Having used the procedure developed and the appropriate program support, it has been established that the non-isothermal thermal decomposition of calcium hydroxide in the acceleratory period (0.004<αc<0.554) can be described by the rate equation: d αc/dT=A/βexp(−E/RT)f(αc), which is based on the concept of the mechanism reaction:f(αc)=2(αc)1/2.The mechanism functions as well as the values of the kinetic parameters are in good agreement with those given in literature.

Fly ash-based geopolymeric adsorbent for copper ion removal from wastewater

AbstractThe batch experiments were carried out to study the removal of copper ions from aqueous solutions by sorption process on geopolymer synthesized from fly ash (FA). The influence of solute concentration and temperature on the sorption process were examined at constant particle size and the sorption capacity of copper ions increases with the initial concentration and temperature. Several kinetic models were used to test the experimental kinetic data: Lagergren pseudo-first-order, the pseudo-second-order (Ho), and Elovich models were analyzed using nonlinear regression technique. A kinetic study has shown that the best fit is achieved when the Ho model was applied. The results obtained indicate that geopolymer synthesized from FA has excellent adsorption ability and could be used as an efficient material for the sorption of copper ions.

Kinetic studies of cobalt ion removal from aqueous solutions using fly ash-based geopolymer and zeolite NaX as sorbents

ABSTRACT This study explored the cobalt ions removal efficiency from aqueous solutions by the sorption process on geopolymer and zeolite NaX. The influence of concentration and temperature on the sorption process was examined. FTIR and SEM analyses were conducted to elucidate the structure of sorbents. An additional goal was to test the experimental kinetic data using several kinetic models. A kinetic study has shown that the best fit is achieved when the Blanchard model was applied, suggesting that the sorption of cobalt ions on geopolymer and zeolite NaX is a second-order reaction.

Synergistic Action of a Ternary System of Portland Cement – Limestone – Silica Fume in Concrete

Some experimental investigations on a synergistic action when a ternary system of Portland cement – silica fume – limestone is used in mortar or concrete are present in this paper. Standard laboratory tests with respect to the pore size distribution, micromorphology, compressive strength and sulphate resistance in both sodium and magnesium sulphate solutions were performed on mortars made with 70% (by mass) of Portland cement (PC), type CEM II/B-S and 30% of cement replacement materials consisted of various combination of fine ground limestone filler (LF) and silica fume (SF). In addition to these ternary systems, binary blends, such as: PC-LF, as well as PC-SF, along with 100 % PC mortars, were investigated for comparison. It is found that SF-blends reach higher compressive strengths than LF-blends for the same replacement of cement. When SF was added together with LF, the mortars show considerable increase in the compressive strength and show a lower expansion than a control, sulphate-resisting mortar, independent of the type of sulphate solutions, due to pore size refinement microstructure of mortars.

Reply to the discussion of the paper “Durability of the hydrated limestone-silica fume Portland cement mortars under sulphate attack”☆

The authors thank Dr. B. Mather for his interest in our paper and for his comments, which we find most helpful. We feel that we should try to clarify some things. Our experiments were performed with industrial, blended Portland cement produced in the Croatian Cement Works, with 30 percent mass blast furnace slag and the 28-day strength class of 45 MPa, marked PC30z45s according to the Croatian standard HRN.B.C1.011-conforming to the European cement standard EN 197.1:cement type: CE II-S, strength class 42, 5. Table 2 lists the potential mineralogical composition (the Bogue calcularion) of cements used. In ad-dition to this, the C_3A content of the cement clinker is 10.5°~o mass, according to Bogue. Many studies have dealt with the ettringite formation mechanism and the damage mechanism associated with ettringite formation in hardened mortar/concrete. We agree with the statement in Dr. Mathers discussion that the mech-anism of the ettringite formation is the result of complex, long-term processes, in which the cement composition, the concentration conditions and pH values in the pore solution, and ambient effects play important roles. Various study re-sults show that the ettringite formed at high pH values is probably microcrystalline (Type II according to Mehta) and less stable, and can therefore gradually dissolve in the pore liquid of a moist concrete and recrystallize in the voids, cracks, or contact zones. As a rule, the occurrence of ettringite crystals in concrete cracks is only a consequence, and seldom the cause, of the cracks. Fig. 6 shows that after the 120-day immersion in Na_2S0_4 solution, the scanning electron microscopy (SEM) of mortar containing 8 percent mass of silica fume detected large, long-needle ettringite crystals (Type I as defined by Mehta) in pores. Based on the experimental results, given in our paper, we have con-cluded that this type of ettringite crystal, formed under condi-tions of low calcium hydroxide concentration, resulting from silica fume addition to the cement mixture, is not expansive. The same photomicrograph also shows a couple of cracks transecting the void, as Dr. Mather points out. In our opinion, however, microcracks observed in Fig. 6 have only resulted from the effect of the high vacuum in the specimen chamber of the SEM. Artefacts (specimen changes, cracks) caused by the method preparation and the effects of high vacuum in the sample chamber are typical and only occur in a conventional high vac-uum scanning electron microscope (conventional SEM), as have been reported by Moser and Stark. The investigations also confirm that any information associated with contraction of the hardened cement paste observed with conventional SEM is to be regarded with caution. The sulphate performance was examined using the ASTM C 452-68 mortar bar expansion test. In conformance to this standard, we did not incorporate additional gypsum into the mortar mix and the expansion difference was measured after 28 days of water storage and thereafter every 30 days within a 6-month period of sulphate immersion. The initial length mea-surements of mortar were carried out by means the length com-parator after the 28-day water storage. We agree with Dr. Mather that the ASTM C 425-68 method applies only to Port-land cement. It is for this reason that we have used only a part of this test, as described, but not emphasized in section 1.2. (Test methods) of our paper, and have adjusted it for testing the sulphate resistance of blended cement (i.e., the Portland ce-ment-silica fume-limestone systems). The experimental pro-cedure used corresponds to the ASTM C 1012-84, for both plain Portland cements and blended cements or composite ce-ments, as Dr. Mather has suggested.

Reply to the discussion by J. Bensted and J. Munn of the paper Durability of the hydrated limestone-silica fume Portland cement mortars under sulphate attack

The authors are thankful to Drs J. Bensted and J. Munn for their interest in our paper and for their comments, which we find most useful and which provide a better insight into this complex phenomenon. The authors agree that the formation of non-bonding thaumasite sulphate attack can also occur at lower temperature conditions and influence the basic chemistry of the phenomenon. For use and improved of application in construction practice, further research is certainly needed to fully elucidate the influence of sulphate environments on the behaviour of Portland cement mortar/concrete containing ground limestone and silica fume at lower temperature conditions.