Tag Archives: Blended cement

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.

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.

Methods to measure the reaction degree of slag in blended cement

The reaction degree of slag in blended cement is difficult to measure using those methods which are successfully applied for Portland cement. Demoulian et al. described a method based on extraction of the constituents other than unreacted slag with a reagent based on Ethylene Diamine Tetraacetic Acid (EDTA), finding it effective for determining reaction degree of slag in unhydrated blended cements.

EDTA has long been the most used and well-known method to measure the reaction degree of slag in blended cement; however, as stated by Luke and Glasser, special care should be taken when applying this method, e.g. systematic error may occur due to the residue of hydrotalcite in the extraction of unreacted slag.

Lumley et al. reported that EDTA extraction method can be used to determine the degree of reaction of slag under certain corrections, but the accuracy is poorer at degrees of reaction above 70%.

Reaction degree

Source: WikiPedia.com

Besides EDTA method, SEM-BSE image analysis could be applied to directly estimate the hydration degree of both plain cement and blended cement paste. This method is based on the fact that cement and slag grains can be distinguished from unhydrated clinkers and paste matrix, though there is some overlap due to the similar gray levels of cement and slag. However, it should be bore in mind that using this method may produces an over-estimated result on the reaction degree of slag due to its fine particles (G. Ye).

To improve the precise of ESEM image analysis determining the reaction degree of slag, V. Kocaba studied SEM-IA-Mg-mapping method to clearly distinguish slag phase from clinkers. The methodology is based on the Mg element of slag does move during the hydration process, thus the presence of Mg can mark the unreacted slag phase. The disadvantage of this method is extremely time consuming, which is the drawback of SEM-BSE image analysis as well.

As a conclusion,there is no perfect method so far to determine the reaction degree of slag in blended cements. The best solution may be using more than one method to validate each other and get reliable result.