Efecto de alcalis en los cementos escorias

8/10/2019 Efecto de alcalis en los cementos escorias

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nonsilicates salts obtained from weak acids (R2CO3,R2SO3, R2S)

silicates salts obtained from weak acids (R2O (m) SiO2)The activation process is influenced by the chemical com-

position of the slag, the glass content, and the activator used(Malolepszy and Nocum-Wezelik 1988).

Experimentation

Materials

Portland cementIt is obtained from grinding the clinker using about 5% of

gypsum. The chemical composition of clinker used for thisresearch can be found in Table 1.

Granulated slag of high furnaceThe slag used in the experiment is provided by the metallur-

gic unit of (El Hadjar) Annaba, Algeria. It is obtained by rapidimmersion in a cold water basin, presenting a base modulus(Mb = 1.074), ground to a specific surface of 3000 cm/g andhas a chemical composition as presented in Table 1.

Activation agents (alkalis constituents)

Several types of solutions having different densities wereprepared (see Table 2) by adding quantities of water to ob-tain desired densities. 1st group of activation agents: (caustic alkalis) NaOH,

KOH 2nd group of activation agents: Na2CO3, K2CO3, Na2SO3 3rd group of activation agents: NaCl, K2O 4th group of activation agents: K2SO4, Na2SO4

Samples preparation

Compressive strength specimens were prepared usingmortar with cement:sand ratio of 1:3).

Compression tests were carried out on 0.787 in. 0.787 in. 0.787 in. cubes and prisms measuring 1.57 in. 1.57 in. 6.30 in. for different percentages of slag in thebinding medium (50%, 60%, 70%, 80%) with different acti-vation agents. The first category of tests was composed ofsix samples and the second was composed of three samples.

Some samples were heat treated, with steam beginning15 min after they had been cast. In heat treatment, the tem-perature was raised from +20 C to 60 C for more than 2 hand was kept constant for 12 h before it was reduced to20 C for more than 2 h. Thus, the steam curing cycle was16 h, after which all samples were stored in water at 20 Cfor the duration of the tests. Other tests were also used toevaluate the activated slag cement systems. These tests in-clude setting time and expansion.

Results and discussion

Effect of activation agents on the mechanical strengthFor an evaluation of the efficacy of the activating agent

the slag percentage was fixed at 80% and the solution to ce-ment ratio is 0.3.

Tests carried out on these cubes show the effect of the acti-vation agents on the development of the compressive strength.Results are presented for ages of 7 and 28 d in Fig. 1.

It was noticed that the activation agents had similar effectson strength development at both 7 and 28 d tests. Thismeans that the activation agents that caused an increase or adecrease in strength at 7 d had the same effect at 28 d.

When 80% of slag is added to the binding medium with-out activation agents, it adversely affected strength becauseof the low quantity of lime that is contained in 20% ordinaryPortland cement (OPC). For the activation agents of group 1

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1st group 2nd group 3rd group 4th group

Solution Water NaOH KOH Na2CO3 K2CO3 Na2SO4 K2O NaCl K2SO4 Na2SO4

Solution A B C D E F G H I J

Density (g/cm3) 1.18 1.18 1.18 1.19 1.17 1.19 1.09 1.19 1.18

Table 2. Densities of used solutions.

Fig. 1. Compressive strength with different activation agents. Fig. 2. Compressive strength of a bending medium, nonactivatedfunction of different percentage of slag.


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(NaOH, KOH), we noticed a slight increase in the strengthcompared with the nonactivated binding medium. The acti-vation agents of group 2, i.e., (RCO3) showed a noticeable

increase in strength, whereas (RSO3) diminishes thestrength.

The increase of strength is related to the increase in pH,which permits the solubility of the binding medium constitu-ents and leads to the formation of the hydration products.

Effect of slag content on the mechanical strength

For this part of the experiment, the efficient activationagents were used, i.e., NaOH, Na2SO4, Na2CO3 with a den-sity of 1.15 g/cm3.

Compressive strength test results at 1, 7, 28, 120, and365 d of hardening are presented in Figs. 25.

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Fig. 5. Compressive strength of a bending medium activated by

Na2SO4 function of different percentage of slag.

Fig. 6. Compressive strength as a function of slag percentage

and activation density (NaOH). (D represents activation density).

Fig. 7. Flexural strength as a function of slag percentage and ac-

tivation density (NaOH). (D represents activation density).


NaOH function of different percentage of slag.


Na2CO3 function of different percentage of slag.


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Fig. 11. Influence of steam curing on the compressive strength

of an nonactivated binder after 1 d of hardening.


of an activated binder by NaOH after 1d of hardening.


of an activated binder by NaOH after 28 d of hardening.

Fig. 8. Tensile strength as a function of slag percentage and acti-

vation density (NaOH). (D represents activation density).

Fig. 9. Compressive strength as a function of percentage of slagand activators (after 1 d of natural hardening).

Fig. 10. Compressive strength as a function of percentage of slag

and activators (after 365 d of natural hardening).


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Figures 2 and 3 show that increasing the percentage of

slag, diminishes compressive strength with or without acti-vation agent (NaOH) at all ages.

When adding Na2CO3 and Na2SO4 with greater percent-ages of slag (70% and 80%), the strength increases, asshown in Figs. 4 and 5. This can be explained by the posi-tive effect of the activation agents, i.e., they permit reactionwith the slag.

For an equal percentage of OPC cement and slag (i.e., thebinding medium has 50% cement and 50% slag), lime that isfreed from the OPC cement during the hydration process issufficient to permit slag hydration and gives significant me-chanical strength. Greater than 70% slag contents require anactivation agent for the reactions to progress.

When the activation agent NaOH is added, the increase instrengths is very significant during the initial days as com-pared with nonactivated binders and activated binders, usingNa2SO4 and Na2CO3. This information can be useful in pre-cast concrete plants using slag.

Effect of density of the activation agent on the strengthdevelopment of the binding medium

For this experiment, three different compositions of bind-ing medium have been chosen: 50%, 70%, and 80% slag, us-ing only one activation agent (NaOH) with densities d1 =1.20 g/cm3 and d2 = 1.25 g/cm

3. Tests were carried out oncementitious mortar systems (cement:sand = 1:3).

The development of compressive strength, flexural strength,and tension strength (by flexure) is represented in Figs. 68.

Figures 68 show an increase in the strengths (compres-sive strength, flexural strength, and tensile strength) with in-creased activation agent density, at both 28 and 60 d ofhydration. Increase in density of NaOH builds up the con-centration of the solution, which is favorable for the rapidhydration process of the slag, and hence develops strength.

The different strength values of activated mortars are im-portant for the different percentages of slag in mortar systems.

Hydration of activated slagTwo graphs Figs. 9 and 10 represent the strength develop-

ment of slag percentage and the activator used at 1 d ofhardening and after a year of natural hardening, respectively.These have been discussed previously for Figs. 25.

Finely ground blast furnace slag mixed with water doesnot possess truly cementious properties because of the for-mation of an acid layer around the slag grain, thus prevent-ing the hydration of the latter (Lea 1971).

The role of the added activator is to ensure that a chemicalcomposition in the solution is capable of dissolving thislayer and accelerating the solubilization of slag.

The hydration process can be divided into stages: dissolution of components (aluminum, silicium, and cal-

cium) in a basic medium. formation of stable hydrates (the activators act not only as

a catalyst, but also as a reagent in hydrate formation but inexclusion to the soda).The use of the caustic soda (NaOH) accelerates the

hydration for all the slag percentages, which indicates partic-ipation in two reaction mechanisms: increased Ca(OH)2 lib-eration during the hydration of Portland cement and of theadded NaOH, forming an amorphous gel calcium-silictae-hydrate (C-H-S), aluminate, and gehlenite.

Figure 10 shows the positive effect of Na2SO4 in the long-term strength development and high slag percentages in thelong-term curing.

Effect of steam curing on the strength evolutionThe slag cements perform favorably in accelerated curing.

Temperature elevation leads to a more active hydration andpromotes the formation of hydrates.

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Fig. 14. Setting time as a function of the slag percentage and the effect of the activation agent: ( a) initial set and (b) final set.

Fig. 15. Expansion as a function of slag percentage.


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Figures 11 and 12 represent the strength development ofthe slag cement activated by NaOH versus nonactivated slag,when the latter is cured with heat with steam.

The results show that the combination of activation andsteam contribute in a complementary fashion with respect tocompressive strength (Fig. 13).

Study of physical characteristics of the activatedbinding medium

For this analysis, only one sample has been studied; abinding medium constituted of 20% cement and 80% slagwith only one activation agent (NaOH).

Setting of activated binding medium

The setting of slag cement systems is usually slower than100% Portland cement systems. The addition of activationagent and the finer grinding of cement can improve settingtime.

The study of Fig. 14 shows that increasing the percentageof slag negatively affects the setting process because of theslow hydration of nonactivated slag cement systems.

Expansion of the activated binding medium

The expansion was measured at elevated temperatures. Itwas noticed that the expansion of slag cement diminishedwith increasing percentages of slag. This indicates that slagcontributes to volume stability at elevated temperatures. Theactivation agent did not appear to influence the expansion(Fig. 15).

Shrinkage test

When cement hydrates, it is usually accompanied with a

volume shrinkage phenomenon. Cement hydration duringsaturated curing diminishes the shrinkage phenomenon. Cer-tain types of cement,mainly slag-based cements, are moresensitive to desiccation mechanisms.

Two different compositions of cement samples were tested:

Shrinkage tests were conducted on prisms measuring1.57 in. 1.57 in. 6.30 in., using caustic soda with a den-sity of 1.25 for activated samples.

50% of OPC and 50% of nonactivated slag and activatedslag (50 and 50b, respectively).

20% of OPC and 80% of nonactivated slag and activatedslag (80 and 80b, respectively).The obtained results are shown in Fig. 16.

Conclusions

It can be stated that the activation of slag improves thephysico-mechanical characteristics of slag cement systems.

Economic improvement of mix designs when using OPCis obvious. It is possible to replace cement with 50% of slagwithout activators. If the quantity of slag is increased to 80%then activation agents are necessary for acceptable strengthscomparable to OPC systems.

Slag activation by NaOH improves the set time of slag ce-ment and gives notable strength gains at early ages, whichcan help in precast concrete applications.

When using the activation agents, the mechanical strengthof slag cement increased by 10% to 60% over the strength ofnonactivated slag cement systems.

Slag activated cements respond well to steam curing andleads to considerable strength gains.

References

Lea, F.-M. 1971. The chemistry of cement and concrete. Chemical

Publishing Co., Inc., New York, N.Y.

Malolepszy, J., and Nocum-Wezelik, W. 1988. Micro-calorimetric

studies of slag alkaline binders. Journal of Thermal Analysis,

33: 431434.

Mezghiche, B., Guettala, A., Chebili, R., and Zeghichi, L. 1999.

Study of the effect of alkalis on the slag-cement systems. Pro-

ceedings of the International Conference on Modern Concrete

Materials: Binders, Additions and Admixtures, Dundee, Scot-land, 810 September 1999. Edited byR.K. Dhir and T.D. Dyer.

Thomas Telford, London, U.K. pp. 287293.

Zeghichi, L., Mezghiche, B., and Merzougui, A. 2002. La valorisa-

tion du laitier dEl-Hadjar dans la confection des ciments.

Compte rendu du sminaire international de Gomatriaux,

Msila, Algrie. Msila University, Algrie. pp. 433439.

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Fig. 16. Linear variation of cement as a function of time and slag percentage: (a) nonimmersed and (b) immersed.

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Efecto de alcalis en los cementos escorias