Fourier – transformed infrared spectroscopy is used to obtain an infrared spectrum of absorption or emission of solid, liquid, or gas. It collects high – spectral- resolution data over a wide range. FT-IR is carried out on Perkin Elmer Frontier equipment using potassium bromide. The range is 4000 – 400 cm−1 with 2 cm−1 resolution. This study was carried out to correlate the curing age with the development of special features in FTIR spectra during the hardened concrete. Concrete cured in portable water for 7day, 14 days, and 28 days and its wavenumbers and its functional groups of hydrated cement are given in Figures 2 and 3.
Control mix concrete
Silica absorption bands are found in the range of 500 – 700 cm-1 due to the v4vibration of the SiO4 group [Mollah et al 2004 and Ylmen et al.2009]. The peak of SiO4rises with curing age and crystallize. Al-OH (800 – 850) cm−1 OH bending stretch occurred and converted into an amorphous condition. The V3 vibration of SiO44− and SiO42− are observed on 980 cm−1 and (1100 – 1200 cm−1) wavenumber, respectively. It converted into crystalline form with the hardening of concrete. The peak at v1 vibration CO3 at 1519 cm−1 was formed due to carbonates, but it decreased at 28 days of hydration due to dilution of Ca+ ion and emission of CO2. The CaCO3 occurred at 2875 –2879 cm−1 and 2983 - 3010 cm−1 wavenumber in vibration form due to calcium carbonate present in cement. The amount of calcium carbonate was observed to decrease due to the hydration reaction. The vibration of v1 + v3 vibration of H2O and capillary water are formed at 3323 shifted to 3400 cm−1 and a peak of 3450 cm−1 wavenumber shifted to 3510 cm−1 at 28days of curing age. Peak of v2 vibration of H2O at the range of 1645cm−1 change into a broad peak from 7days curing age to 28 days. Peak of v vibration of OH- at the range of 3645 cm−1 wave number also decreased with the curing age. The v2 vibration of CO32−, v1 symmetric stretching andv3 asymmetric stretching CO32− were observed at 850cm−1 ,1070 cm−1, and 1550 cm−1wavenumbers, respectively. The stretching of OH was present in a range of more than 3000cm−1 wavenumber. The peak of CaCO3stretching was at the range of 2358 cm−1 wavenumber and it increased with curing age as also observed by Lee and Deventer 2002.
By adding rice husk ash and maize cob ash, the sharp lower peak of SiO44− was in a range of 535 cm−1 wavenumber. The V4 vibration and peak of SiO4− was at the range of 656 and 670 cm−1 wavenumber on 7 days and converts into stretching band and broad peak after 28 days of curing. The small peak of Al-O and Al-OH at the range of 848 cm−1 wavenumber was converted into a low stretching band on 28 days of curing. The stretch of SiO42− had a weak shoulder at 1105 cm−1 wavenumber. Carbonate (CO32−) stretch was at the range of 1400 – 1500 cm−1 wavenumber with a smaller and flatter peak and shifted to more than 1500 cm−1 wavenumbers as has also been confirmed earlier [Lee and Deventer 2002]. This shift indicated the change in hydration products. While the sharp, strong peak and spiral of CaCO3 at the range of 2875 – 3000 cm−1 wavenumber was converted into a small peak within 7 and 14days but was sharper at 28 age curing age. When compared with controller concrete, it becomes obvious that hydration even at 28days was not complete in presence of RHA and MCA. Vibration of v2 of water in sulphate was present at the range of 1600 – 1650 cm−1. The v1 + v3 vibration of H2O, v3 of H2O, and capillary water and v3 vibration of H2O in gypsum was shown in the range between 3325 to 3600cm−1 wavenumber. These types of H2O groups reduced with the time period. The peak of Ca(OH)2 presents a range of 3650 cm−1 was the wavenumber. Ca(OH)2 is present in CSH at this range. Maximum changes are found between 3000 to 4000 cm−1 where calcium hydroxide, gypsum, and capillary water decreased most probably with the pozzolanic reaction.
Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD)
A scanning electron microscope is a tool to study the microstructure of concrete. The experiment was performed by a low vacuum SEM model JOEL JSM – LV/100 with an energy dispersive X-ray (EDS) detector. The splinted piece of concrete was fixed in a sample holder with a carbon-coated with electrically conductive platinum material tape attached to the sampler in the machine. The study was conducted at 7, and 28 days of curing age for both controlled concrete and MR specimens.
The X-ray diffraction test was used to establish the hydration peaks that appeared in the concrete at 7 and 28 days curing age. The collected samples were dipped in anhydrous ethanol to block the further hydration in concrete. The broken concrete pieces were grounded to a size fewer than 75 microns to use for XRD analysis. The mineralogy was studied with monochromatic Cu - Kɑ radiation at a scattering speed of 1.5o (2Ɵ) min-1. The powdered samples were affixed to the sampler and the top surface of the sample was streaked by a glass slide to obtain a uniform surface. The samples were placed in the diffractometer and scanned in continuous mode from 10o – 80o with a scanning rate of 0.05o/ Sec.
Concrete at 7 days of curing period
Figure 4 shows the SEM image after 7 days of hydration of control mix concrete. During the hydration reaction, it has produced glossy quartz (SiO2) low in quantity. White precipitation over particles of portlandite (CH) and CSH like cotton was observed in abundance around the anhydrous particle. In. Figure 5, EDS of microstructure analysis shows that the whitened area consists of the average Ca/Si ratio of 2.03 confirming the formation of portlandite. XRD analysis showed the minerals present and their contribution to the increase in strength and development of concrete. 1- Quartz low, 2 – Portlandite, and 3 – Sinnerite in XRD analysis. Since the age of concrete was 7 days, many voids could also be seen. The compressive strength of concrete is found to be 30.50 N/mm2.
Figure6 (a) shows the SEM image of 7days Maize cob and Rice Husk Concrete. By adding rice husk and maize cob ashes in concrete, hydration reaction produced, the round dark porous structure, needles of silica (Quartz), and bright parts of anhydrous alite (Ca3O5Si) which covered more than 50% of the visible area. Figure 6(b) The XRD analysis showed that peaks of alite were very less and were in an amorphous state. 1- Quartz low, 2 - Alite (Ca3O5Si), 3 – As8 Cu12S18 in XRD analysis. The presence of calcium hydroxide is less quantum when compared with control mix concrete indicated the slow rate of hydration process of MR concrete due to a large amount of silica in ashes and pozzolanic reaction. Black spherical particles both hollow and solid of ashes were also clearly observed. Many voids at this age of 7 days as were in controlled concrete could also be observed. The 7 days compressive strength is found at 20.2 N/mm2. Figure 7, EDS analysis found Ca/Si ratio is 1.8 which was less than 2 indicated the formation of CSH gel.
Concrete at 28 days of curing period
Figures 8 (a) & (b) and 9 show SEM analysis, XRD and EDS analysis of 28 days control mix concrete. When compared with Fig 4, this figure confirms the concrete has now hydrated, the voids were less and CSH particles along with ettringite needles were observed. EDS analysis found that Ca/Si was more than 1.5 which indicated the formation of CSH in crystal form. XRD analysis shows the peaks of quartz become lower and Ca(OH)2 getting higher. Peaks of sinnerite become prominent as compared with 7 days control mix concrete. 28 days compressive strength enhanced up to 45.40N/mm2.
(a) & (b) and 11 show the SEM, XRD and EDS analysis of 28 days MR concrete. SEM image showed that concrete had hydrated, contained quartz in bright portion and very less amount of alite. It became dense and fewer needle-like ettringite crystals were produced as compared to 7 days MR concrete. EDS analysis revealed that Ca/Si ratio is 1.3 indicating the formation of CSH crystal. It was found that the Ca/Si get decreased by adding SCM in concrete as also observed earlier by Ahmadi (2016
). XRD curve indicated reduced quantity of that alite. The quantum of CH was also much less as compared with controlled concrete which is main susceptible from the aspect of durability. The 28 days compressive strength is found to be 44 N/mm2