Test on fresh concrete
Fresh concrete was tested using slump cone test as shown in Fig. 1 to find the workability of each concrete mix. It was observed that the increasing in ratio of basalt as replacement of ordinary Portland cement increasing in workability up to 20% as compared to control concrete. The gradual increase of basalt shows that gradual increasing in workability by 10%, 16%, and 20% of B1, B2, B3 respectively compared to control concrete specimen B0. This is attributed to lower reaction of basalt with water at early ages compared with ordinary Portland cement.
Slump cone test
Decrease in the amount of water demand for consistency of cement paste with partial replacement of ordinary Portland cement type 1 by basalt materials was observed. Due to the non-hydraulic characteristics of basalt compared with ordinary Portland cement where the basalt cannot be hardened when exposed to water like ordinary Portland cement. So, the less water demanding for gauging and then the water of consistency decreases with the increasing of basalt content.
Test on hardened concrete
Compressive strength tests were done as per EN :(12390-3) [9, 13] and were measured at ages 7 days, 28 days, 54 days, 90 days of curing, splitting tensile strength were done as per BS-EN 12390-6/2009 [12, 14] and were measured at ages 28 days, and 90 days, flexural strength tests were done as per BS EN:12390-5/2009) [10] and were measured at ages 28 days, and 90 days, and water absorption at ages 28 days, and 90 days were conducted and monitored also. A different aging time of compressive, splitting, and flexural tests specimens up to 90 days were conducted and monitored due to the basalt powder are pozzolanic material and pozzolanic reactions starts later [4, 15,16,17].
Compressive strength
Concrete specimens at ages 7 days, 28 days, 56 days, and 90 days were tested by compression testing machine in the laboratory as shown in Fig. 2 for its compressive strength having different percentage of mixture of basalt as a replacement of ordinary Portland cement. Level of replacement of cement by mixture of basalt was 7.5%, 15%, and 22.5%. The compressive strength of concrete was tested on three specimens of each concrete mix replaced by basalt, and the average of the three cube sample test results was used to determine the compressive strength of concrete. The compression test results are expressed as the maximum load that the cube specimen can carry before failing. By dividing the highest load by the loaded area corresponding to it, the concrete compressive stress can be calculated.
Concrete compression testing machine
The compressive strength was calculated by formula as follows:
Compressive strength (MPa) = Failure load/cross sectional area.
Table 7 represents all the results of compressive strength and are as a function of percentage basalt content and curing time according to tested days of cube specimens and are graphically plotted in Fig. 3.
Compressive strength as a function of curing time-up to 90 days
The degree of hydration is an essential step towards understanding the rate of strength development. The results show that the compressive strength increases for all cement pastes with curing time up to 90 days. During the hydration processes, cementatious materials were formed such as calcium-silicate hydrate(C-S-H) which is the main binder of cement paste in concrete. It starts forming from the early stages of the contributing to cement hydration and it progressively densifies as cement sets, so can be considered that it is the main source of cement contributing to pastes compressive strength. and consequently, increase in the concrete compressive strength in early ages can be observed. So, the increasing in the amount of basalt percentage as a replacement of ordinary cement leads to decreasing in concrete compressive strength at early ages.
From the tests results in this investigation, the optimum percentage of basalt percentage which was showed a good result related compressive strength of concrete for tested ages of concrete specimens is 15%.
Negative effect by decreasing in compressive strength at early age especially at ages 7 days and 28 days for mixes with varying percentages of basalt as a replacement of ordinary Portland cement (B1, B2, and B3) compared with control concrete mix (B0) up to 27.5%.
Increasing in concrete compressive strength for mixes with varying percentage of basalt up to 15% replacement of cement (B1 and B2) at a long-life age at 56 days and 90 days compared with control concrete mix B0 by up to 20%.
The test results of concrete compressive strength using basalt as a replacement ordinary Portland cement in concrete mix shows that the increasing in percentage of basalt in blended cement over 15% was not given a good result in concrete compressive strength in early and long-life ages although achieved a target compressive strength at age 28 days.
7.5% and 15% of basalt as replacement of ordinary Portland cement in concrete mix was given a good result in concrete compressive strength. So, the optimum percentage of replacement basalt is 15% related with concrete compressive strength.
Decreasing in concrete compressive strength at early ages for concrete mixes samples B1, B2, and B3 compared with control sample B0 is attributed to the basalt are pozzolanic material and the pozzolanic material reactions starts later regarding to calcium silicate hydrate formation
The test results shows that the target concrete compressive strength was obtained by samples contained 7.5% and 15% of basalt replacement ordinary Portland cement in concrete mix B1 and B2 respectively at age 28 days while the target concrete compressive strength was not obtained by sample contained 22.5% of basalt replacement ordinary Portland cement in concrete mix B3 at age 28 days.
Increasing in the concrete compressive strength of sample B1 at age 7 days compared samples B2 and B3 by 36.5% and 38% respectively but less than by 5% compared with control concrete mix sample B0 this is attributed to that the low hydraulic properties of basalt as compared with Portland cement. Which that the basalt does not take some extent in initial hydration as ordinary Portland cement. So; the dilution of cement with basalt leads to delaying the reaction.
Splitting tensile strength
The splitting tensile strengths of basalt blended cement in concrete mixes after 28 days and 90 days of curing were conducted. This test was done on the cylinder-shaped concrete specimen as shown in Fig. 4.
Concrete cylinder specimens
When the cylinder is placed on its side longitudinally and loaded to induce transverse tension, it actually creates tensile stresses as shown in Fig. 5 and relatively high compressive stresses on the cylinder. When the cylinder is given a load of compression along the plate kept parallel on either side of the cylinder, major tensile stress will be created along the diameter, and can be given by the formula below:
$$ {f}_{ct}=\frac{2f}{\pi dl}\kern3em N/{mm}^2 $$
Splitting tensile test samples
Where
F is the applied compressive failure load over the cylinder’s length, d is the cylinder diameter, and L is the cylinder length.
Sixteen of cylinders for total concrete specimens were tested for a different proportion with basalt as a replacement of cement (type 1) in concrete mix after 28 days and 90 days of curing. The test results of specimens as shown in Fig. 6. Two cylinders were examined for each age category, and the average value was recorded.
Tensile strength of specimens at age 28 days and 90 days
The using of basalt as a partially replacement of ordinary Portland cement in concrete mix leads to increase in tensile strength up to 26.86% which is attributed to the strengthening of interfacial transition zone by the basalt content present as a replacement of Portland cement in concrete mix.
It can be clearly seen that the splitting strength value increases basalt content 7.5% and then 15% by 20.29%, and 26.86%; respectively compared with control concrete mix B0. So it was observed that maximum splitting tensile strength was obtained at 15% basalt as a replacement cement.
Slightly increasing in tensile strength of all concrete mixes samples B0, B1, B2, and B3 at age 90 days compared with tensile strength at age 28 days by 2.08%, 4.21%, 2.35%, and 5.71%, respectively as shown in Fig. 6.
Flexural strength
Concrete sample beams have a standard sizes 100 mm width, 100 mm height, and 500 mm length span of 28 days and 90 days were tested for its flexural strength having different percentages of mixture of basalt as a replacement of cement.
The level of replacement of cement by mixture of basalt was 0%, 7.5%, 15%, and 22.5%. Eight beams samples were tested for control mix (B0) and for replacement cement by basalt B1, B2, and B3. Two samples of each replacement were tested by one beam specimen for each age to calculate flexural strength. All concrete beam specimens were cured before testing at age 28 days and 90 days respectively and the flexural test was performed on two points loading system as shown in Fig. 7.
Flexural testing machine
And the flexural strength can be calculated by the formula given below:
$$ {f}_{cf}=\frac{F\ L}{d_1{d}_2^2} $$
Where
fcf = flexural strength
F = is the failure load, in (N)
L = is the distance between the supporting rollers, in (mm)
d1, d2 = are the lateral dimensions of the specimens, in (mm)
The change in the flexural strength with respect to different percentages of basalt is presented in Fig. 8, which shows that the maximum flexural tensile strength was reached corresponding to 15% addition of basalt.
Flexural strength of specimens at age 28 days and 90 days
The flexural strength after 28 days drops by 9.3%, 16%, and 10.7% in case of 7.5%, 15%, and 22.5% basalt powder addition as replacement ordinary Portland cement in concrete mix respectively in later time; i.e., after 90 days, the flexural strength of the reference mortar with 7.5% and 15% basalt powder addition does not differ much. Only slightly decrement was observed, while the flexural strength of mortar with 22.5% addition of basalt powder increases by 4.7% after 90 days.
Decreasing in flexural due to increasing in basalt powder as a replacement ordinary Portland cement in concrete mix was observed. This is attributed to the large irregularity and roughness of the surface of basalt grains which have the ability to mechanical wedging in the cement matrix.
Table 8 shows a comparison of the compressive, splitting, and flexural strengths for all tested specimens at all aging ages in the present investigation
Water absorption
Water absorption and penetration property was determinate by saturated water absorption of concrete cubs have dimensions 100 mm × 100 mm × 100 mm for various of cement pastes which it was blended of various percentages of basalt as a replacement of ordinary Portland cement in concrete mixes after 28 days and 90 days.
In this test, a dried specimen of known weight was immersed in water for a specified period of time after that period, the specimen was weighted a gain and the increase in weight with respect to the percentage of the original weight was expressed as its water absorption (in percentage). Values of water absorbed for different basalt percentage for ages 28 days and 90 days were listed in Table 9.
The water absorption percentage were listed in Table 8, and it was found that the water absorption increases as the percentage of basalt increased; this is attributed to the basalt powder particles is finer than of ordinary Portland cement and also it is hygroscopic in natural. Consequently, using basalt as replacement ordinary Portland cement in concrete mix was created a porous water fills, the pores which increase the water absorption rate. The water absorption percentage was decreasing at ages 90 days as shown in previous table.
SEM analysis
The Microstructure of the B1, B2, and B3 as a pozzolanic basalt specimens and B0 as an ordinary Portland cement specimen (the ones with the lowest and highest strength) was evaluated using the scanning electron microscope (SEM), as shown in Fig. 9. The porosities were marked by yellow squares in the figure. As can be seen, minor porosities are evident for basalt pozzolana while large pores in OPC compared to pozzolanic cement due to the high level of microstructure formation. Accordingly, both the positive impact of chemical composition of basalt and the enhancement of pores formation were confirmed. The results of compressive strength, porosity, and SEM-photographs are harmonious with each other. From an ecological science perspective, the technique used in this work is a CO2 emission-free technique.
SEM micrographs of the fracture surface of samples: a B1, b B2, c B3, and d B0
