Category Archives: Blended cement

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.

How much is the bulk density of hydrating (blended) cement paste?

In the last post, I explained the clear definition of different densities. Among these densities, bulk density and apparent density are the two most important values when performing mercury intrusion porosity (MIP) experiment.

Since samples that undergo MIP are usually irregular, the bulk volume is not possible to measure without being immersed in liquid. By the help of MIP, the bulk volume can be measured, thus the bulk density can be calculated simply dividing the mass by bulk volume.

At the end of MIP, the intruded volume of mercury at corresponding pressure is recorded, which means the pore volume is known. As soon as the bulk volume and pore volume are known, the solid volume including closed fine pores is also known. Then the porosity and apparent density are easily calculated.

The density of cement particles is commonly referred as 3.1 g/cm^3, and that of slag and limestone are 2.6 and 2.7 g/cm^3, respectively. You may be curious to know how about the bulk density of cement paste. Are they higher and lower than the raw materials? Are they stable values as curing ages extend and thus more hydration products formed?

Recently, several cement and blended paste samples of mine have been tested using MIP. I list the bulk density results as below. All the pastes are mixed at water : powder = 0.4, sealed and cured at 20 °C. As each scheduled curing age (1, 3, 7, 14, 28 and 91 days) reaches, the hydration were stopped by liquid nitrogen, and later freeze dried to remove the frozen free water.

porosity-cement
Bulk density of hydrating (blended) cement pastes.

From the results, the bulk density of cement or blended cement ranges between 1.4 to 1.8 g/cm^3, much lower than raw material, and blended cement paste, such as the ternary blended cement paste, constantly has lower bulk density. This show that the hydrating blended cement pastes are more porous, which are further confirmed by tested porosity results below.

bulk-density
Porosity of hydrating (blended) cement pastes.