Hydration Products, Morphology and Microstructure of Activated Slag Cement

The blast furnace slag used in this work was collected from Rourkela steel plant (RSP). The slag was sun dried and mixed thoroughly to bring homogeneity in the sample. The same was ground in a ball mill to Blaine’s fineness value of 410 m² kg⁻¹. The plaster of Paris and hydrated lime used were procured from the local market. The chemical compositions of the slag, lime and plaster of Paris were determined from EDX tests, and the same is shown in Table 1.

Composition	Slag	Plaster of Paris	Lime	Composition	Slag	Plaster of Paris	Lime
MgO	9.52	1.92	2.47	Fe2O3	1.37	0.852	0.381
Al2O3	21.86	1.13	0.68	Na2O	0.088	1.55	1.54
SiO2	30.82	0.916	2.75	MnO	0.14	–	–
K2O	1.04	0.661	0.9	TiO2	1.04	–	–
P2O5	–	0.58	–	SO3	0.66	39.88	–
CaO	32.42	41.45	90.26	Loss on Ignition	1.0	6.25	0.84

The variation of normal consistency values with lime at a given plaster of Paris content is presented in Fig. 1. It is seen that the normal consistency increases with either increase in lime or plaster of Paris contents. The consistency values of slag-lime-plaster of Paris mixes vary over a wide range from 28.89 to 37.7 % compared to the value of about 30 % for ordinary Portland cement (OPC). The increase of water demand or consistency mainly depends on percentage of lime and plaster of Paris content in the mix. This is attributed to the increase of calcium ions in the mixture or increase of (Ca/Si) ratio. Hence an increase in lime and/or plaster of Paris content in the mixture results in an increase in consistency values. A similar trend was also observed by Benghazi et al. (2009).

The effects of lime on initial and final setting times of slag-plaster of Paris mixes are delineated in Figs. 2 and 3 respectively. The setting time of mixes containing no plaster of Paris is too long for cementing materials, as prescribed in Bureau of Indian Standards. A marginal decrease in both initial and final setting times is observed with the increase in lime contents. Similar results were also obtained by Naceri et al. (2011). The reduction in setting time of the mix in addition of lime is due to increase in cation concentration and increase in pH value of the mix. In alkaline activation, the introduction of calcium hydroxide, sodium hydroxide, soda and others in an aqueous solution leads to formation of corresponding silica-hydrate. The calcium silicate is known to be structure forming phase whereas the sodium silicate is soluble. This results in a marginal decrease in setting of the mix. Further, it is observed that at a given lime content, an increase in plaster of Paris greatly reduces the setting time. This quick setting action is attributed to the high concentration of sulfate ions in solution, which reacts quickly with aluminum rich slag forming hydrated products. It is reported by Chandra (1996) that when the calcium sulfate activator is mixed with slag, it interacts directly with the alumina, calcium hydroxide and water to form hydro-sulfate-aluminates 3CaO–Al₂O₃–CaSO₄–12H₂O along with other new phase-formations during the hardening process. This results in a decline in setting times of the mix. In general, it is observed that the initial and final setting times of the mixes containing plaster of Paris are lesser than that of the value prescribed for ordinary Portland cement. In order to ward off or overcome this problem borax was added in these mixes. The effect of borax on the initial and final setting times of slag-plaster of Paris mixes at 20 % lime content is highlighted in Figs. 4 and 5 respectively. From these figures, it is observed that the setting times increased when the borax is added to a mixture of slag-lime-plaster of Paris. Further, it is observed that the retarding effect is not significant before a critical amount of retarder was used; beyond this the setting time increases. This indicates the retarding effect is very sensitive to the amount of retarder. Excess amount borax retards the setting time too much. On the other hand, insufficient amount of retarder cannot retard the setting time to the required workability. Similar results were also obtained by Mehrotra et al. (1982) and Chang (2003). It is well known that the anions and cations present in the activators play a major role in deciding the physical properties of fresh mixtures. The setting times mainly depend on activator type and concentration of the activator. It is found that a borax content of 0.4 % is sufficient to increase the initial setting time from 11 min to a workable range of 23 min and final setting time from 72 to 245 min. Higher borax contents further delay the initial setting than that prescribed for OPC.

The soundness value of various mixes of slag-lime-plaster of Paris is presented in Fig. 6. It is seen that the soundness of these mixes varies between 1 and 2.5 mm for all mixes except for the mix containing 10 % plaster of Paris and 40 % lime. For the said composition, the soundness value is 3 mm. It may be due to the presence of an excess amount of free calcium and magnesia in the mix. According to Bureau of Indian Standards, the soundness of cement shouldn’t exceed 10 mm. Therefore, the above mixes are sound and can be used as building material.

The formation of chemical bonds was studied by using FTIR. The microstructure and hydration products during setting or hydration process were examined using SEM and XRD analysis. The hydrated paste containing 80 % slag, 20 % lime and 10 % plaster of Paris was analyzed by XRD (with the testing condition: CuKα; 7°–70°; 2θ; 2° min⁻¹) after different set periods. The XRD patterns of specimens cured for different setting periods are shown in Fig. 7. From X-ray diffraction analysis, a series of crystalline compounds such as calcium hydroxide, quartz, calcium–sulfate–hydrate, portlandite, calcium silicate hydrate and gypsum was found. As hydration time increased the series of crystalline compounds or phases are intensified. The crystalline peaks of calcium–sulfate–hydrate, calcium carbonate and portlandite appeared at 5 min. Thereafter, that is at 11 min (initial setting time) quartz and gypsum compounds appeared, and the peaks become more intensified. An increase in the hydration period, especially at 72 min (final setting time) the peaks become more intensified. However, the peaks after 24 h of setting and abundance of calcium–silicate–hydrate are observed. The microstructure, surface morphology and hydration products of specimens cured for different periods are studied using SEM and EDX analyzer respectively. Figure 8 shows the surface morphology of specimens cured for different periods. Abundance of needle-like structures is found in specimens cured for 5 min. Usually needle-like crystals appeared during the early period of hydration. As curing period proceeds in, the needle-shaped crystals change to hexagonal platy crystals and some gel-like substances appeared. Further increase in setting period (24 h of curing) results in an increase of crystal concentration and more gel like phase of calcium–alumina–silicate–hydrate appeared. This results in an increase of strength and hardness of specimens. The elemental composition was analyzed using EDX. The EDX output for D10 sample after the curing period of 5, 11 min and 24 h are shown in Fig. 9a, b, c respectively along with the corresponding surface morphology obtained from SEM analysis. At early stage of setting (Fig. 9a) hydrated oxides of Ca, Al, S, and Si are found indicating the presence of compounds of calcium–alumina–sulfate–hydrate that is the C–A–S–H gel. At 11 min (Fig. 9b) abundance of element sulfur is noticed in addition to other elements like Ca, Al and Si, indicating intensification of calcium–alumina–sulfate–hydrate compounds. The C–A–S–H gel is found to form in the early stages of curing in alkali activated slag cement instead of C–S–H gel, as generally found in hydration-products of OPC. A similar observation was also reported by Puertas et al. (2011). Specimen cured for a longer period that is 24 h of curing (Fig. 9c) showed presences of Ca, Al, Si and S indicating the formation of both calcium–alumina–sulfate–hydrate and calcium–silicate–hydrate compounds. At this setting time the atomic percentage of sulfur is much lower than the earlier cases. This may be due to conversion of tri-sulfate alumina (AFt) to mono-sulfate alumina (AFm). The Si/Ca (% atomic ratio) is 1.02, 0.98 and 0.94 at 5, 11 min and 24 h setting of the sample respectively. From this it is concluded that more amount of calcium reacted with un-reacted silicon ion of slag. The Al/Ca (% atomic ratio) is 0.54, 0.71, and 0.79 at 5, 11 min and 24 h setting of the sample respectively and in all the cases the concentration of Al is lower as compared to Si. These parameters indicate the formation of C–S–H phase with an increase in curing period. The presence of this phase results in imparting hardness and strength to the mass. The elemental analyses of samples, cured for different periods, show compounds that are identified earlier from XRD analysis. Hence, the EDX analysis confirms the XRD results. The FTIR spectra of the D10 specimen are given in Fig. 10 for different curing periods. Analysis of the results showed the bands of O–H at wave numbers 1,780 and 3,345–3,500 respectively. The minor band range 570–715 cm⁻¹ indicate presences of small amounts of siliceous and alumina-silicate material. The stretching vibration band of O–H is banded at wave number of 3,345–3,500 cm⁻¹ due to calcium hydroxide phase. The presences of peak at 1,410 cm⁻¹ is due to the bonding in CO₃²⁻ ions. This indicates the presence of carbonated minerals, possibly due to the absorption of CO₂ from the atmosphere. The bending vibration band of O–H is observed at wave numbers 875 cm⁻¹. The quick setting of paste is due to S-rich compounds such as gypsum. Prominent peaks found at 1,650 cm⁻¹ indicates the formation of S–O bonds in the paste. The presences of this bond indicated the formation of calcium–sulfate–hydrate phase. The S–O and O–H groups are found to be shifted right with the increase in curing period. This indicates that the hydration process continues with setting time and more amount of calcium–sulfate–hydrate gel is formed during the hydration process.