Effect of Temperature vis-à-vis curing time on the hydration products of OPC paste

Article history: Received 22 October 2017 Received in revised form 11 March 2018 Accepted 20 March 2018 The curing temperature and time are the basic fundamental factors that contribute to the hydration of Ordinary Portland Cement (OPC) paste. The purpose of this work is to investigate the effect of temperature vis-à-vis curing time on the hydration products: Portlandite (CH), calcium silicate hydrate (C-S-H), ettringite (AFt) and monosulfoaluminate (AFm) of Japan OPC paste using the water-cement ratio (w/c) of 0.40 and following intensive procedures in sample preparation and testing. Results showed that the proportion of CH and C-S-H phases dominated the hydration products at higher curing temperature and longer time as revealed in BEI/SEM analysis. The XRD/Rietveld analysis showed that the CH had the highest peaks intensity compared to other crystalline phases cured at 40oC for 28 days. It has a mineralogical composition equivalent to 20.06% compared to 17.80% as OPC cured at room temperature. There was no C-S-H phase detected since it is non-crystalline/amorphous in nature with composition equivalent to 61.75%. The conversion of AFt to AFm phase from 0.12% to 3.10% was also detected at higher curing temperature and longer time. The CH is the major crystalline phase identified in this study.


silicate hyd
ate, hydro ferrite and calcium hydro aluminate (Jumate and Manea, 2011;Klug and Alexander, 1954).

The curing temperature and time are the basic fundamental factors that contribute to the hydration.The increase in temperature enhances the acceleration of hydration reaction and hardening of cement paste and concrete.To quantify the hydration pro ucts, the powder X-ray diffraction (XRD)/Rietveld analysis has been employed as a technique for the direct identification of crystalline and mineral phases of cement such as Portlandite, ettringite and other phases present during the hydration of OPC paste (Rietveld, 1969;Klug and Alexander, 1954).However, the identification of these crystalline phases is more complicated due to the presence of amorphous substance which results in a weak diffraction peak.

The major cement compounds that are easily affected by heat are the following: (a) Alite (C3S)colorless equi-sized grain mineralogical compound in cement.It has the highest hydration product and reacts easily with water and gives cement its high init al and final mechanical strengths.(b) Belite (C2S) -a mineralogical compound known to take three even four polymorphous forms, which reacts with water and transforms into hydrated dicalcium silicate.The C2S slowly contributes to the strengths of cement paste in at most seven days.After 28 days, the mechanical strength is very close to that of C3S.(c) Aluminate (C3A) -a mineralogical compound present as crystals in cement clinker.Pure C3A react quickly with water that leads to the instantaneous setting of the cement paste.(d) Ferrite (C4AF) -acts as a flux during cement manufacturing and contributes very slightly to strength gain.

Gypsum (CaSO42H2O) is added in a controlled amount when grinding the cement clinker.This material acts as a flux and lowers the burning temperature of the clinker.Too much amount of gypsum leads to expansion and damage the setting of the cement paste and co crete (Young et al., 1998;Jumate and Manea, 2011).

During hydration, the ferrite reacts with gypsum, and forms into ettringite (AFt) phase and monosulfoaluminate (AFm) phase whose presence can accelerate the hydration of calcium silicates.The mineralogical component begins to lose hydrates and crystallize.Con equently, the cement paste transforms into a rigid material with very high strength (Young et al., 1998;Jumate and Manea, 2011).

In this study, the effect of temperature vis-à-vis curing time on the hydration products of Japan OPC paste was investigated.


Materials and methodology


The materials and sample preparation

The composition and properties of Japan ordinary Portland cement us d in making cement paste is shown in Table 1.The cement was mixed with deionized water using the rotary mixer at a constant sp

r/cement ratio (w/c) of 0.40, cast in
50mm diameter x 100mm height cylindrical stainless steel mold and covered with Para film to avoid water evaporation for 24hrs at room temperature.After the cement paste hardened, this was removed from the mold and cured at 20 and 40 o C temperatures in water bath for three and 28 days curing period (Dejeto and Kurumisawa, 2015).


The morphological analysis

Samples were cut into cubes of 5x5x5 mm using a diamond saw.Initial weights were measured.The samples were oven dried at 105 o C for 24 hrs, then immersed in acetone to interrupt hydration reaction and subsequently placed in a vacuum

siccator for 24 hrs.These w
re molded into epoxy resin and hardened for at least 48hrs.The surfaces were polished using the course to fine abrasive paper.

Finally, a 0.25 μm diamond paste was used for final polishing.These were then freeze-dried using Eyela Freeze Dryer model FD-1000 for 24hrs.

Prior to Backscattering Electron Images (BEI)/ Scanning Electron Microscope (SEM) analysis, the samples were coated with Plat num (Pt) ions using Jeol Auto fine coater model JFC-1600.The BEI were obtained using Shimadzu model SSX-550 super scan SEM to separate each ph se namely: unhydrated cement particle (UH), Portlandite (CH), calcium silicate hydrate (C-S-H) and pores.Each phase has different density; therefore the brightness of each phase was different.White appearance is identified as UH, bright gray is CH, dark gray is C-S-H and the black spots are pores (Dejeto and Kurumisawa, 2015;Kurumisawa and Nawa, 2016).


The mineralogical analysis

The samples used in BEI/SEM analysis were crushed and screened to 2.5-5 mm sized aggregates using a hammer, and then ground using Fritsch pulverisette 5 ball mills at 300 rpm for 3mins.The powdered samples were mixed with 10% Al2O3, an ultra-p

e agglomerate-free precisio
aluminum oxide by Baikalox, Baikowski International Corporation as standard reference material for XRD/Rietveld analysis.Siroquant software ver. 3 was employed to interpret the Rigaku X-ray Diffractometer (XRD) data (Dejeto and Kurumisawa, 2015).


Results and discussion

The comparative BEI/SEM images of Japan OPC paste with a water/cement ratio (w/c) of 0.40 cured at different temperature and time is shown in Fig. 1.

It can be seen that there are UH: unhydrated cement particle (white color), CH: Portlandite (light

ray), C-S-H: calcium si
icate hydrate (dark gray), and P: pores (black), as revealed in Figs.1a-1d.The main characteristic features observed are the presence of Aft (Fig. 1b) and AFm (Fig. 1d), reduction of pores and unhydrated cement particles despite curing at higher temperature and longer time.

The comparative composition and mineralogical analysis of OPC before and after hydration are shown in Table 2.The residual unhydrated cement refers to the sum of C3S, C2S, C3A, C4AF, and gypsum product (Bassanite and Anhydrite).The CH and AFt denote the proportions of respect ve hydration products; AFm denotes the sum of AFm, Ms, and H hydrates (Table 2).The CH is one of the major hydration product develop at 40 o C for 28 days curing period.It has a mineralogical composition equivalent to 20.06% compared to 17.80% as OPC cured at room temperature (Table 2).The C-S-H denotes the proportion of amorphous substances determined by α-Al2O3 in the internal standard (Kumar et al., 2014).There was no substantial difference in C-S-H composition which is equal to 61.75% at 20 o C and 40 o C curing temperature in 28 days curing period.

The conversion of ettringite (AFt) to monosulfoaluminate (AFm) phase from 0.12% to 3.10% was also detected at higher curing temperature and longer time.This was due to low concentration of sulfate present that resulted to less stable ettringite at early hydration period and converted to monosulfoaluminate as shown in Table 2.The AFt phase is stable as long as there is sufficient amount of gypsum available (Young et al., 1998).Once gypsum is depleted, the AFt and C3A react further to form the AFm phase.The AFm is the stable phase in mature cement paste and concrete.The proportion of AFt and AFm formed during hydration at lower curing temperature are smaller than those cured at higher temperature due to stagnation in the reaction of C3A and C4AF in the early hardening cement paste (Young et al., 1998).

The Portlandite is the major crystalline phase identified all throughout the study at different magnitudes (Fig. 2).Detection of C-S-H phase is not possible since it is non-crystalline/amorphous in nature.Both CH and C-S-H shows greater proportion of hydration product at higher curing temper tures and time.

The unhydrated cement compounds (alite, belite, and calcite), traces of calcium aluminum hydrate and hemicarbonate were detected in the XRD/Rietveld analysis.These have trends similar with the reaction rates of CaO/SiO2 mole ratio of C-S-H from the mass balance of CaO and SiO2 which was determ ned from the proportion of CH and the reaction rate of C3S and C2S at longer curing period (Young et al., 1998;Feng et al., 2004).

The strength development of cement paste is entirely owed by the hydration of calcium silicates (C3S and C2S) in the early days of hydration.The major component responsible for the strength generation is the C-S-H (Feng et al., 2004).The rise in heat particularly in the early hydration reaction affects the C3S and C3A which are characterized by highly exothermic reactions.The fast hydration in the initial stage leads to a more heterogeneous distribution of the hydration products as the hydrated precipitate around the clinker particles and build up a dense inner shell around the clinker.At low temperature, for instance, hydration starts very slowly which allows the dissolved ions more time for diffusion before the hydrates precipitate (Lothenbach et al., 2007;Elkhadiri et al., 2009).


Conclusion

The hydration reaction product of ordinary Portland cement (OPC) paste cured at 40 o C produced greater amount of hydrated compounds compared to cement paste cured at room temperature (20 o C) in both three days and 28 days period.The Portlandite phase, for example, shows the greatest

ount formed
at higher curing temperature compared to curing at room temperature.This is due to the acceleration of hydrates which is characterized by a highly exothermic reaction.The hydration reaction starts very slowly at low temperatures that allow the dissolved ions more time for diffusion before the hydrates precipitate.For fast curing period, it is possible to work with cement paste and concrete at elevated temperatures.

Fig. 1 :
1
Fig. 1: The BEI/SEM images (150x200µm) of Japan OPC paste with a water/cement ratio (w/c) of 0.40 cured at different temperature and time: three days (a) 20 o C, (b) 40 o C and 28 days (c) 20 o C, (d) 40 o C. Note: UH: unhydrated cement particle (white color), CH: Portlandite (light gray), C