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Influence of production parameters on calcium sulfoaluminate cements
Construction and Building Materials 239 (2020) 117866
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of production parameters on calcium sulfoaluminate cements Og˘ulcan Canbek ⇑, Sinan T. Erdog˘an Department of Civil Engineering, Middle East Technical University, Dumlupınar Bulvarı, Ankara 06800, Turkey
h i g h l i g h t s 40% limestone, 40% bauxite, and 20% gypsum yield a satisfactory CSA clinker. Raw meal composition and kiln temperature greatly influence clinker phases. Optimum gypsum amount for a CSA clinker is determined with isothermal calorimetry. Addition of gypsum greatly reduces the heat evolved in the first few days. Ye’elimite increases early strength and belite contributes to ultimate strength.
a r t i c l e
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Article history: Received 7 August 2019 Received in revised form 30 November 2019 Accepted 12 December 2019
Keywords: Alternative binders Calcium sulfoaluminate clinker Cement Hydration mechanism Microstructure
a b s t r a c t The main appeal of calcium sulfoaluminate (CSA) cements is the possibility of reducing CO2 emissions. CSA clinkers can generally be produced at lower kiln temperatures and with lower limestone contents than required for portland cement clinker. However, it is important to assess the effects of various production parameters on the properties of CSA clinkers and cements. The influence of kiln maximum temperature, kiln retention time, and raw mixture proportioning on clinker properties, and those of water-to-cement ratio (W/C) and gypsum addition on cement hydration, were investigated. CSA clinkers were prepared in a laboratory furnace, with limestone, bauxite, and gypsum. Clinker and cement properties were explored with X-ray diffraction, isothermal calorimetry, thermogravimetric analysis, and scanning electron microscopy. Kiln temperatures as low as 1250 °C and retention times as low as 90 min. yielded satisfactory clinkers. Raw meal composition and calcination temperature have a greater effect on clinker phases than retention time. Hydration heat is affected mostly by raw meal composition. Hydration and strength gain were rapid until 3 d, after which they slowed down due to ettringite and AH3 coating the clinkers particles. Mortars with W/C = 0.6, achieved using citric acid as a retarder, gained 50 MPa strength at 28 d, 50–60% higher than mortars with W/C = 0.7. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Portland cement gives concrete its remarkable technical properties that make it suitable for many construction applications; useful working time, rapid hydration and strength gain, high ultimate compressive strength, and reasonable shrinkage being a few. However, the annual production of Portland cement (>4 Gt per year [1]) is responsible for 5% of global industrial energy consumption and 5–10% of anthropogenic CO2 emissions [2–4]. Hence, a search for alternative, environmentally advantageous but also technically adequate binders is ongoing. Alkali-activated materials and geopolymers, supersulfated cements, calcium sulfoaluminate cements, magnesium oxide binders, calcined clay-based binders, ⇑ Corresponding author. E-mail address: [email protected] (O. Canbek). https://doi.org/10.1016/j.conbuildmat.2019.117866 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
and many more have been proposed [5–9], each with its own advantages and disadvantages. CO2 emissions related to Portland cement (PC) stem from the decarbonation of the 80% limestone in the kiln raw meal and the burning of fuel needed to maintain the high temperatures (1450 °C) required for the formation of alite, the primary phase in PC clinker. As such, an environmentally preferable alternative binder (or its main phases) should avoid using limestone partly or completely, and should be produced at lower temperatures. Calcium sulfoaluminate (CSA) cements were first proposed in the 1960s and are reported to have been produced industrially in China [10]. The main compound of CSA clinker, ye’elimite (or Klein’s compound) differs from alite in that it contains about half the amount of CaO. As the kiln temperatures required for the production of CSAs are also lower than needed for PC, its production can release less than half the CO2. Ye’elimite also reacts (with
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gypsum added to the clinker) rapidly and can compensate the loss in early-strength due to the absence of alite. Among the other main clinker phases, belite contributes to later strength [11], and the hydration of ferrite is less understood, some studies reporting a slow contribution to hydration and others reporting it as inactive [12,13]. The relative proportions of these clinker phases determine the overall behavior of the cement [14,15]. The proportions of the clinker phases depend on the production parameters, such as the calcination conditions and the relative amounts and chemical compositions of the main raw materials; limestone, bauxite, and gypsum [10]. While the composition of limestone often shows little variation between sources, gypsum can be of different states of hydration, and especially bauxite can vary greatly in its Al2O3 and Fe2O3 contents [14]. This study investigated the influence of varying raw material proportions, kiln temperatures and kiln retention times on the mineralogy of CSA clinker, and varying added gypsum content on the hydration and strength development of cements made with these clinkers. 2. Experimental
Table 2 Phase composition of the gypsum used determined by XRD quantitative analysis. Mineral
Content in the gypsum used (%)
Calcite Hemihydrate Gypsum dihydrate Quartz Rwp (agreement factor [16], %)
5.4 63.0 28.6 3.0 8.5
mixer, following the method in ASTM C 305 [17] for pastes. After mixing, the paste was rolled by hand until the desired thickness (1 cm) was achieved and crisscrossed on a refractory plate, as shown in Fig. 1. The slender paste shape was chosen to allow uniform clinkering. 1 kg of clinker could be produced per batch. A heating rate of 7 °C/min was used to heat the furnace to the selected maximum temperature, the clinker was then kept at this temperature for a chosen duration (retention time), the furnace was shut off, and the clinker cooled naturally inside the furnace. Although desired, rapid cooling or quenching could not be applied in order not to damage the heating elements of the furnace. The whole process was completed in <24 h.
2.1. Materials The raw materials used were limestone and gypsum, obtained from Votorantim Hasanog˘lan Cement Plant, in Ankara, and bauxite obtained from Seydisßehir Eti Aluminum Plant, in Konya. All raw materials were oven-dried (100 °C) prior to grinding in a laboratory mill until > 90% passed the 150-mm sieve. Oxide compositions of these materials, determined using X-ray fluorescence spectrometry (XRF), are given in Table 1. Limestone and gypsum provide the CaO and SO3, and the bauxite provides the Al2O3 needed to form the main CSA clinker phase ye’elimite (4CaO.3Al2O3.SO3). Since the water demand and strength gain of CSA cements is affected by the hydration state of the added calcium sulfate, the mineral phase composition of the as-received gypsum was also determined using quantitative Xray diffraction (XRD) and is given in Table 2. The gypsum is mostly hemihydrate, with some impurities which may be related with the drying step used. As the setting of some mortars mixtures was very rapid, citric acid monohydrate (BDH Laboratory Supplies, Ankara) was also used, in some mixtures, as a retarder. 2.2. Methods 2.2.1. Raw material proportioning and firing Pre-determined proportions of raw materials were mixed at a water-to-powder ratio of 0.25 with a standard laboratory mortar Table 1 Oxide composition of natural raw materials used. Oxide (%)
Limestone
Bauxite
Gypsum
SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O TiO2 Na2O Loss on ignition (%) Density (g/cm3) Blaine (cm2/g)
2.96 1.44 0.61 49.86 0.73 0.16 0.17 – – 44 2.69 –*
12.06 51.14 17.53 1.10 0.33 0.44 0.31 2.68 – 14 3.15 –*
4.55 1.64 1.07 34.86 0.33 42.18 0.26 0.13 – 15 2.51 8300
* Finenesses of limestone and bauxite were not determined since they were not used as an added agent like gypsum and desulfogypsum.
2.2.2. Production of clinkers and cements Proportions of raw materials and calcination parameters for a ‘‘reference clinker” were chosen with the goal of obtaining adequate quantities of both ye’elimite and belite (for both early and late strength development), and low free lime or unreacted raw material minerals. 40% limestone (LS), 40% bauxite (B) and 20% gypsum (G) were combined, by mass. A kiln maximum temperature of 1300 °C and a retention time of 120 min. were selected. Several other clinkers and cements were also prepared (Table 3). Kiln maximum temperatures and retention times higher and lower than those selected for the reference clinker were tried. The influence of different raw material proportions on the composition of the CSA clinker was also investigated, by preparing one other clinker (N6). The proportions of the raw materials in this clinker were selected using an optimization process. A set of ‘‘modified” Bogue’s equations [18] were used to estimate the contents of ye’elimite and other CSA clinker phases, and the Solver tool in Microsoft Excel was run with the objectives of maximizing the amount of ye’elimite and minimizing the free lime content in the clinker produced. The limestone and bauxite contents calculated for N6 were higher and the gypsum content was lower than in the reference clinker (N2). A high amount of B is necessary to provide Al2O3 for ye’elimite formation, however increasing B causes a decrease in the LS and/or G that can be used. As LS and G provide the CaO, and SO3 needed for ye’elimite, an optimum combination is reached for the maximum amount of ye’elimite to be produced. It should be underlined that the ‘‘modified” Bogue’s equations, similar to the original Bogue’s equations (for Portland cement clinker), consider only the oxide composition of the raw meal and ignore factors like calcination time or temperature, impure compounds, etc. All CSA clinkers obtained were ground in a ball mill and sieved through a 1 mm sieve prior to testing. A Blaine fineness of 4000 cm2/g was achieved for N1, N2, N3 and N4. An equal milling time yielded 2800 cm2/g Blaine fineness for N6. This difference can be related with the higher bauxite content of N6 which makes it relatively harder to grind. Then, the crystalline phase compositions of the clinkers were determined using quantitative XRD and Rietveld refinement, with MAUD [19] as the analysis program. N5, made at 1350 °C, melted partly and stuck to the refractory plate, allowing only a very small amount to be recovered. As its grinding
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Fig. 1. C S A clinker raw material pastes: (a) before firing, (b) after firing (image widths are 20 cm).
þ 2CSH 2 þ 34H ! C3 A:3CS:32H C4 A3 S þ 2AH3
Table 3 Raw mixture proportions and calcination parameters of C S A clinkers produced.
*
Clinker ID
Natural Materials (%) Limestone
Bauxite
Gypsum
N1 N2* N3 N4 N5 N6
40 40 40 40 40 45
40 40 40 40 40 45
20 20 20 20 20 10
Kiln Temperature (°C)
Retention Time (min.)
1300 1300 1300 1250 1350 1300
90 120 150 120 120 120
Clinker N2 is the ‘‘reference clinker”.
process contrasted with those of the other clinkers, a meaningful Blaine fineness value cannot be reported for N5. A ‘‘reference cement” was prepared with the reference clinker, by adding 19% separately-ground gypsum (by mass of clinker + gypsum). This amount was chosen to obtain a rapid-hardening (or high-early-strength) CSA cement without harmful expansions, based on a formula proposed by Zhang [20], which suggests that the optimum ratio of gypsum-to-clinker (Ct), based on stoichiometric calculations, needed for a CSA cement of desired nature is:
Ct ¼ 0:13 m A=S
ð1Þ
where A is the mass of ye’elimite in the clinker, S is the mass of SO3, 0.13 is a conversion coefficient between mass and molar ratios, and ‘‘m” is the molar ratio of gypsum to ye’elimite. m values lower than 1.5 give rapid-hardening cements, and higher m values give expansive or self-stressing cements. Using the oxide compositions of the raw materials (Table 1) and their proportions in the clinker (Table 2) in the modified Bogue’s equations, the potential ye’elimite content of N2 was estimated as 39% (by mass). Using this value and m = 2 in Eq. (1) gave Ct = 0.13*2*39/42 = 0.24. A gypsum-toclinker ratio of 0.24 corresponds to a gypsum-to-cement ratio of 0.19; hence 19% gypsum was added to 81% N2 clinker to obtain the reference cement. m = 2 was chosen, as it corresponds to the theoretical ratio of gypsum-to-ye’elimite needed to form ettringite during hydration and not monosulfate, according to Eqn. (2).
ð2Þ
Lower and higher m values both result in the formation of some monosulfate. As such, the ‘‘reference cement” could be expected to be rapid setting and hardening without excessive expansion. Cements with higher and lower added gypsum contents than in the reference cement were also prepared with some of the clinkers to investigate the influence of added gypsum on hydration (Table 4). It should be noted that the amount of ye’elimite in N2 quantified with XRD (49%, Section 3.1.1) indicates the need for a higher amount of added gypsum (23%) to give m = 2. Also, the reactivity of the gypsum used influences the effectiveness of Eqn. (1) in predicting CSA cement performance. The effect, on clinker phase and hydrated cement phase formation, of kiln maximum temperature was investigated using N2, N4, and N5 while that of kiln retention time was studied using N1, N2 and N3 (see Table 3). The effects of changing clinker raw mixture proportions and the amount of gypsum added to the clinker to prepare cements, were investigated using N2 and N6. The compressive strength development of each cement was investigated using 4x4x16 cm mortar specimens made with a sand-to-powder ratio of 2.75 and water-to-cement ratio (W/C) = 0.7. This value was chosen because preliminary mixtures with lower W/C exhibited low flow. The high hydration water requirement of ye’elimite and CSA A cements has been reported [21] and is known to increase with increasing added gypsum content [12]. The workabilities of the cement mortars were assessed using a flow table, according to ASTM C 1437 [22]. The heat evolution of the pastes were determined up to 48 h, using an isothermal calorimeter, on pastes with W/C = 0.4 to avoid bleeding in the fresh paste. The nature of the hydration products in the pastes were investigated using quantitative XRD, as well as thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), using 3-d and 28-d-old specimens. Scanning electron microscopy was also used, to study the microstructure of the hydrated pastes. For the mineralogical analyses, 20 g paste samples were taken, and ground to <150 mm in an agate mortar just before the tests. They were kept under same conditions as the mortars until the age of testing.
Table 4 C S A cements prepared to investigate the effect of production parameters. Investigated Parameter
Clinkers Used
Added Gypsum (wt.% of cement)
Added Citric Acid (wt.% of cement)
Water/ Cement
Calcination conditions Clinker raw mixture proportioning and added gypsum amount Water/Cement
N1, N2, N3, N4, N5 N2, N6 N2
19 14, 19, 24 24
– – 0, 0.5
0.7 0.7 0.6, 0.7
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3. Results 3.1. Influence of production parameters on clinker composition 3.1.1. Influence of kiln maximum temperature The influence of the kiln maximum temperature was investigated by calcining the reference clinker raw meal at 1250, 1300, and 1350 °C. The phase compositions of the different clinkers obtained are compared in Fig. 2 and Table 5. All three clinkers appear to contain the same main phases, ye’elimite, belite, and ferrite, but at different contents. The content of ye’elimite, the primary phase in CSA cements, is higher than that estimated by the modified Bogue’s equations for all three clinkers, and by 10% for N2, the reference clinker. Calcination at 1250 °C yields the lowest ye’elimite. In return, the belite contents are 6–9% lower than estimated, with N2 containing the least. Although the initial selection of the 40% LS: 40% B: 20% G proportions was intended to form some anhydrite in the clinker, the actual amount formed was low, especially for the reference clinker. It could be that a rise in temperature from 1250 °C to 1300 °C promotes further formation of ye’elimite at the expense of anhydrite but that some of the ye’elimite decomposes at 1300–1350 °C [23]. The calculated Brownmillerite contents were close to the estimated values. Free lime, intended to be zero was indeed quite low, <1.1% for all clinkers. Low free lime content is an indicator of adequate proportioning, calcination and clinkering. Just like in PC, its
Fig. 3. X-ray diffractograms for C S A clinkers produced using different kiln retention times (Legend: Be – Belite; Br – Brownmillerite; Y – Ye’elimite).
presence can lead to unsoundness due to the formation of Ca (OH)2 [15,24], however it is not considered a problem at <2%, and could even contribute to early strength [25]. The clinker produced at 1300 °C also contains the lowest amount of minor phases like gehlenite and merwinite. Clinker N5, made at 1350 °C, actually melted partly and stuck to the refractory plate. 1250 °C or 1300 °C appear to be suitable for CSA clinker preparation. 3.1.2. Influence of kiln retention time The phase compositions of the clinkers made with 40% LS : 40% B: 20% G, at 1300 °C for 90 min, 120 min, and 150 min, are compared in Fig. 3 and Table 6. There appears to be little difference (<1%) between the ye’elimite contents of clinkers calcined at the same kiln maximum temperature but exposed to it for different durations. The contents of other main clinker phases are also similar, with the most significant observation being an increase in the content of merwinite and a decrease in gehlenite, with increasing retention time. 3.1.3. Influence of raw mixture proportions The phase compositions of the clinkers made with 40% LS: 40% B: 20% G, and with 45% LS: 45% B: 10% G, by calcining for 120 min at 1300 °C, are compared in Fig. 4 and Table 7. Clinker N6, with greater limestone and bauxite content, was proportioned (Section 2.2.2) to yield the maximum amount of ye’elimite possible with these raw materials, 43.8%. The Al2O3 content of the selected bauxite (the only one available locally),
Fig. 2. X-ray diffractograms for C S A clinkers produced at different kiln maximum temperatures (Legend: Be – Belite; Br – Brownmillerite; Y – Ye’elimite).
Table 5 Phase composition of C S A clinkers produced at different kiln maximum temperatures. Phase
Belite Brownmillerite Ye’elimite Anhydrite Periclase Free Lime Gehlenite Perovskite Dolomite Tricalcium aluminate Merwinite Rwp / %
Amount (%)
Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Calculated Calculated Calculated Calculated Calculated Calculated
1250 °C (N4)
1300 °C (N2)
1350 °C (N5)
25.2 16.0 26.4 28.6 39.2 41.8 8.7 3.7 0.7 0.8 0 0.2 2.1 1.9 1.4 0.6 3.0 7.7
25.2 14.6 26.4 29.2 39.2 49.1 8.7 1.4 0.7 0.6 0 0.3 1.7 0.4 1.5 0.2 1.1 8.5
25.2 19.5 26.4 27.5 39.2 45.5 8.7 2.0 0.7 0.1 0 1.1 0 0.8 0.2 2.3 1.0 8.1
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Table 6 Phase composition of C S A clinkers produced using different kiln retention times. Phase
Amount (%)
Belite
Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Calculated Calculated Calculated Calculated Calculated Calculated
Brownmillerite Ye’elimite Anhydrite Periclase Free Lime Gehlenite Perovskite Dolomite Tricalcium aluminate Merwinite Rwp/%
90 min.(N1)
120 min.(N2)
150 min.(N3)
25.2 15.6 26.4 29.0 39.2 48.2 8.7 1.8 0.7 0 0 0 3.2 0.1 1.6 0.1 0 7.7
25.2 14.6 26.4 29.2 39.2 49.1 8.7 1.4 0.7 0.6 0 0.3 1.7 0.4 1.5 0.2 1.1 8.5
25.2 16.7 26.4 25.5 39.2 50.6 8.7 1.0 0.7 1.0 0 0 1.1 0.1 0.5 0 3.5 8.7
17.5% Fe2O3. The simultaneous increase of limestone and decrease of gypsum in N6 may have caused the complexing of ferric ions with calcium, later incorporating aluminum and iron at higher temperatures, in the sulfate-deprived environment [26]. 3.2. Influence of production parameters on cement pastes and mortars
Fig. 4. X-ray diffractograms for C S A clinkers produced using different raw mixture proportions (Legend: Be – Belite; Br – Brownmillerite; Y – Ye’elimite).
Table 7 Phase composition of C S A clinkers produced using different raw mixture proportions. Phase
Belite Brownmillerite Ye’elimite Anhydrite Periclase Free Lime Gehlenite Perovskite Dolomite Tricalcium aluminate Merwinite Rwp / %
Amount (%)
Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Calculated Calculated Calculated Calculated Calculated Calculated
40%LS : 40%B : 20%G (N2)
45%LS : 45%B : 10%G (N6)
25.2 14.6 26.4 29.2 39.2 49.1 8.7 1.4 0.7 0.6 0 0.3 1.7 0.4 1.5 0.2 1.1 8.5
26.4 15.4 29.2 39.5 43.8 40.5 0 0 0.8 0.4 0 0.3 0.8 1.4 0 0 1.9 7.2
51%, limits the maximum amount of ye’elimite that can be produced. However, this clinker did not even contain as much ye’elimite as the reference clinker (N2), instead it contained greater brownmillerite. This could be because the bauxite also contains
3.2.1. Influence of kiln maximum temperature and retention time on heat evolution The differences in the rates and cumulative amounts of heat evolution of cement pastes prepared by adding 19% gypsum (by mass of cement) to the various clinkers are shown in Fig. 5. Short induction times (1 h) are noticeable in Fig. 5a. The first peak (in the first few minutes) in all pastes is due to wetting and possibly early reactions like the conversion of hemihydrate to dehydrate. The second (main) peak, following the dormant period, is due to the reaction of ye’elimite and gypsum, with water. The total heat production of the cements at 48 h varies between 140 and 200 J/g, lower than for a typical PC paste. The pastes prepared from clinkers of the same raw meal (40%LS:40%B:20%G) but fired at 1250 or 1300 °C (N1, N2, N3, N4) gave similar main peak times, corresponding roughly to their final setting times. The paste of the clinker fired at 1350 °C (N5) shows an earlier and larger peak. This could be attributed to its slightly higher C3A and free lime contents. The heights of the rate of heat evolution peaks do not appear closely linked with the contents of ye’elimite in the clinkers, as one might expect. The cumulative heat evolved up to 48 h, however, appears to increase with increasing calcination temperatures [27]. 3.2.2. Influence of added gypsum content and raw mixture proportions on heat evolution There exists an optimum gypsum content for a given CSA clinker, to ensure that ye’elimite reacts with gypsum and water to form ettringite rather than react simply with water and form monosulfate. This optimum gypsum is the amount that gives a molar ratio of gypsum to ye’elimite of 2, based on the commonly assumed reaction equation [15,27,28]. This ratio corresponds to slightly different mass ratios, depending on the hydration state of the gypsum used. For hemihydrate, it is 0.475. So, assuming the gypsum used were purely hemihydrate (from Table 2), for N2 and N6, with 49% and 41% ye’elimite, respectively, the optimum clinker-to-gypsum ratios would be expected to be 81:19, and 84:16. Figs. 6 and 7 show the influence of increasing added gypsum content (from 0% to 24%, comprising gypsum contents surely lower
6
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Fig. 5. a) Rate of heat evolution; b) Cumulative heat evolved for C S A cement pastes containing 19% gypsum.
Fig. 6. a) Rate of heat evolution; b) Cumulative heat evolved for C S A cement pastes containing N2 and different amounts of added gypsum.
and higher than the optimum contents of the two cements) on the heat evolution of clinkers N2 and N6. The N2 clinker-only paste shows an initial peak immediately following the addition of water, attributed to wetting and initial dissolution of the anhydrous clinker [29,30]. This is followed by a rather long dormant period of 12 h, which could be explained by the surfaces of clinker particles being covered by early hydration products [31]. Winnefeld and Barlag [32] also reported 15 h induction periods for CSA clinkers hydrated at a water/ cement ratio of 0.7. The main peak is due to the formation of mostly monosulfate, some ettringite (due to the anhydrite in N2, Table 7) and possibly some calcium aluminate hydrates [29,33]. The pastes with 14–24% added gypsum also show the initial wetting peak. Their main peaks are all similar in magnitude and occur at 5 h, much earlier than for the clinker-only paste. A secondary/ shoulder peak is visible on all curves, except the paste containing 24% gypsum. This shoulder shifts slightly to later ages [34] and its intensity decreases with increasing gypsum content, and it is barely noticeable for 19% gypsum. Hence, the ‘‘optimum” gypsum content for N2 (to form ettringite rather than monosulfate [35]), is 19 or slightly higher. Heat evolution for the pastes with greater gypsum contents is prolonged (Fig. 6b). The paste with 24% gypsum, despite having the lowest clinker content, evolves greater heat than the others and accordingly, the mortar prepared with the 24% gypsum paste achieves the highest strength at 28 d (Section 3.2.6). Unlike the N2 clinker-only paste, the N6 clinker-only paste gives a main hydration heat rate peak at about the same time as its gypsum-containing counterparts. Doval et al. [31] reported a similar heat flow peak for a synthetic ye’elimite hydrated at 25 °C and W/C = 0.7, but did not provide specifics about the clinker phase contents. The main peak is also quite wide and appears to contain within it smaller heat flow peaks, which may be related
Fig. 7. a) Rate of heat evolution; b) Cumulative heat evolved for C S A cement pastes containing N6 and different amounts of added gypsum.
with the hydration of C4AF and some minor phases. Addition of gypsum gives heat flow behavior similar to those of pastes made with N2, with shoulder peaks clearly visible up to and including
O. Canbek, S.T. Erdog˘an / Construction and Building Materials 239 (2020) 117866
19% gypsum, disappearing only for the case with 24% gypsum. Hence, the optimum gypsum content for N6 appears to be 19–24%, slightly higher than for N2. This is rather surprising, considering the lower ye’elimite content of N6, and could be related with some gypsum being consumed for the hydration of C4AF, which also produces monosulfate and ettringite in the absence and presence, respectively, of added sulfates [13]. N6 contains 10% more C4AF than N2, due to the reduced gypsum and increased bauxite in its raw meal. 3.2.3. Influence of kiln maximum temperature and retention time on hydration products The formation of hydration products was investigated using XRD and TGA/DSC. Fig. 8 shows the change in the X-ray diffractograms of the reference cement paste (N2 with 19% added gypsum), from 1 to 28 d. The main hydration product is ettringite, from the reaction of ye’elimite and gypsum, with water. There also exist unhydrated clinker phases in the hydrated paste. A large decrease in the intensity of the ye’elimite peak is observed between the unhydrated paste and the 1-d paste. This is accompanied by the growth of the ettringite peaks. Then, beyond 1 d, not much change is observed in the diffractograms, consistent with the limited corresponding strength gains (Section 3.2.5). The diffractograms of hydrating pastes made from N1-N5 are all quite similar (not shown), showing peaks for the same hydration products as in Fig. 8, with slight differences in intensity. The formation of hydration products was also investigated using thermal analysis. Fig. 9 shows the heat flow and mass loss upon heating of cement pastes made with the various clinkers and 19% added gypsum, at 3 d and 28 d, respectively. Mass loss between 80 and 150 °C is due to the decomposition of ettringite. Conversion of any unreacted gypsum to anhydrite occurs at 100–160 °C. AH3 expected to form simultaneously with ettringite, but not observed in XRD, is identified between 250 and 280 °C [28,29,36]. Little mass loss is observed at higher temperatures, until at 700 °C. This loss is attributed to the breakdown of calcium carbonate, formed due to the carbonation of the paste specimens. As such, the loss amount grows considerably from 3 to 28 days. CSA cements are known to carbonate much faster than PC mortars [37], particularly at high W/C. Additionally, the anhydrite content of the CSA pastes strongly affects the ye’elimite
7
reaction kinetics which plays an important role in imparting carbonation resistance to CSA cements [38]. 3.2.4. Influence of added gypsum content and raw mixture proportions on hydration products Fig. 10 shows the change in the X-ray diffractograms of the cement pastes made of N2 and N6 with different amounts of added gypsum, from 1 to 28 d. Higher added gypsum content in the cements resulted in more intense gypsum peak. Ettringite peaks of N2 are larger than for N6, at equal clinker:gypsum ratios, due to a greater amount of initial ye’elimite reacting. A greater amount of ettringite forms for N2 with clinker:gypsum = 81:19 (close to the optimum gypsum content) than with higher or lower added gypsum. Thermal analysis was performed to elucidate the hydration mechanism. Fig. 11 shows the heat flow and mass loss of cement pastes made with N2 and N6, and with various amounts of added gypsum, at 3 d and 28 d, respectively. Hydration products identified are identical for all mixtures as ettringite, gypsum, AH3 and calcite. Monosulfate, an expected phase for undersulfated mixtures, is not discernible with TGA (contrary to calorimetry) at its decomposition temperature of 200 °C [33,39]. Mass loss observed in 700–800 °C at 28 d is indicative of carbonation of the cement pastes. Fig. 12 shows SEM images of cement pastes made with N2 and N6 and 24% gypsum, at 3 d. In both pastes, ettringite crystals are surrounded by some unhydrated particles and amorphous hydration products which could be AH3. Also, the sponge-like fraction (Fig. 12b) can be attributed to the ye’elimite with ettringite formed in its pores [40]. 3.2.5. Influence of kiln maximum temperature and retention time on strength The strength developments of cement mortars prepared with the various clinkers (from Table 3), and 19% added gypsum are compared in Fig. 13. The strength gain of the mortars made with N2 (the ‘‘reference mortar”) and N4, containing clinkers made at different kiln temperatures, were similar, despite ye’elimite contents that differed by more than 7%. The 1-d strengths of both were 18 MPa, but their strength developments beyond 1-d were very little. Since not all the ye’elimite in a cement can react, particularly up
Fig. 8. Hydration product development of N2 up to 28 days (Legend: Be – Belite; Br – Brownmillerite; E – Ettringite; G – Gypsum; Y – Ye’elimite).
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O. Canbek, S.T. Erdog˘an / Construction and Building Materials 239 (2020) 117866
Fig. 9. Heat flow and mass loss of C S A cement pastes containing 19% gypsum, by mass, a) 3 d; b) 28 d.
Fig. 10. XRD patterns of cement pastes with clinker-to-gypsum ratios of 86:14, 81:19, 76:24, at 28 days of hydration: a) N2; b) N6 (Legend: Be – Belite; Br – Brownmillerite; E – Ettringite; G – Gypsum; Y – Ye’elimite).
to 1 d, the effect of ye’elimite contents of the clinkers becomes insignificant, probably as long as the clinker contains more than a minimum amount of it. The small increase in the strength of N2 (5 MPa) from 1 d to 28 d could be explained by the rather low amounts of belite and the lack of reactivity of the high
amounts of ferrite they contain. N1 and N3, with their slight higher belite contents, gain greater strength in this time period. Mortars with N5 could not be made, as an amount sufficient to prepare mortars could not be obtained due to the production problems mentioned in Section 3.1.1.
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Fig. 11. Heat flow and mass loss of C S A cement pastes consisting of N2 and N6 with various amounts of added gypsum, by mass, a) N2 at 3 d; b) N2 at 28 d; c) N6 at 3 d; d) N6 at 28 d.
Fig. 12. SEM images, at 3 d, of hydrated cement pastes: a-b) N2; c-d) N6.
10
O. Canbek, S.T. Erdog˘an / Construction and Building Materials 239 (2020) 117866 Table 8 Flow table values of C S A cement mortars. Added Gypsum Amount (%)
Flow (%)
14 19 24
N2
N6
93 65 55
90 80 50
continues for a longer time (or is delayed) which may damage the microstructure [15,32], resulting in a drop in strength. Strength is recovered beyond age 7 d, indicating the prior cessation of ettringite formation.
Fig. 13. Compressive strength development of C S A cement mortars containing 19% gypsum.
3.2.6. Influence of raw mixture proportions and added gypsum amount on strength Fig. 14 shows the change in strength development of mortars made using clinkers with different compositions (N2 and N6), and varying amounts of added gypsum, at W/C = 0.7. The 1-d strengths appear to slightly increase as the added gypsum content increases. At 28 d, the strength of the 24% gypsum mortars is highest for both clinkers but at 14% and 19% gypsum, the opposite is true. This outcome could be related with the differences in the workabilities of the mortars, which decreased with gypsum addition (Table 8). The spread of the mortar on the flow table decreases as added gypsum is increased from 14% to 19% which may have caused some compaction problems that overcame the benefit of the added gypsum in the early stages of hydration. The relative changes in the flows of the mortars of the two clinkers also reflect the difference in their optimum gypsum contents. For N2, with an optimum gypsum content closer to 19% (Section 3.2.2), the decrease in flow is greater from 14 to 19% but not so great afterwards. For N6, which has an optimum gypsum content higher than 19%, the change in flow is greater from 19 to 24%. A slight drop is observed in the strength of the 24% gypsum N6 mortar (Fig. 12b) between 3 d and 7 d. This drop is attributed to delayed (or continued) ettringite formation [41]. In systems with low added gypsum, ettringite formation is terminated shortly after setting. In systems with higher added gypsum content (or when the added calcium sulfate has low reactivity), ettringite formation
3.2.7. Influence of W/C on the strength of CSA mortars The water need of CSA cements is known to be higher than that of PC and to be dependent on added gypsum content. The strength development of mortars made with the reference clinker and 24% added gypsum are compared at WC = 0.6 and W/C = 0.7, in Fig. 15. A large increase (50%) in 1-d strength, and an even larger increase (60%) in 28-d strength is noticed when W/C is reduced from 0.7 to 0.6. The mortar with W/C = 0.6 is not only more compact initially but contains some citric acid. Samples containing citric acid have been reported to yield more compact pore
Fig. 15. Strength development of C S A cement mortars prepared at different W/C using N2.
Fig. 14. Compressive strength development of C S A cement mortars, with different amounts of added gypsum, a) N2; b) N6.
O. Canbek, S.T. Erdog˘an / Construction and Building Materials 239 (2020) 117866
structures and better distribution of ettringite, leading to increased strength [41,42]. A slight decrease in strength between 3 d and 7 d is observed for N2 with 24% gypsum, similar to that in Fig. 14b for N6, when W/C is reduced from 0.7 to 0.6. This can also be attributed to the damage due to delayed ettringite formation [41] in the relatively denser matrix formed with lower W/C. Addition of 0.5% citric acid rendered the lower-W/C N2 mortars more fluid than the higher-W/C mortars without citric acid.
4. Conclusions The influence of varying production parameters on certain properties of CSA clinkers and cements were investigated experimentally. The following conclusions were reached: The amounts of clinker phases predicted using the set of modified (for CSA) Bogue’s equations [18] differed significantly from those determined with quantitative XRD phase analysis. Lower amounts than predicted, of belite and anhydrite and greater amounts of ye‘elimite were observed to form. Clinker raw meal proportions of 40% limestone, 40% bauxite, and 20% gypsum appear to be suitable to produce a clinker with 49% ye’elimite and 15% belite, with the natural raw materials chosen. The Al2O3 content of the chosen bauxite limits the maximum amount of ye’elimite that can be produced. A maximum kiln temperature of 1250 °C and a retention time of 90 min. are sufficient to produce a clinker with strength development similar to that achieved using higher temperatures and longer durations, at equal added gypsum contents. Raw meal composition and kiln temperature have a greater influence on clinker phases than retention time. The amount of gypsum to be added to a CSA clinker for formation of ettringite and not monosulfate (19–24% in this study), can be determined using isothermal calorimetry, by observing the formation of a shoulder on the descending branch of the main rate of heat evolution peak. The main hydration products formed in hydrated CSA pastes are ettringite, gypsum and AH3. Hydration is rapid until 3 d after which it slows down due to the coating of unhydrated particles with ettringite and AH3. Compressive strength gain of CSA mortars is similarly rapid, 20 MPa at 1 d, subsequently slowing down, 25 MPa at 28 d, with 19% gypsum added to the clinker. Use of citric acid allows lowering the W/C from 0.7 to 0.6, resulting in strength increases of 50–60%, up to 50 MPa at 28 d with 24% added gypsum. Both the early and late-age compressive strengths of mortars increase with increasing amounts of gypsum blended with the CSA clinker. Higher gypsum also translates to less clinker. However, increasing gypsum too much can cause workability and dimensional stability issues in CSA systems. The compositions of CSA clinkers and cements influence their hydration heat evolution, whereas the influence of kiln parameters is insignificant. Similarly, addition of gypsum greatly reduces the heat evolved in the first few days. Calcite determined in the hydrated pastes indicates the tendency of all produced CSA cement pastes to carbonate.
CRediT authorship contribution statement Og˘ulcan Canbek: Writing - original draft. Sinan T. Erdog˘an: Writing - original draft.
11
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to thank The Scientific and Technologi_ cal Research Council of Turkey (TÜBITAK) for funding of this research through project 116 M233. References [1] USGS, Mineral Commodity Summaries, Cement. https://prd-wret.s3-us-west2.amazonaws.com/assets/palladium/production/s3fs-public/atoms/files/mcs2019-cemen_0.pdf, 2019 (last accessed 1 August, 2019). [2] C.A. Hendricks, E. Worrell, L. Price, N. Martin, L. Ozawa Meida, Greenhouse Gases from Cement Production, prepared for the IEA Greenhouse Gas R&D Programme, ECOFYS, Utrecht, The Netherlands, 1999. [3] C. Shi, A. Fernández Jiménez, A. Palomo, New cements for the 21st century: The pursuit of an Alternative to Portland Cement, Cem. Concr. Res. 41 (2011) 750– 763. [4] T.A. Boden, G. Marland, R.J. Andres, 2016. Global, Regional, and National FossilFuel CO2 Emissions, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., USA, available at: https://cdiac.ess-dive.lbl.gov/trends/emis/meth_reg.html, 2017 (last accessed 1 August, 2019). [5] M.C.G. Juenger, F. Winnefeld, J.L. Provis, J.H. Ideker, Advances in alternative cementitious binders, Cem. Concr. Res. 41 (12) (2011) 1232–1243. [6] E. Gartner, Industrially interesting approaches to ‘‘low-CO2” cements, Cem. Concr. Res. 34 (9) (2004) 1489–1498. [7] K.L. Scrivener, V.M. John, E.M. Gartner, 2016. Eco-efficient cements: Potential economically viable solutions for a low-CO2 cement-based materials industry. United Nations Environment Program. https://spiral.imperial.ac.uk/handle/ 10044/1/51016, last accessed 1 August, 2019. [8] J.L. Provis, Alkali-activated materials, Cem. Concr. Res. 114 (2018) 40–48. [9] E. Gartner, T. Sui, Alternative cement clinkers, Cem. Concr. Res. 114 (2018) 27– 39. [10] L. Zhang, M. Su, Y. Wang, Development of the Use of Sulfo- and Ferroaluminate Cements in China, Adv. Cem. Res. 11 (1999) 15–21. [11] V. Kasselouri, P. Tsakiridis, C. Malami, B. Georgali, C. Alexandridou, A Study on the Hydration Products of a Non-Expansive Sulfoaluminate Cement, Cem. Concr. Res. 25 (1995) 1726–1736. [12] F.P. Glasser, L. Zhang, High-Performance Cement Matrices Based on Calcium Sulfoaluminate-Belite Compositions, Cem. Concr. Res. 31 (2001) 1881–1886. [13] G. Álvarez-Pinazo, I. Santacruz, L. León-Reina, M.A.G. Aranda, A.G. De la Torre, Hydration reactions and mechanical strength developments of iron-rich sulfobelite eco-cements, Ind Eng Chem Res. 52 (2013) 16606–16614. [14] P.K. Mehta, Investigations on Energy Saving Cements, World Cement Technology 11 (1980) 166–177. [15] I. Odler, Special Inorganic Cements, E&FN Spon, New York, NY, 2000. [16] K. Scrivener, R. Snellings, B. Lothenbach, A Practical Guide to Microstructural Analysis of Cementitious Materials, Taylor & Francis Group, LLC, Boca Raton, 2016. [17] ASTM C 305, Standard Practice for Mechanical Mixing of Hydraulic Cement Pastes and Mortars of Plastic Consistency, American Society of Testing and Materials, West Conshohocken, PA, 2014. [18] J. Majling, S. Sahu, M. Vlna, D.M. Roy, Relationship between Raw Mixture and Mineralogical Composition of Sulfoaluminate Belite Clinkers in the System CaO-SiO2-Al2O3-Fe2O3-SO3, Cem. Concr. Res. 23 (1993) 1351–1356. [19] L. Lutterotti, S. Matthies, H. Wenk, MAUD (Material Analysis Using Diffraction): A User-Friendly Java program for Rietveld Texture Analysis and More, in: Proceeding of the Twelfth International Conference on Textures of Materials (ICOTOM-12). Vol. 1, 1999, p. 1599. [20] L. Zhang, Microstructure and Performance of Calcium Sulfoaluminate Cements (PhD Thesis), University of Aberdeen, Aberdeen, 2000. [21] G. Bernardo, A. Telesca, G.L. Valenti, A Porosimetric Study of Calcium Sulfoaluminate Cement Pastes Cured at Early Ages, Cem. Concr. Res. 36 (2006) 1042–1047. [22] ASTM C 1437, Standard Test Method for Flow of Hydraulic Cement Mortar, ASTM International, West Conshohocken, PA, 2015. [23] F. Puertas, M.T.B. Varela, S.G. Molina, Kinetics of Thermal Decomposition of C4A3S in Air, Cem. Concr. Res.. 25 (1995) 572–580. [24] P.K. Mehta, P.J.M. Monteiro, Concrete: Microstructure, Properties and Materials, 3rd ed., McGraw-Hill, New York, 2006. [25] G. Sudoh, T. Ohta, H. Harada, High Strength Cement in the CaO-Al2O3-SiO2-SO3 System and its Application, in: Proceedings of the 7th International Congress on Chemistry of Cements. Paris, 1980, pp. 152–157. Vol. 3. [26] N. Ukrainczyk, N. Frankovic Mihelj, J. Šipušic, Calcium Sulfoaluminate EcoCement from Industrial Waste, Chem. Biochem. Eng. Q. 27 (1) (2013) 83–93.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of production parameters on calcium sulfoaluminate cements Og˘ulcan Canbek ⇑, Sinan T. Erdog˘an Department of Civil Engineering, Middle East Technical University, Dumlupınar Bulvarı, Ankara 06800, Turkey
h i g h l i g h t s 40% limestone, 40% bauxite, and 20% gypsum yield a satisfactory CSA clinker. Raw meal composition and kiln temperature greatly influence clinker phases. Optimum gypsum amount for a CSA clinker is determined with isothermal calorimetry. Addition of gypsum greatly reduces the heat evolved in the first few days. Ye’elimite increases early strength and belite contributes to ultimate strength.
a r t i c l e
i n f o
Article history: Received 7 August 2019 Received in revised form 30 November 2019 Accepted 12 December 2019
Keywords: Alternative binders Calcium sulfoaluminate clinker Cement Hydration mechanism Microstructure
a b s t r a c t The main appeal of calcium sulfoaluminate (CSA) cements is the possibility of reducing CO2 emissions. CSA clinkers can generally be produced at lower kiln temperatures and with lower limestone contents than required for portland cement clinker. However, it is important to assess the effects of various production parameters on the properties of CSA clinkers and cements. The influence of kiln maximum temperature, kiln retention time, and raw mixture proportioning on clinker properties, and those of water-to-cement ratio (W/C) and gypsum addition on cement hydration, were investigated. CSA clinkers were prepared in a laboratory furnace, with limestone, bauxite, and gypsum. Clinker and cement properties were explored with X-ray diffraction, isothermal calorimetry, thermogravimetric analysis, and scanning electron microscopy. Kiln temperatures as low as 1250 °C and retention times as low as 90 min. yielded satisfactory clinkers. Raw meal composition and calcination temperature have a greater effect on clinker phases than retention time. Hydration heat is affected mostly by raw meal composition. Hydration and strength gain were rapid until 3 d, after which they slowed down due to ettringite and AH3 coating the clinkers particles. Mortars with W/C = 0.6, achieved using citric acid as a retarder, gained 50 MPa strength at 28 d, 50–60% higher than mortars with W/C = 0.7. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Portland cement gives concrete its remarkable technical properties that make it suitable for many construction applications; useful working time, rapid hydration and strength gain, high ultimate compressive strength, and reasonable shrinkage being a few. However, the annual production of Portland cement (>4 Gt per year [1]) is responsible for 5% of global industrial energy consumption and 5–10% of anthropogenic CO2 emissions [2–4]. Hence, a search for alternative, environmentally advantageous but also technically adequate binders is ongoing. Alkali-activated materials and geopolymers, supersulfated cements, calcium sulfoaluminate cements, magnesium oxide binders, calcined clay-based binders, ⇑ Corresponding author. E-mail address: [email protected] (O. Canbek). https://doi.org/10.1016/j.conbuildmat.2019.117866 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
and many more have been proposed [5–9], each with its own advantages and disadvantages. CO2 emissions related to Portland cement (PC) stem from the decarbonation of the 80% limestone in the kiln raw meal and the burning of fuel needed to maintain the high temperatures (1450 °C) required for the formation of alite, the primary phase in PC clinker. As such, an environmentally preferable alternative binder (or its main phases) should avoid using limestone partly or completely, and should be produced at lower temperatures. Calcium sulfoaluminate (CSA) cements were first proposed in the 1960s and are reported to have been produced industrially in China [10]. The main compound of CSA clinker, ye’elimite (or Klein’s compound) differs from alite in that it contains about half the amount of CaO. As the kiln temperatures required for the production of CSAs are also lower than needed for PC, its production can release less than half the CO2. Ye’elimite also reacts (with
O. Canbek, S.T. Erdog˘an / Construction and Building Materials 239 (2020) 117866
2
gypsum added to the clinker) rapidly and can compensate the loss in early-strength due to the absence of alite. Among the other main clinker phases, belite contributes to later strength [11], and the hydration of ferrite is less understood, some studies reporting a slow contribution to hydration and others reporting it as inactive [12,13]. The relative proportions of these clinker phases determine the overall behavior of the cement [14,15]. The proportions of the clinker phases depend on the production parameters, such as the calcination conditions and the relative amounts and chemical compositions of the main raw materials; limestone, bauxite, and gypsum [10]. While the composition of limestone often shows little variation between sources, gypsum can be of different states of hydration, and especially bauxite can vary greatly in its Al2O3 and Fe2O3 contents [14]. This study investigated the influence of varying raw material proportions, kiln temperatures and kiln retention times on the mineralogy of CSA clinker, and varying added gypsum content on the hydration and strength development of cements made with these clinkers. 2. Experimental
Table 2 Phase composition of the gypsum used determined by XRD quantitative analysis. Mineral
Content in the gypsum used (%)
Calcite Hemihydrate Gypsum dihydrate Quartz Rwp (agreement factor [16], %)
5.4 63.0 28.6 3.0 8.5
mixer, following the method in ASTM C 305 [17] for pastes. After mixing, the paste was rolled by hand until the desired thickness (1 cm) was achieved and crisscrossed on a refractory plate, as shown in Fig. 1. The slender paste shape was chosen to allow uniform clinkering. 1 kg of clinker could be produced per batch. A heating rate of 7 °C/min was used to heat the furnace to the selected maximum temperature, the clinker was then kept at this temperature for a chosen duration (retention time), the furnace was shut off, and the clinker cooled naturally inside the furnace. Although desired, rapid cooling or quenching could not be applied in order not to damage the heating elements of the furnace. The whole process was completed in <24 h.
2.1. Materials The raw materials used were limestone and gypsum, obtained from Votorantim Hasanog˘lan Cement Plant, in Ankara, and bauxite obtained from Seydisßehir Eti Aluminum Plant, in Konya. All raw materials were oven-dried (100 °C) prior to grinding in a laboratory mill until > 90% passed the 150-mm sieve. Oxide compositions of these materials, determined using X-ray fluorescence spectrometry (XRF), are given in Table 1. Limestone and gypsum provide the CaO and SO3, and the bauxite provides the Al2O3 needed to form the main CSA clinker phase ye’elimite (4CaO.3Al2O3.SO3). Since the water demand and strength gain of CSA cements is affected by the hydration state of the added calcium sulfate, the mineral phase composition of the as-received gypsum was also determined using quantitative Xray diffraction (XRD) and is given in Table 2. The gypsum is mostly hemihydrate, with some impurities which may be related with the drying step used. As the setting of some mortars mixtures was very rapid, citric acid monohydrate (BDH Laboratory Supplies, Ankara) was also used, in some mixtures, as a retarder. 2.2. Methods 2.2.1. Raw material proportioning and firing Pre-determined proportions of raw materials were mixed at a water-to-powder ratio of 0.25 with a standard laboratory mortar Table 1 Oxide composition of natural raw materials used. Oxide (%)
Limestone
Bauxite
Gypsum
SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O TiO2 Na2O Loss on ignition (%) Density (g/cm3) Blaine (cm2/g)
2.96 1.44 0.61 49.86 0.73 0.16 0.17 – – 44 2.69 –*
12.06 51.14 17.53 1.10 0.33 0.44 0.31 2.68 – 14 3.15 –*
4.55 1.64 1.07 34.86 0.33 42.18 0.26 0.13 – 15 2.51 8300
* Finenesses of limestone and bauxite were not determined since they were not used as an added agent like gypsum and desulfogypsum.
2.2.2. Production of clinkers and cements Proportions of raw materials and calcination parameters for a ‘‘reference clinker” were chosen with the goal of obtaining adequate quantities of both ye’elimite and belite (for both early and late strength development), and low free lime or unreacted raw material minerals. 40% limestone (LS), 40% bauxite (B) and 20% gypsum (G) were combined, by mass. A kiln maximum temperature of 1300 °C and a retention time of 120 min. were selected. Several other clinkers and cements were also prepared (Table 3). Kiln maximum temperatures and retention times higher and lower than those selected for the reference clinker were tried. The influence of different raw material proportions on the composition of the CSA clinker was also investigated, by preparing one other clinker (N6). The proportions of the raw materials in this clinker were selected using an optimization process. A set of ‘‘modified” Bogue’s equations [18] were used to estimate the contents of ye’elimite and other CSA clinker phases, and the Solver tool in Microsoft Excel was run with the objectives of maximizing the amount of ye’elimite and minimizing the free lime content in the clinker produced. The limestone and bauxite contents calculated for N6 were higher and the gypsum content was lower than in the reference clinker (N2). A high amount of B is necessary to provide Al2O3 for ye’elimite formation, however increasing B causes a decrease in the LS and/or G that can be used. As LS and G provide the CaO, and SO3 needed for ye’elimite, an optimum combination is reached for the maximum amount of ye’elimite to be produced. It should be underlined that the ‘‘modified” Bogue’s equations, similar to the original Bogue’s equations (for Portland cement clinker), consider only the oxide composition of the raw meal and ignore factors like calcination time or temperature, impure compounds, etc. All CSA clinkers obtained were ground in a ball mill and sieved through a 1 mm sieve prior to testing. A Blaine fineness of 4000 cm2/g was achieved for N1, N2, N3 and N4. An equal milling time yielded 2800 cm2/g Blaine fineness for N6. This difference can be related with the higher bauxite content of N6 which makes it relatively harder to grind. Then, the crystalline phase compositions of the clinkers were determined using quantitative XRD and Rietveld refinement, with MAUD [19] as the analysis program. N5, made at 1350 °C, melted partly and stuck to the refractory plate, allowing only a very small amount to be recovered. As its grinding
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Fig. 1. C S A clinker raw material pastes: (a) before firing, (b) after firing (image widths are 20 cm).
þ 2CSH 2 þ 34H ! C3 A:3CS:32H C4 A3 S þ 2AH3
Table 3 Raw mixture proportions and calcination parameters of C S A clinkers produced.
*
Clinker ID
Natural Materials (%) Limestone
Bauxite
Gypsum
N1 N2* N3 N4 N5 N6
40 40 40 40 40 45
40 40 40 40 40 45
20 20 20 20 20 10
Kiln Temperature (°C)
Retention Time (min.)
1300 1300 1300 1250 1350 1300
90 120 150 120 120 120
Clinker N2 is the ‘‘reference clinker”.
process contrasted with those of the other clinkers, a meaningful Blaine fineness value cannot be reported for N5. A ‘‘reference cement” was prepared with the reference clinker, by adding 19% separately-ground gypsum (by mass of clinker + gypsum). This amount was chosen to obtain a rapid-hardening (or high-early-strength) CSA cement without harmful expansions, based on a formula proposed by Zhang [20], which suggests that the optimum ratio of gypsum-to-clinker (Ct), based on stoichiometric calculations, needed for a CSA cement of desired nature is:
Ct ¼ 0:13 m A=S
ð1Þ
where A is the mass of ye’elimite in the clinker, S is the mass of SO3, 0.13 is a conversion coefficient between mass and molar ratios, and ‘‘m” is the molar ratio of gypsum to ye’elimite. m values lower than 1.5 give rapid-hardening cements, and higher m values give expansive or self-stressing cements. Using the oxide compositions of the raw materials (Table 1) and their proportions in the clinker (Table 2) in the modified Bogue’s equations, the potential ye’elimite content of N2 was estimated as 39% (by mass). Using this value and m = 2 in Eq. (1) gave Ct = 0.13*2*39/42 = 0.24. A gypsum-toclinker ratio of 0.24 corresponds to a gypsum-to-cement ratio of 0.19; hence 19% gypsum was added to 81% N2 clinker to obtain the reference cement. m = 2 was chosen, as it corresponds to the theoretical ratio of gypsum-to-ye’elimite needed to form ettringite during hydration and not monosulfate, according to Eqn. (2).
ð2Þ
Lower and higher m values both result in the formation of some monosulfate. As such, the ‘‘reference cement” could be expected to be rapid setting and hardening without excessive expansion. Cements with higher and lower added gypsum contents than in the reference cement were also prepared with some of the clinkers to investigate the influence of added gypsum on hydration (Table 4). It should be noted that the amount of ye’elimite in N2 quantified with XRD (49%, Section 3.1.1) indicates the need for a higher amount of added gypsum (23%) to give m = 2. Also, the reactivity of the gypsum used influences the effectiveness of Eqn. (1) in predicting CSA cement performance. The effect, on clinker phase and hydrated cement phase formation, of kiln maximum temperature was investigated using N2, N4, and N5 while that of kiln retention time was studied using N1, N2 and N3 (see Table 3). The effects of changing clinker raw mixture proportions and the amount of gypsum added to the clinker to prepare cements, were investigated using N2 and N6. The compressive strength development of each cement was investigated using 4x4x16 cm mortar specimens made with a sand-to-powder ratio of 2.75 and water-to-cement ratio (W/C) = 0.7. This value was chosen because preliminary mixtures with lower W/C exhibited low flow. The high hydration water requirement of ye’elimite and CSA A cements has been reported [21] and is known to increase with increasing added gypsum content [12]. The workabilities of the cement mortars were assessed using a flow table, according to ASTM C 1437 [22]. The heat evolution of the pastes were determined up to 48 h, using an isothermal calorimeter, on pastes with W/C = 0.4 to avoid bleeding in the fresh paste. The nature of the hydration products in the pastes were investigated using quantitative XRD, as well as thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), using 3-d and 28-d-old specimens. Scanning electron microscopy was also used, to study the microstructure of the hydrated pastes. For the mineralogical analyses, 20 g paste samples were taken, and ground to <150 mm in an agate mortar just before the tests. They were kept under same conditions as the mortars until the age of testing.
Table 4 C S A cements prepared to investigate the effect of production parameters. Investigated Parameter
Clinkers Used
Added Gypsum (wt.% of cement)
Added Citric Acid (wt.% of cement)
Water/ Cement
Calcination conditions Clinker raw mixture proportioning and added gypsum amount Water/Cement
N1, N2, N3, N4, N5 N2, N6 N2
19 14, 19, 24 24
– – 0, 0.5
0.7 0.7 0.6, 0.7
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3. Results 3.1. Influence of production parameters on clinker composition 3.1.1. Influence of kiln maximum temperature The influence of the kiln maximum temperature was investigated by calcining the reference clinker raw meal at 1250, 1300, and 1350 °C. The phase compositions of the different clinkers obtained are compared in Fig. 2 and Table 5. All three clinkers appear to contain the same main phases, ye’elimite, belite, and ferrite, but at different contents. The content of ye’elimite, the primary phase in CSA cements, is higher than that estimated by the modified Bogue’s equations for all three clinkers, and by 10% for N2, the reference clinker. Calcination at 1250 °C yields the lowest ye’elimite. In return, the belite contents are 6–9% lower than estimated, with N2 containing the least. Although the initial selection of the 40% LS: 40% B: 20% G proportions was intended to form some anhydrite in the clinker, the actual amount formed was low, especially for the reference clinker. It could be that a rise in temperature from 1250 °C to 1300 °C promotes further formation of ye’elimite at the expense of anhydrite but that some of the ye’elimite decomposes at 1300–1350 °C [23]. The calculated Brownmillerite contents were close to the estimated values. Free lime, intended to be zero was indeed quite low, <1.1% for all clinkers. Low free lime content is an indicator of adequate proportioning, calcination and clinkering. Just like in PC, its
Fig. 3. X-ray diffractograms for C S A clinkers produced using different kiln retention times (Legend: Be – Belite; Br – Brownmillerite; Y – Ye’elimite).
presence can lead to unsoundness due to the formation of Ca (OH)2 [15,24], however it is not considered a problem at <2%, and could even contribute to early strength [25]. The clinker produced at 1300 °C also contains the lowest amount of minor phases like gehlenite and merwinite. Clinker N5, made at 1350 °C, actually melted partly and stuck to the refractory plate. 1250 °C or 1300 °C appear to be suitable for CSA clinker preparation. 3.1.2. Influence of kiln retention time The phase compositions of the clinkers made with 40% LS : 40% B: 20% G, at 1300 °C for 90 min, 120 min, and 150 min, are compared in Fig. 3 and Table 6. There appears to be little difference (<1%) between the ye’elimite contents of clinkers calcined at the same kiln maximum temperature but exposed to it for different durations. The contents of other main clinker phases are also similar, with the most significant observation being an increase in the content of merwinite and a decrease in gehlenite, with increasing retention time. 3.1.3. Influence of raw mixture proportions The phase compositions of the clinkers made with 40% LS: 40% B: 20% G, and with 45% LS: 45% B: 10% G, by calcining for 120 min at 1300 °C, are compared in Fig. 4 and Table 7. Clinker N6, with greater limestone and bauxite content, was proportioned (Section 2.2.2) to yield the maximum amount of ye’elimite possible with these raw materials, 43.8%. The Al2O3 content of the selected bauxite (the only one available locally),
Fig. 2. X-ray diffractograms for C S A clinkers produced at different kiln maximum temperatures (Legend: Be – Belite; Br – Brownmillerite; Y – Ye’elimite).
Table 5 Phase composition of C S A clinkers produced at different kiln maximum temperatures. Phase
Belite Brownmillerite Ye’elimite Anhydrite Periclase Free Lime Gehlenite Perovskite Dolomite Tricalcium aluminate Merwinite Rwp / %
Amount (%)
Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Calculated Calculated Calculated Calculated Calculated Calculated
1250 °C (N4)
1300 °C (N2)
1350 °C (N5)
25.2 16.0 26.4 28.6 39.2 41.8 8.7 3.7 0.7 0.8 0 0.2 2.1 1.9 1.4 0.6 3.0 7.7
25.2 14.6 26.4 29.2 39.2 49.1 8.7 1.4 0.7 0.6 0 0.3 1.7 0.4 1.5 0.2 1.1 8.5
25.2 19.5 26.4 27.5 39.2 45.5 8.7 2.0 0.7 0.1 0 1.1 0 0.8 0.2 2.3 1.0 8.1
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Table 6 Phase composition of C S A clinkers produced using different kiln retention times. Phase
Amount (%)
Belite
Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Calculated Calculated Calculated Calculated Calculated Calculated
Brownmillerite Ye’elimite Anhydrite Periclase Free Lime Gehlenite Perovskite Dolomite Tricalcium aluminate Merwinite Rwp/%
90 min.(N1)
120 min.(N2)
150 min.(N3)
25.2 15.6 26.4 29.0 39.2 48.2 8.7 1.8 0.7 0 0 0 3.2 0.1 1.6 0.1 0 7.7
25.2 14.6 26.4 29.2 39.2 49.1 8.7 1.4 0.7 0.6 0 0.3 1.7 0.4 1.5 0.2 1.1 8.5
25.2 16.7 26.4 25.5 39.2 50.6 8.7 1.0 0.7 1.0 0 0 1.1 0.1 0.5 0 3.5 8.7
17.5% Fe2O3. The simultaneous increase of limestone and decrease of gypsum in N6 may have caused the complexing of ferric ions with calcium, later incorporating aluminum and iron at higher temperatures, in the sulfate-deprived environment [26]. 3.2. Influence of production parameters on cement pastes and mortars
Fig. 4. X-ray diffractograms for C S A clinkers produced using different raw mixture proportions (Legend: Be – Belite; Br – Brownmillerite; Y – Ye’elimite).
Table 7 Phase composition of C S A clinkers produced using different raw mixture proportions. Phase
Belite Brownmillerite Ye’elimite Anhydrite Periclase Free Lime Gehlenite Perovskite Dolomite Tricalcium aluminate Merwinite Rwp / %
Amount (%)
Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Modif. Bogue Calculated Calculated Calculated Calculated Calculated Calculated Calculated
40%LS : 40%B : 20%G (N2)
45%LS : 45%B : 10%G (N6)
25.2 14.6 26.4 29.2 39.2 49.1 8.7 1.4 0.7 0.6 0 0.3 1.7 0.4 1.5 0.2 1.1 8.5
26.4 15.4 29.2 39.5 43.8 40.5 0 0 0.8 0.4 0 0.3 0.8 1.4 0 0 1.9 7.2
51%, limits the maximum amount of ye’elimite that can be produced. However, this clinker did not even contain as much ye’elimite as the reference clinker (N2), instead it contained greater brownmillerite. This could be because the bauxite also contains
3.2.1. Influence of kiln maximum temperature and retention time on heat evolution The differences in the rates and cumulative amounts of heat evolution of cement pastes prepared by adding 19% gypsum (by mass of cement) to the various clinkers are shown in Fig. 5. Short induction times (1 h) are noticeable in Fig. 5a. The first peak (in the first few minutes) in all pastes is due to wetting and possibly early reactions like the conversion of hemihydrate to dehydrate. The second (main) peak, following the dormant period, is due to the reaction of ye’elimite and gypsum, with water. The total heat production of the cements at 48 h varies between 140 and 200 J/g, lower than for a typical PC paste. The pastes prepared from clinkers of the same raw meal (40%LS:40%B:20%G) but fired at 1250 or 1300 °C (N1, N2, N3, N4) gave similar main peak times, corresponding roughly to their final setting times. The paste of the clinker fired at 1350 °C (N5) shows an earlier and larger peak. This could be attributed to its slightly higher C3A and free lime contents. The heights of the rate of heat evolution peaks do not appear closely linked with the contents of ye’elimite in the clinkers, as one might expect. The cumulative heat evolved up to 48 h, however, appears to increase with increasing calcination temperatures [27]. 3.2.2. Influence of added gypsum content and raw mixture proportions on heat evolution There exists an optimum gypsum content for a given CSA clinker, to ensure that ye’elimite reacts with gypsum and water to form ettringite rather than react simply with water and form monosulfate. This optimum gypsum is the amount that gives a molar ratio of gypsum to ye’elimite of 2, based on the commonly assumed reaction equation [15,27,28]. This ratio corresponds to slightly different mass ratios, depending on the hydration state of the gypsum used. For hemihydrate, it is 0.475. So, assuming the gypsum used were purely hemihydrate (from Table 2), for N2 and N6, with 49% and 41% ye’elimite, respectively, the optimum clinker-to-gypsum ratios would be expected to be 81:19, and 84:16. Figs. 6 and 7 show the influence of increasing added gypsum content (from 0% to 24%, comprising gypsum contents surely lower
6
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Fig. 5. a) Rate of heat evolution; b) Cumulative heat evolved for C S A cement pastes containing 19% gypsum.
Fig. 6. a) Rate of heat evolution; b) Cumulative heat evolved for C S A cement pastes containing N2 and different amounts of added gypsum.
and higher than the optimum contents of the two cements) on the heat evolution of clinkers N2 and N6. The N2 clinker-only paste shows an initial peak immediately following the addition of water, attributed to wetting and initial dissolution of the anhydrous clinker [29,30]. This is followed by a rather long dormant period of 12 h, which could be explained by the surfaces of clinker particles being covered by early hydration products [31]. Winnefeld and Barlag [32] also reported 15 h induction periods for CSA clinkers hydrated at a water/ cement ratio of 0.7. The main peak is due to the formation of mostly monosulfate, some ettringite (due to the anhydrite in N2, Table 7) and possibly some calcium aluminate hydrates [29,33]. The pastes with 14–24% added gypsum also show the initial wetting peak. Their main peaks are all similar in magnitude and occur at 5 h, much earlier than for the clinker-only paste. A secondary/ shoulder peak is visible on all curves, except the paste containing 24% gypsum. This shoulder shifts slightly to later ages [34] and its intensity decreases with increasing gypsum content, and it is barely noticeable for 19% gypsum. Hence, the ‘‘optimum” gypsum content for N2 (to form ettringite rather than monosulfate [35]), is 19 or slightly higher. Heat evolution for the pastes with greater gypsum contents is prolonged (Fig. 6b). The paste with 24% gypsum, despite having the lowest clinker content, evolves greater heat than the others and accordingly, the mortar prepared with the 24% gypsum paste achieves the highest strength at 28 d (Section 3.2.6). Unlike the N2 clinker-only paste, the N6 clinker-only paste gives a main hydration heat rate peak at about the same time as its gypsum-containing counterparts. Doval et al. [31] reported a similar heat flow peak for a synthetic ye’elimite hydrated at 25 °C and W/C = 0.7, but did not provide specifics about the clinker phase contents. The main peak is also quite wide and appears to contain within it smaller heat flow peaks, which may be related
Fig. 7. a) Rate of heat evolution; b) Cumulative heat evolved for C S A cement pastes containing N6 and different amounts of added gypsum.
with the hydration of C4AF and some minor phases. Addition of gypsum gives heat flow behavior similar to those of pastes made with N2, with shoulder peaks clearly visible up to and including
O. Canbek, S.T. Erdog˘an / Construction and Building Materials 239 (2020) 117866
19% gypsum, disappearing only for the case with 24% gypsum. Hence, the optimum gypsum content for N6 appears to be 19–24%, slightly higher than for N2. This is rather surprising, considering the lower ye’elimite content of N6, and could be related with some gypsum being consumed for the hydration of C4AF, which also produces monosulfate and ettringite in the absence and presence, respectively, of added sulfates [13]. N6 contains 10% more C4AF than N2, due to the reduced gypsum and increased bauxite in its raw meal. 3.2.3. Influence of kiln maximum temperature and retention time on hydration products The formation of hydration products was investigated using XRD and TGA/DSC. Fig. 8 shows the change in the X-ray diffractograms of the reference cement paste (N2 with 19% added gypsum), from 1 to 28 d. The main hydration product is ettringite, from the reaction of ye’elimite and gypsum, with water. There also exist unhydrated clinker phases in the hydrated paste. A large decrease in the intensity of the ye’elimite peak is observed between the unhydrated paste and the 1-d paste. This is accompanied by the growth of the ettringite peaks. Then, beyond 1 d, not much change is observed in the diffractograms, consistent with the limited corresponding strength gains (Section 3.2.5). The diffractograms of hydrating pastes made from N1-N5 are all quite similar (not shown), showing peaks for the same hydration products as in Fig. 8, with slight differences in intensity. The formation of hydration products was also investigated using thermal analysis. Fig. 9 shows the heat flow and mass loss upon heating of cement pastes made with the various clinkers and 19% added gypsum, at 3 d and 28 d, respectively. Mass loss between 80 and 150 °C is due to the decomposition of ettringite. Conversion of any unreacted gypsum to anhydrite occurs at 100–160 °C. AH3 expected to form simultaneously with ettringite, but not observed in XRD, is identified between 250 and 280 °C [28,29,36]. Little mass loss is observed at higher temperatures, until at 700 °C. This loss is attributed to the breakdown of calcium carbonate, formed due to the carbonation of the paste specimens. As such, the loss amount grows considerably from 3 to 28 days. CSA cements are known to carbonate much faster than PC mortars [37], particularly at high W/C. Additionally, the anhydrite content of the CSA pastes strongly affects the ye’elimite
7
reaction kinetics which plays an important role in imparting carbonation resistance to CSA cements [38]. 3.2.4. Influence of added gypsum content and raw mixture proportions on hydration products Fig. 10 shows the change in the X-ray diffractograms of the cement pastes made of N2 and N6 with different amounts of added gypsum, from 1 to 28 d. Higher added gypsum content in the cements resulted in more intense gypsum peak. Ettringite peaks of N2 are larger than for N6, at equal clinker:gypsum ratios, due to a greater amount of initial ye’elimite reacting. A greater amount of ettringite forms for N2 with clinker:gypsum = 81:19 (close to the optimum gypsum content) than with higher or lower added gypsum. Thermal analysis was performed to elucidate the hydration mechanism. Fig. 11 shows the heat flow and mass loss of cement pastes made with N2 and N6, and with various amounts of added gypsum, at 3 d and 28 d, respectively. Hydration products identified are identical for all mixtures as ettringite, gypsum, AH3 and calcite. Monosulfate, an expected phase for undersulfated mixtures, is not discernible with TGA (contrary to calorimetry) at its decomposition temperature of 200 °C [33,39]. Mass loss observed in 700–800 °C at 28 d is indicative of carbonation of the cement pastes. Fig. 12 shows SEM images of cement pastes made with N2 and N6 and 24% gypsum, at 3 d. In both pastes, ettringite crystals are surrounded by some unhydrated particles and amorphous hydration products which could be AH3. Also, the sponge-like fraction (Fig. 12b) can be attributed to the ye’elimite with ettringite formed in its pores [40]. 3.2.5. Influence of kiln maximum temperature and retention time on strength The strength developments of cement mortars prepared with the various clinkers (from Table 3), and 19% added gypsum are compared in Fig. 13. The strength gain of the mortars made with N2 (the ‘‘reference mortar”) and N4, containing clinkers made at different kiln temperatures, were similar, despite ye’elimite contents that differed by more than 7%. The 1-d strengths of both were 18 MPa, but their strength developments beyond 1-d were very little. Since not all the ye’elimite in a cement can react, particularly up
Fig. 8. Hydration product development of N2 up to 28 days (Legend: Be – Belite; Br – Brownmillerite; E – Ettringite; G – Gypsum; Y – Ye’elimite).
8
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Fig. 9. Heat flow and mass loss of C S A cement pastes containing 19% gypsum, by mass, a) 3 d; b) 28 d.
Fig. 10. XRD patterns of cement pastes with clinker-to-gypsum ratios of 86:14, 81:19, 76:24, at 28 days of hydration: a) N2; b) N6 (Legend: Be – Belite; Br – Brownmillerite; E – Ettringite; G – Gypsum; Y – Ye’elimite).
to 1 d, the effect of ye’elimite contents of the clinkers becomes insignificant, probably as long as the clinker contains more than a minimum amount of it. The small increase in the strength of N2 (5 MPa) from 1 d to 28 d could be explained by the rather low amounts of belite and the lack of reactivity of the high
amounts of ferrite they contain. N1 and N3, with their slight higher belite contents, gain greater strength in this time period. Mortars with N5 could not be made, as an amount sufficient to prepare mortars could not be obtained due to the production problems mentioned in Section 3.1.1.
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Fig. 11. Heat flow and mass loss of C S A cement pastes consisting of N2 and N6 with various amounts of added gypsum, by mass, a) N2 at 3 d; b) N2 at 28 d; c) N6 at 3 d; d) N6 at 28 d.
Fig. 12. SEM images, at 3 d, of hydrated cement pastes: a-b) N2; c-d) N6.
10
O. Canbek, S.T. Erdog˘an / Construction and Building Materials 239 (2020) 117866 Table 8 Flow table values of C S A cement mortars. Added Gypsum Amount (%)
Flow (%)
14 19 24
N2
N6
93 65 55
90 80 50
continues for a longer time (or is delayed) which may damage the microstructure [15,32], resulting in a drop in strength. Strength is recovered beyond age 7 d, indicating the prior cessation of ettringite formation.
Fig. 13. Compressive strength development of C S A cement mortars containing 19% gypsum.
3.2.6. Influence of raw mixture proportions and added gypsum amount on strength Fig. 14 shows the change in strength development of mortars made using clinkers with different compositions (N2 and N6), and varying amounts of added gypsum, at W/C = 0.7. The 1-d strengths appear to slightly increase as the added gypsum content increases. At 28 d, the strength of the 24% gypsum mortars is highest for both clinkers but at 14% and 19% gypsum, the opposite is true. This outcome could be related with the differences in the workabilities of the mortars, which decreased with gypsum addition (Table 8). The spread of the mortar on the flow table decreases as added gypsum is increased from 14% to 19% which may have caused some compaction problems that overcame the benefit of the added gypsum in the early stages of hydration. The relative changes in the flows of the mortars of the two clinkers also reflect the difference in their optimum gypsum contents. For N2, with an optimum gypsum content closer to 19% (Section 3.2.2), the decrease in flow is greater from 14 to 19% but not so great afterwards. For N6, which has an optimum gypsum content higher than 19%, the change in flow is greater from 19 to 24%. A slight drop is observed in the strength of the 24% gypsum N6 mortar (Fig. 12b) between 3 d and 7 d. This drop is attributed to delayed (or continued) ettringite formation [41]. In systems with low added gypsum, ettringite formation is terminated shortly after setting. In systems with higher added gypsum content (or when the added calcium sulfate has low reactivity), ettringite formation
3.2.7. Influence of W/C on the strength of CSA mortars The water need of CSA cements is known to be higher than that of PC and to be dependent on added gypsum content. The strength development of mortars made with the reference clinker and 24% added gypsum are compared at WC = 0.6 and W/C = 0.7, in Fig. 15. A large increase (50%) in 1-d strength, and an even larger increase (60%) in 28-d strength is noticed when W/C is reduced from 0.7 to 0.6. The mortar with W/C = 0.6 is not only more compact initially but contains some citric acid. Samples containing citric acid have been reported to yield more compact pore
Fig. 15. Strength development of C S A cement mortars prepared at different W/C using N2.
Fig. 14. Compressive strength development of C S A cement mortars, with different amounts of added gypsum, a) N2; b) N6.
O. Canbek, S.T. Erdog˘an / Construction and Building Materials 239 (2020) 117866
structures and better distribution of ettringite, leading to increased strength [41,42]. A slight decrease in strength between 3 d and 7 d is observed for N2 with 24% gypsum, similar to that in Fig. 14b for N6, when W/C is reduced from 0.7 to 0.6. This can also be attributed to the damage due to delayed ettringite formation [41] in the relatively denser matrix formed with lower W/C. Addition of 0.5% citric acid rendered the lower-W/C N2 mortars more fluid than the higher-W/C mortars without citric acid.
4. Conclusions The influence of varying production parameters on certain properties of CSA clinkers and cements were investigated experimentally. The following conclusions were reached: The amounts of clinker phases predicted using the set of modified (for CSA) Bogue’s equations [18] differed significantly from those determined with quantitative XRD phase analysis. Lower amounts than predicted, of belite and anhydrite and greater amounts of ye‘elimite were observed to form. Clinker raw meal proportions of 40% limestone, 40% bauxite, and 20% gypsum appear to be suitable to produce a clinker with 49% ye’elimite and 15% belite, with the natural raw materials chosen. The Al2O3 content of the chosen bauxite limits the maximum amount of ye’elimite that can be produced. A maximum kiln temperature of 1250 °C and a retention time of 90 min. are sufficient to produce a clinker with strength development similar to that achieved using higher temperatures and longer durations, at equal added gypsum contents. Raw meal composition and kiln temperature have a greater influence on clinker phases than retention time. The amount of gypsum to be added to a CSA clinker for formation of ettringite and not monosulfate (19–24% in this study), can be determined using isothermal calorimetry, by observing the formation of a shoulder on the descending branch of the main rate of heat evolution peak. The main hydration products formed in hydrated CSA pastes are ettringite, gypsum and AH3. Hydration is rapid until 3 d after which it slows down due to the coating of unhydrated particles with ettringite and AH3. Compressive strength gain of CSA mortars is similarly rapid, 20 MPa at 1 d, subsequently slowing down, 25 MPa at 28 d, with 19% gypsum added to the clinker. Use of citric acid allows lowering the W/C from 0.7 to 0.6, resulting in strength increases of 50–60%, up to 50 MPa at 28 d with 24% added gypsum. Both the early and late-age compressive strengths of mortars increase with increasing amounts of gypsum blended with the CSA clinker. Higher gypsum also translates to less clinker. However, increasing gypsum too much can cause workability and dimensional stability issues in CSA systems. The compositions of CSA clinkers and cements influence their hydration heat evolution, whereas the influence of kiln parameters is insignificant. Similarly, addition of gypsum greatly reduces the heat evolved in the first few days. Calcite determined in the hydrated pastes indicates the tendency of all produced CSA cement pastes to carbonate.
CRediT authorship contribution statement Og˘ulcan Canbek: Writing - original draft. Sinan T. Erdog˘an: Writing - original draft.
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