Bond strength of granulated foam glass with binder in foam glass concrete

. This paper investigates the contact zone between the binder material, foam glass and its analogues such as block foam glass and sheet glass, using the detachment method. Data is presented on the detachment pattern and bond strength between the foam glass and the binder material, which in turn allows the quality of the contact zone to be assessed. The results of research on the use of such binders as cement, gypsum and modified gypsum binder with the addition of amorphous silica and lime in foam glass concrete are presented. It is shown that the use of modified gypsum binder significantly increases the strength of bond of the aggregate (granulated foam glass) with the binder. The obtained data can be used to improve the quality of the contact zone and, as a consequence, to increase the quality of expanded glass concrete.


Introduction
Nowadays, the requirements for building envelopes have increased considerably.They should have not only the necessary strength characteristics and low heat conductivity coefficient, but also resistance to atmospheric influences.In this regard, there is a growing need for new materials for wall constructions, which can provide the necessary set of characteristics.Foamed glass concrete is one of the promising materials for use as wall materials.It consists [1,2] of a binder and granulated foam glass as an aggregate (Fig. 1).This material has not only low density, but also high strength and resistance to weathering.Granulated foam glass is a highly porous cellular heat-insulating material of inorganic origin.It is obtained by sintering finely ground glass with the addition of a blowing agent [3,4].Foam glass has many advantages such as high strength, durability, environmental friendliness and low values of density and water absorption [5,6].Due to the high strength and low thermal conductivity of the material, expanded glass pellets are well suited as aggregate for the production of expanded glass concrete [7,8].Varying the fractional composition of expanded glass (from 0.16 to 20 mm), density of expanded glass (100-300 kg/m3) and binder consumption allows to obtain a material with a wide range of properties and different functional purpose.
However, the use of granulated foam glass in combination with cement can lead to the occurrence of alkali-silica corrosion.This corrosion is caused by a reaction between amorphous silica (SiO 2 ), the main component of the expanded glass, and potassium and sodium hydroxides (NaOH and KOH) found in the cement.This reaction results in the formation of a viscous soluble gel of sodium silicate (Na 2 SiO 3 -nH 2 O), which expands upon water absorption and reduces the bond between the binder and aggregate.
To improve the performance of concrete, it is possible to replace cement with a less corrosive substance such as gypsum [9,10,11].Gypsum has a number of advantages such as relative ease of production, high curing rate, which reduces the specific material intensity of production and accelerates the time of manufacturing or erection of structures and products.It is also characterized by low density and low thermal conductivity.In addition, gypsum has enhanced aesthetic and hygienic qualities, as well as allows the use of mineral pigments to create a variety of colors.It is especially important that gypsum binder does not contain alkali NaOH and KOH, and this excludes the development of alkali-silica corrosion.
However, gypsum binders and concretes based on them have low water resistance.Water resistance is characterized by softening factor.The softening coefficient is the ratio of the strength of the material in the water saturated state to the strength of the material in the dry state.The softening factor of gypsum is in the range of 0.3-0.45[12], which does not provide the necessary water resistance for the material of exterior walls, as for exterior building envelopes the softening factor should be at least 0.6 [13].
To solve the problem of low water resistance, part of gypsum is replaced by a complex mineral additive -lime combined with amorphous silica, the reaction products of which are calcium hydrosilicates [14,15].The resulting calcium hydrosilicate is insoluble in water regardless of its composition, which improves water resistance and mechanical properties of gypsum concrete.

Materials and Methods
To evaluate the strength of the contact zone, a number of experiments were conducted using the tear-off method.In the series of the experiments the tear-off of granulated foam glass and its analogues (block foam glass and sheet glass) from the hardened binding material (cement, gypsum and modified gypsum with the addition of amorphous silica and lime) was made.
When making the samples, the faces of the foam glass and its analogues, which are in contact with the mold were covered with paraffin to prevent the binder material from getting on them.Then the samples were placed on the bottom of the mold, and the binding material was placed on top of them (Fig. 2).Online references will be linked to their original source, only if possible.To enable this linking extra care should be taken when preparing reference lists.
Fig. 2. Schematic of test specimen fabrication humidity treatment chamber.After demolding, a metal stamp was glued on the foam glass or its analog.Then the samples were placed in a special clamping device and tested for tear-off on a universal electromechanical machine Instron 3382 (Fig. 3).

Results and discussion
In order to obtain a modified gypsum binder with maximum water resistance, the optimum ratio between SiO 2 and Ca(OH) 2 was selected, where amorphous silica and lime acted in their roles, respectively.For the research samples were made of gypsum, in which part of gypsum was replaced by amorphous silica and lime in different ratios from 1 to 1.3), and their softening factor was determined as a criterion, since it characterizes the water resistance of the hardened gypsum binder (Table 1).As a result of stoichiometric calculation for the complete reaction of SiO 2 and Ca(OH) 2 it is necessary the ratio of these components in the range of 1-1.2, which was confirmed in practice.In further work we use the ratio of SiO 2 /Ca(OH) 2 equal to 1.2, because at this ratio of amorphous silica and lime was obtained the maximum softening factor of 0.81, and with a softening factor ≥ 0.8 we can say that the material is water resistant.
After selecting the optimum composition of the modified gypsum binder, a tear-off study was carried out where the granulated foam glass and its analogues were torn away from the binder material.This provided data on the nature of the detachment and bond strength and allowed conclusions to be drawn about the quality of the contact zone.Granular foam glass, consisting mainly of amorphous silica, reacts with lime to form hydrous silicates at the contact interface with the binder material.This should increase the bond strength between the granules and the binder material, and create a protective shell of hydrosilicates around the foam glass granules.
A series of experiments were conducted to refine (refine) the methodology for determining the bond strength.In the beginning, granulated foam glass was pressed into the binder to a depth of 2 mm using special molds.However, due to the impossibility to select granules of the same size, not all granules could be equally glued to the metal die.The variation of the results was about 70%, which did not allow to use the obtained data.In this connection it was decided to replace granulated foam glass with its analogues (block foam glass and sheet glass), as they are similar to granulated foam glass have the same chemical composition, and they can be more easily and reliably attached to the metal die, which reduces the probability of errors and random deviations during the experiment.
To ensure the tightest possible bond between the block foam glass and the binder, the block foam glass should be coated with paraffin wax at the points of contact with the mold, and then placed on the bottom of the mold before pouring the binder.Then the binder was poured on top of it, after which the specimen was held until complete hardening, then a metal die was glued on the block foam glass, and the specimen was tested for tear-off.However, when using gypsum and gypsum with complex mineral additive, the bond strength was higher than the strength of the block foam glass itself.As a result, the specimen broke not at the point of bond to the binding material, but along the "body" of the foam glass, which indicates the low tensile strength of the block foam glass.
The bond strength in the foam glass samples with cement binder material was close to zero.This indicates the negative effect of alkali-silica corrosion and shows that NaOH and KOH alkalis in the cement binder material significantly deteriorate the contact zone between cement stone and foam glass [16,17,18].(Fig. 4  Since as a result of this experiment it was not possible to compare the bonding strength of foam glass with gypsum and modified gypsum binder due to the fact that the strength of the contact zone of foam glass and binder was greater than the strength of foam glass itself, it was decided to replace foam glass with sheet glass, because, in general, foam glass is made of finely ground glass mixed with a blowing agent and it also consists of amorphous silica.Just like with block foam glass the sheet glass was placed on the bottom of the mold and then the binder was placed on top of it.The specimen was held until complete hardening, then a metal stamp was glued on the sheet glass and the specimens were tested for tear-off.The replacement of foam glass with sheet glass made it possible to compare the bond strength when using modified gypsum binder with gypsum binder (Fig. 5 and Table 3).As shown by experimental studies, the bond strength of modified gypsum binder with sheet glass exceeds the bond strength with conventional gypsum binder by 1.5 times and significantly exceeds the bond strength with cement binder.Based on this, it can be concluded that the improvement of the contact zone is due to the reaction of lime with glass and the formation of hydrosilicates in the contact zone [19,20].

Conclusions
1.The results of the study revealed that the use of cement together with granulated foam glass is ineffective.The alkaline-silica reaction that occurs with this combination of materials leads to a decrease in the bond between the foam glass granules and the binder.
2 The use of modified gypsum binder in combination with granulated foam glass significantly (1.5 times) increases the bond strength compared to conventional gypsum binder.

Fig. 3 .
Fig. 3. Schematic diagram of the special clamping device

Fig. 4 .
Fig. 4. Tear-off character of block foam glass a) from modified gypsum binder b) from cement binder

6 E3SFig. 4 .
Fig. 4. Tear-off character of sheet glass a) from cement binder b) from modified gypsum binder c) from gypsum binder

Table 2 .
Bond strength and tear-off character of block foam glass from different types of binders

Table 3 .
Bond strength and tear-off character of sheet glass from different types of binders