Compressive Strength Investigations Of Foamed Mortar Incorporating Sandblasting Waste As Supplementary Cementitious Materials

Silica sand is one of the abrasive materials commonly used in the sandblasting process. The production of sandblasting waste is raising yearly, linear with the rapid development of the shipping industries. Silica sand was produced as by-product waste, approximately 400 ton per year. This research focused on compressive strength investigations of the foamed mortar incorporating silica sand as a supplementary cementitious material (SCM). Foamed mortar is a lightweight mortar made from a mixture of water, cement, sand, and foam, which can reduce the density of the mortar for construction purposes. Prior to use as SCM, the silica sand was pre-treated by using mechanical grinding to produce finer materials similar to cement. The observations were applied to pre-treated silica sand, such as chemical compositions, particle size analysis, normal consistency test, and setting time test. The specimens used in this research were mortar concrete with dimensions 5 x 5 x 5 cm and tested according to ASTM C579-01. The pre-treated silica sand varied from 10%, 20%, and 30% by weight of cement, were applied in this investigation. The compressive strength and spesific gravity were also observed. The results show that 20% cement replacement with pre-treated silica sand is the optimum composition and has 52.2 MPa in compressive strength at 28th days. These investigations conclude that pre-treated silica sand is potentially used as SCM in foamed mortars for sustainable concrete materials.


Introduction
Silica sand is one of the abrasive materials commonly used in the sandblasting process in the shipyard industries.This silica sand waste has a relatively finer particle size than sand in general because of the contact with the abrased during the sandblasting process.As a commodity, silica sand is a low-priced product resulting from combining one atom of silicon with two atoms of oxygen.It consists of silicon 1265 (2023) 012015 IOP Publishing doi:10.1088/1755-1315/1265/1/012015 2 in crystalline form with a high specific surface.Silica sand is a major compound of sand, rock, and mineral ores [1].Sandblasting waste (silica sand) is pozzolanic with a high SiO2 content (more than 85%) making it useful for cement mixtures [1][2][3][4][5][6][7].
Mortars may be defined as composite materials obtained by adding water to a mixture of two primary components, a binder and a filler.The binder is the substance that gives consistency and adhesiveness to the mortar.Whereas, foamed mortar is a lightweight mortar made from a mixture of water, cement (binder), sand (filler), and the addition of foam which reduces the specific gravity of the mortar.The foamed mortar may have served as (1) a bonding material (for bedding, jointing, and pointing stones and bricks, or for infilling masonry structures); (2) a finishing material (for internal plastering, external rendering, flooring, waterproofing, and sealing); or (3) a decorative material (for casting, molding, modeling, and fixing or supporting architectural ornaments) [8].

Methodology 2.1 Raw Materials
Raw materials in this research were obtained from one of the shipbuilding industry in East Java, Indonesia.Silica sand (SS) as waste materials were produced from the blasting process in the shipyard industry.SS were pre-treated prior to the specimens production and sieved until passed through the sieve with a diameters of 2.36 mm.After that process, SS were grounded for 4 hours.The spesific gravitiy of SS is 2.69 gr/cm 3 .SS were applied as supplementary cementitious materials in this research.

Mix Proportions and Test Method
Tabel 1 and Table 2 below shows the mixing proportion for this research.This compositions according to previous research and eliminate PP fiber content [20].Test methods in this research were divided into two groups.The first group is the test for raw materials SS that used in this research, such as chemical compositions, particle size analysis, normal consistency test, and setting time test.The second group is the test applied for mortar specimens, such as spesific gravity, compressive strength, and strength activity index analysis.The specimens for the second group testing are cubes, as seen below in Figure 1.The specimen names in this research are SS0 for 0%, SS10 for 10%, SS20 for 20%, and SS30 for 30%.The percentage represents the cement replacement using pre-treated silica sand of sandblasting waste.

Chemical Compositions of SS
The chemical compositions of the SS oxides were obtained using X-Ray Fluorescence (XRF) analysis [2].The amount of the oxides were listed in Table 3 below.Generally, the results in Table 2 show that the content of SiO2 is 93%, followed by 2% of Fe2O3, 1.02% of Al2O3, and another oxide listed before.These results are in accordance with the chemical content obtained based on XRF testing and several previous studies which stated that the dominant chemical content was Silicon Oxide.The content of chemical compositions from SS can be used as a pozzolanic material to replace cement as material constructions [2][3][4][5][6][7][8][9][10].

Particle Size of SS
Particle size was observed in this research to obtain the size of SS particles.This test was applied using the laser diffraction particle size analyzer Horiba LA-960.The result of SS particle size is seen in Figure 2. The pre-treatment applied to the SS of sandblasting waste affects the particle size.After the grounded process, the size of silica sand particles becomes 0.363 µm.The fineness and surface area of particle size affected the requirements of dissolved silica in concrete mixtures [2].

Normal Consistency
Normal consistency tests were conducted according to ASTM C187 to obtain the water consumption for the binder mixtures.The binder in this research is combination of Ordinary Portland Cement (OPC) and Silica Sand of Sandblasting Waste (SS).The volume of water consumption influences the workability of mixtures.The process of the normal consistency test and the test result can be seen in Fig 3b and Table 4, respectively.The normal consistency results as listed before, describe the water-to-binder ratio adding to the mixtures obtains good workability.The water requirement is 25% for OPC100, 25.80% for OPC90 SS10, 26.30% for OPC70 SS30, and followed by 27.30% for OPC80 SS20, serially.These results show that the OPC80 SS20 has the highest requirement of water consumption to develop good workability of binder mixtures.Water-to-binder ratio could affect the workability of the mixture during the mixing process [10].5

Setting Time
Setting time was tested according to ASTM C191 to obtain the setting time of binder composition.The process of the normal consistency test as below in Figure 3a.The setting time test result for binder compositions as seen in Table 5 below.In general, test results show that the OPC100 starts to harden in the first 87 minutes, and fully hardens at 210 minutes after the initial time as listed.The OPC80 SS20 has the longest time to harden, but ended up at 225 minutes as recorded.As regulated in ASTM C191, the minimum requirement for initial and final setting times are 60 minutes and 90 minutes, respectively.The setting time of all mixtures was appropriate to ASTM C191.These results show that the using of SS as supplementary cement materials affects the setting time of binder at all [3][4][5][6][7][8][9][10].

Spesific Gravity of Foamed Mortars
Specific gravities of the specimens were obtained at the 28 th day of age.The specific gravities test was conducted according to ASTM C127.The test results for SS0, SS10, SS20, SS30, and SS40, are 2.318 gr/cm 3 , 2.098 gr/cm 3 , 2.342 gr/cm 3 , and 2.032 gr/cm 3 , respectively.The results were plotted as the graphic in Figure 4.In the result above, the highest specific gravities is SS20, and the lowest is SS10.Additional of foam and SS in the specimens were not significant to affect the specific gravities of foamed mortars.

Compressive Strength of Foamed Mortars
The compressive strengths were obtained at 14 th and 28 th days of age according to ASTM C579.These tests were conducted using Universal Testing Machine in 55 ton capacities.The set up of this compressive strength test as seen in Figure 5 below.4 below.In the table below, there are mixture codes and compressive strengths that obtained at 14 th and 28 th days related to the age of specimens.This test represent the strength development of the specimens related to the utilizations of sandblasting waste as supplementary cementitious materials in foamed mortar.The results of compressive strength in Table 6 were plot as graphic in Figure 6  Figure 6 shows that compressive strength increases followed by the aging of the specimens.As the results above, the 20% cement replacement is the optimum composition and has 52.2 MPa in compressive strength at 28 th day of age.The SS20 has the highest strength, followed by SS0, SS10, and SS30, respectively.The compressive strength result were related to spesific gravity reslut.Compressive strength and spesific gravities have linear relationship according to the development strength of mixture, and affect the closed porosities [21].

Conclusion
Based on the data analysis and discussion, the conclusions of this research as listed below : 1. Pre-treated silica sand is potentially used as SCM in foamed mortars for sustainable concrete materials.2. The 20% cement replacement with pre-treated silica sand is the optimum composition and has 52.2 MPa in compressive strength at 28 th days.

Figure 2 .
Figure 2. Silica Sand of Sandblasting Waste's Particle Size

Figure 3a .
Figure 3a.Set up of Setting Time Test

Figure 5 .
Figure 5. Set up of Compressive Strength Test

Table 1 .
Composition of Solid Material for in 1m 3 Mixtures

Table 2 .
Composition of Liquid Material in 1m 3 Mixtures

Table 3 .
Chemical Compositions of The Silica Sands of Sandblasting Waste

Table 4 .
Normal Consistency of Mixtures

Table 6 .
Compressive Strength of Mixtures Figure 6.Strength Development of Mixtures