Tag Archives: Hydration degree

Influence of Particle Size on the Early Hydration of Slag Particle Activated by Ca(OH)2 Solution

Authors: Zhijun Tan, Geert De Schutter, Guang Ye, Yun Gao, Lieven Machiels

Abstract:

This paper investigates the influence of slag particle size on its hydration speed at particle level in the early age. Slags were separated with sieves into groups of different size fractions, considering a wide range of sizes. The chemical compositions of each group were analyzed by X-ray Fluorescence (XRF). Activated by 15% Ca(OH)2 (by mass) at water/powder ratio 1:1, the hydration heat evolution was recorded by isothermal calorimetry up to 84 hours and converted to hydration degree. Based on the hydration degrees and particle size distributions, the rate of increase of hydrating layer thickness of each single slag particle (k value) was calculated. Results reveal that k values of coarse particles are higher than that of fine particles. Coarse particles contain higher content of CaO but relatively lower content of MgO, Al2O3 and SiO2, resulting in higher reactivity index of (CaO+Al2O3+MgO)/SiO2.

Keywords: Blast-furnace slag, Particle size distribution, Reactivity, Hydration degree

Full paper link.

The Influence of Slag on the Hydration of Cement

These following conclusions are summarized from the work of Kocaba’s PhD thesis.

  • alite: no influence is shown on the consumption of alite measured by XRD.
  • belite: the substitution of cement by both slags seems to result in a delay in the hydration of belite in the first days.
  • aluminate phase: there is a filler effect using inert filler at about 12 hours of hydration, which shows slag can also has filler effect in the early hydration period. Transformation of AFt to AFm causes cumulative heat shoulder at about 60 hours.

For all systems, slags did not have a strong influence on hydration of C3A phases. Taking into account the low content of C3A and the corresponding error, it was difficult to highlight any relevant difference between blended paste and corresponding pure pastes.

There was no evidence of slag itself reacting and the effect of slag on aluminate phases can be only attributed to a filler effect.

The raw calorimetry curves of pure cement system showed a peak (called IV) which was attributed to monosulfoaluminate reaction just around 60 hours of reaction. In this way, calcium hemicarboaluminate and monocarboaluminate could be some possible AFm phases corresponding to the second peak of aluminate. But there is no evidence of that and it could be some monosulfate. The corresponding XRD patterns did not show any peaks corresponding to AFm phases at early ages which indicate a very low content if they are present.

  • Ferite: The slags seem to favour the hydration of the ferrite phase.

Influence of slag on the degree of reaction of cement

From XRD-Rietveld refinement and SEM-IA, the degree of reaction of cement did not seem to be strongly affected by the slag.

Determining the Amount of Reacted Slag in Blended Cement Using EDTA Method

EDTA-slag

1 Procedure of EDTA

A typical chemical reaction of a slag could be like the following [NIST, D.P Bentz],

C7.88S7.39M3A + 2.6CH + bH → 7.39C1.42SHmA0.046 + 0.66M4.6AHd

C=CaO, S=SiO2, M=MgO, A=Al2 O3, CH=Ca(OH)2, H=H2O.

The method EDTA (Ethylenediaminetetraacetic acid) has been described by Erntroy [Erntry, 1987]. The following is a short description of the method.

  • 1. 93.0g of disodium EDTA 2H2O is dissolved in a mixture of 250 ml triethanolamine and 500 ml water. The solution is transferred to an volumetric flask;
  • 2. 173 ml of diethylamine added and the mixture made up to 1000 ml with water;
  • 3. For the extraction, 50 ml of the above solution is pipetted into a beaker and diluted to approximately 800 ml with water;
  • 4. The solution is brought to a temperature of 20.0±2 ◦C and 0.5 g of the dried and powdered sample paste, weighed to the nearest 0.0001 g, sprinkled over its surface;
  • 5. The solution is stirred for 120 ± 5 min while maintaining the stated temperature and is then filtered under vacuum through a 90 mm diameter Whatman GF/C filter which had been previously washed with 100 ml of distilled water, dried and weighed;
  • 6. The residue is then washed 5 times with 10 ml lots of distilled water, fried at 105 ◦ C for 1 hour and weighed to the nearest 0.0001 g.
  • 7. Calculation of reaction degree of slag.

2 Calculation of reaction degree of slag

Take Msl g slag and MCH g Ca(OH)2 and MH g H2O as reactant. When the test age reaches, say 3 days, take two pieces of paste, dry and weigh
them, then,

  • 1. one is ground to powder for EDTA test;
  • 2. the other piece undergoes a ignition loss test (LOI), from the LOI, the slag mass fraction in the dried paste powder can be determined, the ignition loss of CH should be taken into account, for Ca(OH)2 is decomposed to CaO and H2O.

The LOI fraction is: fLOI, thus, 1 − fLOI is the mass fraction of CaO+slag, so the original slag content in the powder can be determined.

2.1 Correction of hydrotalcite

h= Mass of dried hydrotalcite formed from 1 g of MgO in the slag glass.

The value of h could be 2.35 g based on the assumption in the paper “Degrees of reaction of the slag in some blends with Portland cement”, there the hydrotalcite is considered as M5ACH7.

Therefore, the residue of slag should minus the residue formed from MgO. The content of MgO can be determined by the content of slag of the tested powder.

2.2 Correction of slag

A small amount of slag may be solved in the reagent EDTA solution.

It is better do a pure unreacted slag test using EDTA, to determine the fraction of slag dissolution.

If the dissolution of slag in EDTA is quite small, then there is no need to take account for correction for solved unreacted slag in EDTA reagent, assuming no unreacted slag is solved in the EDTA solution.

A well formatted file (PDF) of the content on this webpage may be downloaded via this link.