Mixing, casting and curing

The standard ASTM C305 describes a procedure for the mechanical mixing of hydraulic cement pastes and mortars of plastic consistency. Taking this standard as a basis, different mixing procedures have

63 been developed for the alkali-activated binders, as these binders do not behave like the hydraulic cement mortars.

So, the mixing procedure described at the standard ASTM C305 is the following: 1. Place all the mixing water in the bowl.

2. Add the cement to the water; then start the mixer and mix at slow speed for 30 seconds. 3. Slowly add the entire quantity of sand over a 30 seconds period, while mixing at slow speed. 4. Stop the mixer, change to medium speed, and mix for 30 seconds.

5. Stop the mixer and let the mortar stand for 90 seconds. 6. Finish by mixing for 60 seconds at medium speed.

In the case of the OPC mortars, the mixing procedure used is exactly as described in the ASTM C305 standard. Upon completion of the mixing period, the flow was measured in accordance with procedure given in standard ASTM C1437. Afterwards, the mortar was properly cast into the desired mould (depending on the test, see chapter 3.4 and 3.5). Once the mortar was cast into the molds, the molds were covered by a plastic film, in order to prevent water evaporation, and were placed in a humidity chamber at 23ºC and 100% RH. After 24 hours, the specimens were demolded and kept in the same conditions until tested.

In the case of the AAS mortars, the mixing procedure had some modifications according to procedure described at the ASTM C305. In this manner, the period of 90 seconds where we let the mortar stand was eliminated, and replaced by another 90 seconds of mixing at medium speed. It needs to be pointed out that for all the alkali-activated systems, the chemical activator used was prepared 1 day before the mortar mixing, in order to have a thermally and chemically stable dissolution. The mixing procedure results in:

1. Place all the mixing water (activators plus free water) in the bowl.

2. Add the slag to the liquid; then start the mixer and mix at slow speed for 30 seconds.

3. Add the entire quantity of sand slowly over a 30 seconds period, while mixing at slow speed. 4. Stop the mixer, change to medium speed, and finish by mixing for 180 seconds.

Upon completion of the mixing period, the flow, molding and curing were carried out in the same way as described for OPC mortars.

The mixing procedure of the AAFA mortars was similar to that of the AAS mortars, but with another modification. In this case, the dissolution period (step 2 in all the procedures) was extended from 30 seconds to 60 seconds. Thus, the mixing procedure results in:

64 1. Place all the mixing water (activators plus free water) in the bowl.

2. Add the fly ash to the liquid; then start the mixer and mix at slow speed for 60 seconds. 3. Slowly add the entire quantity of sand over a 30 seconds period, while mixing at slow speed. 4. Stop the mixer, change to medium speed, and finish by mixing for 180 seconds.

Upon completion of the mixing period, the flow was measured in accordance with procedure given in standard ASTM C1437. Afterwards, the AAFA mortar was cast into the moulds, in the same way as OPC and AAS mortars, as the mortar consistency was similar. Nevertheless, in this case all the mold surface that was in contact with the mortar was covered with a film (tape) prior casting (see figure 3.8), especially for metallic molds, due to the stickiness of AAFA mortars. Without the film, the mortars were glued to the surface and was undemoldable. For concretes, it was proceeded in the same manner except for cylindrical concrete specimens (100Øx200mm), where disposable carton molds were used, recovering their internal surface with a plastic film to avoid the concrete to be glued to the carton.

1.- Recovering of the internal faces of the mold to be in contact with the mortar

2.- Mold finished and ready to be used

Figure 3.8 Example of preconditioning of the molds with tape

Moreover, the curing for AAFA specimens was different to the others. In this kind of binders, the hardening mechanisms are very slow at normal curing temperature. Hence, once the mortars were cast into the moulds, they were thermally activated putting the moulds in an oven at 80ºC during 24 hours (Palomo, Grutzeck, et al. 1999; Fernandez-Jimenez & Palomo 2007). In order to prevent the high evaporation at this temperature, the moulds were covered first with a plastic film, and then introduced in a well closed bags with enough water inside (see figure 3.9). After the first 24 hours at 80ºC, specimens were demoulded and placed into a humidity chamber at 23ºC and 100% RH until test.

65 a) Moulds covered with plastic film b) Covered moulds inside the bags with water

containers keeping humidity constant

Figure 3.9 Curing procedure of AAFA mortars

For the AAGP mortars, the mixing procedure is practically the same than for AAFA, but with a minor modification. In this case, as the binder is composed of two materials, glass powder and metakaolin, the dissolution period was enlarged to 90 seconds. During the first 60 seconds, the glass powder was added to the activator liquid for a first dissolution, and during the next 30 seconds the metakaolin was added to the already formed glass powder paste. This mixing procedure was according to the recommendations of Balaguer findings (Balaguer Pascual 2014). Thus, the mixing procedure results in:

1. Place all the mixing water (activators plus free water) in the bowl. If it is the case, add to the liquid the admixture.

2. Add the glass powder to the liquid; then start the mixer and mix at slow speed for 60 seconds. 3. Slowly add the metakaolin powder during 15 seconds, and keep mixing another 15 seconds at

slow speed.

4. Add the entire quantity of sand slowly over a 30 seconds period, while mixing at slow speed. 5. Stop the mixer, change to medium speed, and finish by mixing for 180 seconds.

Upon completion of the mixing period, the flow was measured in accordance with procedure given in standard ASTM C1437. Afterwards the AAGP mortar was cast into the moulds, in the same way as the other formulations. Like the AAFA mortars, the hardening mechanism is very slow at normal curing temperature. The mortars were thus thermally activated by introducing the molds in an oven at 60ºC during 48 hours, following the recommendations of Balaguer’s work (Balaguer Pascual 2014). After the first 48 hours at 60ºC, specimens were demolded and placed into a chamber at 23ºC and 56% RH until test.

66

3.3.2.2 Concretes

For concretes, the mixing procedure described at the standard ASTM C192 (Making and curing

concrete test specimens in the laboratory) has been used as a basis:

1. Add the aggregates to the mixer and mix in dry conditions during 30 seconds to homogenize them.

2. Add half the total water to the dry aggregates and mix during for at least 30 seconds. 3. Add the cement powder to the mixer and mix during 30 seconds.

4. Add the rest of the water to the mixer and mix during 120 seconds. 5. Stop the mixer, and let the concrete stand for 180 seconds.

6. Finish with another 120 seconds of mixing.

This procedure has been used as described at the standard for the production of OPC concretes. For AAS and AAFA concretes, similar mixing procedure was used eliminating the stand period. As well as for mortars, the activating solution was prepared one day before the mixing and was added in the step 4, with the second half of the water.

After mixing, the flow was measured according to ASTM C143 and the fresh density and the air content (if required) were measured according to ASTM C231. OPC and AAS concretes were similarly cast according to the requirements of each test. After cast into the molds, the molds were covered with plastics in order to prevent evaporation, and stored at laboratory temperature (23ºC) during 24 hours. Afterwards, specimens were demolded and stored in a moist chamber at 23ºC (100% RH) until used. In the case of AAFA concretes, after cast into the molds, the molds were covered with a plastic film and placed inside a tightly closed bags with water inside, and thermally activated at 80ºC in an oven during 24 hours. After this period, specimens were demolded and stored in a chamber at 23ºC and 100% RH until use.