GUNITECH®: An Innovative Pumice Based Dry Shotcrete Application "2279

Pumice quarried by LAVA MINING AND QUARRYING SA from Yali Island, Dodecanese, is used in domestic and foreign markets mainly as concrete lightweight aggregate, masonry unit constituents, road substrate, and loose soil stabilization. It is a porous natural volcanic rock with low density, low thermal and noise transmission, and the highest strength among all the natural or artificial lightweight materials of mineral origin. Nowadays, pumice is of additional interest as it has a reduced CO2 footprint because thermal energy is not needed for its expansion compared with the artificial lightweight aggregates. In this context, HERACLES GROUP in collaboration with Sika Hellas has launched a new product containing pumice stone under the brand name GUNITECH®. GUNITECH® is an innovative bagged material for spraying concrete applications. It is a ready lightweight concrete, for building repairs certified as EN 1504-3.


Introduction
Dry spraying is a concreting technique (gunite) that consists in blowing dry concrete material with high momentum out of a hose with compressed air, injecting water at the nozzle as the material was released. This technique aims to form a compact, dense, and firmly adhering layer with the lowest possible rebound loss [1]. As in ACI 506R [2], the density of high-quality gunite is usually between 2230 and 2390 kg/m 3 .
In situ preliminary trials, laboratory investigations, and discussions with contractors and structure design engineers indicated that there is a market need for a lightweight gunite with a wet density of around 1800 kg/m 3 (around 1600 kg/m 3 dry oven) and modulus of elasticity not less than 15 GPa. HERACLES Group and Sika Hellas cooperated to develop in their R&D facilities, GUNITECH ® , a two-component sprayed cementitious mortar containing a special pumice mixture, cement, and concrete chemicals used for high-performance structural repairs and reinforcements and meeting the requirements of class R3 according to EN 1504-3 [3]. It is designed for spray application using the dry-spray method.

Components and Applications
GUNITECH ® consists of two components, which are packed in two separate bags marked A and B and mixed at the job site in 1:1 bag proportions.
The constituents of Component A, packed in bags of 20 kg, are: • Pumice of special granulometric gradation and a controlled range of moisture from 18 to 22% on a wet basis (HERACLES Group).
Pumice quarried by LAVA MINING & QUARRYING SA (HERACLES Group) from Yali Island, Dodecanese, is a porous pozzolanic natural volcanic rock with low density, low thermal and noise transmission, and the highest strength among all the natural or artificial lightweight materials of mineral origin. Pumice is of additional interest as it has a reduced CO 2 footprint because no thermal energy is needed for its expansion compared with artificial lightweight aggregates.
Particle size distribution of component B is shown in Table 1. The content of fine particles passing the 0.063 mm sieve is less than 15% w/w. Although pumice is classified in fractions with particles more than 40 mm, the fraction 0/3 mm was chosen, providing higher strength than the other fractions with larger particles and better particles, resulting in a dense adhering layer. Pumice exhibits the highest crushing resistance (EN 1055-1 A) (4.9 MPa) among all natural lightweight aggregates, improving lightweight concrete strength. Pumice an amorphous silicate rock with typical chemical composition, as shown in Table 2. The content of reactive SiO 2 is in the range of 45-65%, with pozzolanic activity resulting in chemical bonding with cement paste and long-term increase of strength. It is noticed that sulfate content of pumice is enough low due to its natural expansion (no use of combustion gases).
Porosity-related properties of pumice 0/3 mm are presented in Table 3. Pumice particles contain closed and open pores. Closed pores decrease the apparent density of particles to 2.10 g/cm 3 (determined according to EN 1097-6) against the apparent particle density of conventional limestone particles, which is about 2.65 g/cm 3 . The specific volume of closed pores is around 9.1% v/v. Open pores present a specific volume of around 7.7% v/v and contain water providing a continuous internal curing of the hardened material [4,5]. The pre-wetted particles with a close range of moisture provide good regulation of the mixing water and particle momentum during spraying and repeatability of the properties of the wet and hardened GUNITECH ® .  Table 3. Porosity related properties.

Property Magnitude
Skeleton density, ρ s g/cm 3 2.33 Apparent particle density, ρ a g/cm 3 2.10 Oven dry particle density, ρ rd g/cm 3 1.94 Specific volume of open pores 1/ρ rd − 1/ρ a cm 3 /g 0.040 Specific volume of closed pores V' cl,m = 1/ρ a − 1/ρ s cm 3 /g 0.047 Specific volume of open pores V' op,v % 7.7 Specific volume of closed pores V' cl,v % 9.1 The components A and B are mixed at the jobsite and ejected from the hose of the gunite machine. GUNITECH ® covers a wide range of applications:

Usage Quality
Usage quality of GUNITECH ® includes properties desirable for the builders and contractors during and after the application of the product:

Technical Properties
GUNITECH ® is certified conforming EN 1504 R3 classification. Its technical properties are summarized in Table 4. Compressive strength of GUNITECH ® is determined more than 40 MPa at age of 28 days using lab techniques with prismatic specimens 40 × 40 × 160 mm conforming EN 1504 R3 as a repair material. In addition, for technical evaluation and structural design, compressive strength is determined with hardened cores conforming LC20/22 lightweight concrete class. Modulus of elasticity indicates a material compatible for masonry repair.
The use of pumice does not make the hardened material lag behind in performance. Water capillary absorption is less than 0.5 kg/m 2 x √ h, and resistance to carbonation is equivalent of the reference mortar with water to cement ratio 0.45.

Resistance to Chloride Permeabity
Although a porous material, pumice increases the resistance of GUNITECH ® to chloride permeability. Figure 1 shows the decreasing permeability with the age using the test ASTM C1202-19 [6]. This test method consists of monitoring the amount of electrical current passed through 50-mm thick slices of 100-mm nominal diameter cores or cylinders during a 6-h period. A potential difference of 60 V dc is maintained across the ends of the specimen, one of which is immersed in a sodium chloride solution, the other in a sodium hydroxide solution. The total charge passed, in coulombs, has been found to be related to the resistance of the specimen to chloride ion penetration.
Mater. Proc. 2021, 5, 46 5 of 5 specimen, one of which is immersed in a sodium chloride solution, the other in a sodium hydroxide solution. The total charge passed, in coulombs, has been found to be related to the resistance of the specimen to chloride ion penetration. At the age of 90 days after concreting, GUNITECH ® cores cured in controlled conditions (21 °C, ≥95% relative humidity) show "Very low" permeability to chlorides according to ASTM C1202-19 [6] (Table 1). At the age of 90 days after concreting, GUNITECH ® cores cured in controlled conditions (21 • C, ≥95% relative humidity) show "Very low" permeability to chlorides according to ASTM C1202-19 [6] (Table 1).