Tag Archives: Hydration

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.

Factors influence the reaction degree of slag in slag blended cement

The hydration kinetics of slag is generally divided into three stages: (1) a nucleation period during which product growth is accelerating, (2) a phase boundary controlled stage, and (3) a diffusion controlled stage.

The reaction degree of slag in blended cement is influenced by many factors. The main factors affecting the reaction of slag in blended system include the reactivity of slag that could be defined as (C+A+M)/S, the fineness of grinding (specific surface area), the vitreous fraction of slag, the replacement level of slag in blended system, the hydration temperature and the water/solids ratio.

Precisely, the rate of reaction of slag decreases with the decreasing water/solids ratio and with increasing proportion of slag in the blend. Higher hydration temperatures increase the reactivity of slag. However, the composition of cement and the incorporation of additional gypsum in slag blended cement have no influence on the extent of reaction of slag at ages of 28 days to 1 year, though variations in Portland cement can affect early strength. The following figure is a representation of the effect of hydration conditions and slag characteristics on the reactivity of slag.

factors-influence-hydration-slag
Representation of the effect of hydration conditions and slag characteristics on the reactivity of slag.