Fast set GGBS Binder

To reduce CO2 emission producing Ordinary Portland cement (every ton of Portland cement accounts for roughly 850 kg CO2) the use of supplementary cementing materials (SCM's) are becoming todays standard in the cement and construction industry.


Introduction
To reduce CO2 emission producing Ordinary Portland cement (every ton of Portland cement accounts for roughly 850 kg CO2) the use of supplementary cementing materials (SCM's) are becoming todays standard in the cement and construction industry.
Although some SCMs are used on their own, most of them are used in combination with Portland cement clinker.SCM's can be divided in latent hydraulic or pozzolanic: In all cases, SCM's/OPC blends should comply with the minimal cement standards for making concrete.In most cases, the reduction of Portland cement clinker content leads to reduced initial strength.
GGBS (Ground Granulated Blast-furnace Slag) is a cementitious material, a by-product from the production of iron through a blast furnace.
Silicates and alumino-silicates of calcium are the main minerals.
Molten slag is quenched rapidly with water giving so-called granules < 5 mm.
After drying these are ground in a ball mill to a fineness similar to that of Portland cement.
On its own GGBS is latent hydraulic, showing poor early strength caused by a slow hydration.
Crystalline aluminosilicate glass is formed on the surface of the GGBS particles slowing down hydration, i.e. it needs some form of activator.
Normally an alkaline or caustic solution; sodium-, potassium hydroxides or sodium silicate will break up the aluminosilicate glass layer, allowing hydration to start.
In our quest in finding an optimum replacement for fastsetting Portland cement, we explored the effect of adding a finer ground special GGBS containing the elevated amount of aluminium to a regular ground slag.
Aim was to accelerate the setting time of a pure GGBS binder and improve its early strength development.From different publications, we learned that fast-setting cements contain C11A7.CAF2 similar to the phases calculated from our Type II GGBS.

Ground Granulated Blast Furnace Slag
The chemical composition of GGBS varies depending on the composition of the raw materials and flux agents used.
The so-called granules are then ground to a fineness similar to that of Portland cement and can be considered as part replacement.
In our study, two different types are being used.Tables 2  and 3 show the chemical constituents.
The aim is to obtain a fast-setting, high early-strength cementitious binder without the use of Portland cement

Production
European standard EN 197-1 limits the amount of replacement in Portland slag cement to not more than 35%.Generally, this type of cement has better sulphate and chloride resistance and generates less heat on hydration than Portland cement on its own.On its own, however, the material is latent hydraulic and developing poor strength, and need some form of activator.Alkali released on the hydration of cement is the most effective and commonly used.Separate added non-cement alkali activators in the form of sodium-or potassium oxide (Na2O -K2O) or derivates like sodium hydroxide (NaOH), and sodium silicate (Na2SiO3) are being used.
Possible downsides in using GGBS need mentioning are: 1. reduced availability of roughly 350 Mt/y 2.
increased demand to reduce carbon footprint (cement industry) 3.
forced closure of furnaces due to high energy cost (Arcelor Mittal plans closure of two blast furnaces in Germany and Spain) 4.
increased use of recycled steel reduces the availability Today calcium aluminate cement or calcium sulfo aluminate cement is also used in combination with GGBS to improve early strength and reactivity.
In this case, we refer to earlier published studies we have undertaken on "Amorphous Calcium Aluminate Cement" and "Flash Calcined Alumina".The focus of this investigation is on developing a complete cement-free binder having the same characteristics as fast-setting Portland cement.To obtain this our study is on blending a regularly available slag (Table 2) with a special slag having a considerably higher amount of alumina (see Table 3).It is known that this type also contains a certain amount of Calcium Fluoride. 1.
In the first stage of our investigation, we searched for the optimum combination of the two types to see if we could come to a satisfactory setting time and early strength development.

2.
In the second part of our investigation, we took the best-performing blends from stage 1 and tested the best type and addition rate of calcium sulfate.

3.
In the third stage of our work, we made a mortar according to EN-196 -binder:normsand ratio 1:3 parts by weight.4.
Stage IV looked at the effect the addition of calcium sulfate had on the change in volume 4

Materials and Methods
Over the years we have been involved in the development of fast-setting binders with a close eye on consistency and environment.In these cases, we investigated the use of forms of calcium aluminate and calcium sulfate.Now our goal is to propose a fast-setting binder consisting completely of industrial by-products -GGBS marked RG and Type XF (Table 2 and Table 3) and additional studies were done including a residual calcium sulfate (Table 4).It is known that solutions in the presence of lime, alumina and calcium sulfate react to lead to the initial formation of different, often complex hydrates but mainly: This formation improves the early strength and accelerates the drying of the binder.As this chemical reaction takes place expansion occurs, compensating for drying shrinkage.

XRD Analysis
XRD analysis shows both types of GGBS are mainly crystalline.GGBS XF showed a content around the double amount of Al2O3 -and a significantly lower amount of SiO2 -CaO is slightly higher.
Markable is the content of calcium fluoride.

Experimental
The compositions tested all included both types of GGBS.We added 1% of citric acid as a set retarder and 0,45% as a super plasticizing agent as a water reducer to facilitate the use.Testing various different ratio of these three constituents following formulation was chosen to be tested further.4.
The mentioned formulation was mixed with 1350 gr.CEN Standard sand EN-196-1Normsand and mixed according to the standard.Noted is that a higher water/cement ratio was used.

Shrinkage/Expansion
In order to measure the shrinkage or expansion of the mixture as mentioned were placed into 40x40x160mm prisms according to EN196-1.These prisms were connected to a micrometre, which digitally measured the linear length change every hour, and followed for 7 days and after.The results for up to 7 days are hereby presented.

FINDINGS
Regular GGBS marked RG showed no initial reactivity on its own and clearly needs an activating.
This study shows that regarding strength development, the fine ground special form of GGBS marked XF achieved the highest flexural and compressive strength.
Blending the two types lead to more than satisfying results.
Data from the isothermal calorimetric analysis indicate that the addition of sulphate helps in improving early strength development.Formation ettringite gave a minimal expansion leading to a reduction of shrinkage.A small retardation in set times was noticed.
Lastly, the XRD analysis of fine GGBS-XF showed the presence of calcium fluoride.It is expected this contributed to activating the regular slag used.Forming C11A7 phase is expected to have the most impact on type XF.Grinding this type to a finer particle size further boosted the reactivity further.It increased water consumption.However, pore structure and pore size were only marginally affected, and strength development was satisfactory.

CONCLUSIONS
In conclusion, data shows that the optimum blends are the GGBS RG and XF types combined with anhydrous sulfate.
GGBS can be activated by Type XF.Depending on the requirements the addition can be varied and depends largely on the chemistry of the slag available.
GGBS XF + calcium sulphate outperforms in all areas, achieving the highest compressive and flexural strengths, and the most stable shrinkage compensation.This is confirmed by the amount of ettringite formed found in the XRD analysis.
The research showed that fineness too has a significant impact on the end result, which can only in part be compensated by increasing the dosage.
In all cases, it is essential to find the right equilibrium of all components in the matrix.
Effects aimed to be achieved depend to a large extent to the type of raw materials being used.

Figure 1
Figure 1 Process of producing iron/steel by blast furnace process.Calcium carbonate of dolomite acts as a purifying flux agent.In some cases, calcium fluoride is added removing further phosphor and sulfur GGBS has a similar mineralogy as Ordinary Portland cement and is often used as part replacement of Portland cement.Its "carbon footprint" is considerably lower i.e.:  Portland cement 850 kg/to. GGBS 42 kg/to.
Strength testing was done at 24 hours, 2 and 7 days on 40x40x160mm prims confectioned in compliance with EN 196-1, Methods of testing cement -Part 1: Determination of strength.The strength results are shown in Table

Table 2
Data for regular GGBS-RG used in this study

Table 3
Data for regular GGBS-XF used in this study

Table 4
Data for anhydrous calcium sulfate

Table 5
Test slagFollowing test results are with an addition of 10% by weight of calcium sulphate.

Table 6
Test slag + calcium sulphate

Table 7
Initial chosen formulation

Table 8
Mortar test

Table 9
Mortar test results