The effect of synthetic CAH (130°C; 8 h; CaO/(SiO2 + Al2O3) = 0.55; Al2O3/(SiO2 + Al2O3) = 0.1, 0.15) with different crystallinity on the hydration kinetics of OPC at early stages of hydration was investigated. Also, the formation mechanism of compounds during OPC hydration was highlighted. It was determined that the synthetic CAH accelerated the initial reaction and shortened the induction period. Also, the second and third exothermic reactions begun earlier, and, during the latter reaction, the higher values of the heat flow were obtained in comparison with pure OPC samples. At later stages of hydration, synthetic CAH affect the OPC hydration as the usual pozzolanic additives; moreover, the larger values of cumulative heat were reached. It should be noted that the nature of synthetic CAH samples accelerated the dissolution of gypsum and stimulates the earliest C3S hydration.
Supplementary cementitious materials (SCMs), including fly ash, ground granulated blast furnace slag, silica fume, calcined clays, and natural pozzolans, are commonly blended with clinker to make Portland cement or are used as a partial replacement of this component in concrete [
It is known that SCMs can be of natural and synthetic origin or used as industrial by-products [
For this reason, due to the high early strength, thermal and chemical stability, and excellent durability with respect to ordinary Portland cement (OPC) the synthetic SCMs have attracted a great scientific attention, when employed in building and ceramic industries [
In previous work [
Aluminium readily enters the calcium silicate hydrate phase (C–S–H) of Portland cement, and this substitution is expected to play a significant role in many aspects of the chemical behavior of cement paste, including the cation and anion exchange behavior, solubility, and the progress of the reactions that occur during delayed ettringite formation [
In previous work [
For this reason, the main objective of this study was to determine the effect of synthetic CAH with different crystallinity on the hydration kinetics of OPC at early stages of hydration. Also, the compound formation mechanism is highlighted.
In this paper the following reagents were used: SiO2·
Dry primary mixtures with CaO/(SiO2 + Al2O3) = 0.55 and Al2O3/(SiO2 + Al2O3) = 0.1; 0.15 were mixed with water to reach the water/solid ratio of the suspension equal to 10.0. The hydrothermal synthesis has been carried out in unstirred suspensions in 25 mL volume PTFE cells, which were placed in “Parr instruments” (Germany) autoclave, under saturated steam pressure at 130°C temperature for 8 h (the temperature was reached within 2 h). After hydrothermal treatments, the autoclave was quenched to room temperature. The suspensions after synthesis were filtered and products were rinsed with ethanol to prevent carbonization of materials, dried at 50 ± 5°C temperature for 24 h, and sieved through a sieve with a width of 80
Samples of OPC were prepared in a laboratory grinding mill by grinding cement clinker (JSC “Akmenes cementas,” Lithuania) with a 4.5% additive of gypsum (“Sigma-Aldrich,” Germany) up to
Chemical and mineralogical composition of clinker.
Oxides | SiO2 | Al2O3 | Fe2O3 | CaO | MgO | Na2O | K2O | | Ignition of losses | Insoluble particles |
---|---|---|---|---|---|---|---|---|---|---|
Amount, % | 19.72 | 5.41 | 4.21 | 62.76 | 3.41 | 0.16 | 1.08 | 2.08 | 0.93 | 0.24 |
Minerals | 3CaO·SiO2 | 2CaO·SiO2 | 3CaO·Al2O3 | 4CaO·Al2O3·Fe2O3 | ||||||
Amount, % | 63.19 | 8.89 | 7.21 | 12.81 |
In the next stage of experiment, synthesized CAH samples were added as a partial replacement of the OPC at levels of 10% by weight of the total cementitious material. The additives of hydrated calcium aluminate were labeled as OPC-CAH1 ((CaO/(SiO2 + Al2O3) = 0.55, Al2O3/(SiO2 + Al2O3) = 0.1)) and OPC-CAH2 (CaO/(SiO2 + Al2O3) = 0.55, Al2O3/(SiO2 + Al2O3) = 0.15) depending on the composition of primary mixtures.
The X-ray powder analysis (XRD) was performed on the D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) operating at the tube voltage of 40 kV and tube current of 40 mA. The X-ray beam was filtered with Ni 0.02 mm filter to select the CuK
Simultaneous thermal analysis (STA: differential scanning calorimetry, DSC, and thermogravimetry, TG) was also employed for measuring the thermal stability and phase transformation of samples at a heating rate of 15°C min−1; the temperature ranged from 30°C up to 900°C under air atmosphere. The test was carried out on a Linseis instrument STA PT1000. The ceramic sample handlers and crucibles of Pt were used.
Differential scanning calorimetry (DSC) analysis was performed by a Netzsch DSC 214 Polyma instrument. This method was employed for measuring the thermal stability and phase transformation of samples at a heating rate of 10°C min−1, and the temperature ranged from 30°C up to 600°C under air atmosphere. Ceramic sample handlers and Al crucibles were used.
An eight-channel TAM Air III isothermal calorimeter was used to investigate the heat evolution rate of the samples. Glass ampoules (20 mL) each containing 3 g dry cementitious material were placed in the calorimeter and the injection units for each ampoule filled with amounts of water equivalent to a W/(OPC and additive) ratio of 0.5. After a steady temperature of 25°C had been reached, the water was injected into the ampoules and mixed inside the calorimeter with the dry material for 20 s (frequency 2-3 s−1). The heat evolution rate was then measured over a period of 72 h. Repetition of the measurements showed deviations in total heat below 3% for samples of similar type. Apart from the first minutes of water additive and mixing, the heat evolution rates were essentially identical. The rate of heat evolution was calculated on the basis of a unit weight of OPC.
In order to investigate the minerology and chemical composition of compounds formed during hydration, the heat evolution experiments were repeated at 25°C for different time periods (1.8, 3, 9.5, 13, and 24 h), which corresponded to the onset/peak/end duration of different early hydration periods. Hydration of samples was stopped by using acetone. Later on, the samples were crushed to powder, dried at the temperature of
The data of cumulative heat of hydration as well as the rate of heat evolution of the binary blended pastes are presented in Figure
The heat evolution rate (a–c) and cumulative heat (d) of OPC (1), OPC-CAH1 (2), and OPC-CAH2 (3) samples during the early stage of hydration.
The heat of hydration curves for pure OPC and OPC with additives shows the typical five stages of the hydration reaction (the initial reaction, the induction period, the acceleratory period, the deceleratory period, and the period of slow continued reaction) as described in the literature [
It was determined that the additives of synthetic CAH samples in OPC samples accelerated the initial reaction (1-2 min) because an increase in the maximum heat evolution rate was observed from 0.005 Wg−1 to 0.011 Wg−1 (OPC-CAH2) and 0.018 Wg−1 (OPC-CAH1) (Figure
At later stages of hydration, synthetic CAH samples affect the OPC hydration as the usual pozzolanic additive; moreover, the largest value of cumulative heat was reached in the OPC-CAH2 sample (Figure
It was determined that, during early OPC hydration, the stability and reactivity of OPC-CAH1 and OPC-CAH2 significantly depend on the primary mixture composition used for hydrothermal synthesis. OPC-CAH1 additive fully reacted already after 1.8 h of hydration; meanwhile, in a case of OPC-CAH2, only 33% (14.65 J/g) of calcium aluminium hydrate reacted (Figure
The main characteristics of thermal effects typical of CAH.
Composition | Hydration time (h) | Onset, °C | Peak, °C | Heat of process, J/g | Unreacted CAH, wt.% |
---|---|---|---|---|---|
OPC-CAH2 | 0 | 250.2 | 268.6 | 21.81 | 100 |
1.8 | 253.8 | 271.0 | 14.65 | 67.17 | |
3 | 254.5 | 273.3 | 14.51 | 66.53 | |
5.5 | 256.7 | 276.3 | 13.67 | 62.68 | |
9.5 | 261.6 | 280.5 | 12.24 | 56.12 | |
13 | 265.2 | 282.5 | 11.94 | 54.75 | |
16.5 | 264.1 | 286.0 | 11.71 | 53.69 | |
24 | 267.7 | 286.8 | 10.39 | 47.64 |
DSC curves OPC with 10 wt.% additive after 1.8 h (a) and 16.5 h (b): 1, OPC; 2, OPC-CAH1; 3, OPC-CAH2.
However, when the duration of OPC hydration was extended to 24 h, the amount of unreactive compounds was decreased (Figure
It should be noted that the nature of CAH accelerated the dissolution of gypsum. In the pure system, the latter compound fully reacted only after 16.5 hours of hydration, whereas in the samples with additives it fully reacted already after 13 h. It is clearly visible in DSC curve: the endothermic effect in a 105–135°C temperature range, which corresponded to the dehydration of gypsum, disappeared (Figure
The main characteristics of thermal effects typical of CSH and gypsum by DSC method.
Composition | Hydration time (h) | CSH | Gypsum | |||||
---|---|---|---|---|---|---|---|---|
Onset, °C | Peak, °C | Heat of process, J/g | Onset, °C | Peak, °C | Heat of process, J/g | Unreacted gypsum, wt.% | ||
OPC | 1.8 | 45.8 | 66.7 | 14.89 | 112.5 | 122.4 | 16.23 | 69.01 |
3 | 48.0 | 64.4 | 17.22 | 108.1 | 118.3 | 16.69 | 70.96 | |
5.5 | 52.4 | 73.6 | 28.12 | 111.1 | 119.4 | 10.97 | 46.64 | |
9.5 | 54.7 | 76.9 | 40.88 | 113.3 | 119.8 | 6.86 | 29.15 | |
13 | 57.5 | 82.0 | 53.01 | 116.2 | 121.3 | 0.80 | 3.36 | |
16.5 | 61.0 | 87.8 | 71.84 | — | — | — | — | |
24 | 58.8 | 86.3 | 77.42 | — | — | — | — | |
| ||||||||
OPC-CAH1 | 1.8 | 49.0 | 66.8 | 18.61 | 113.6 | 121.0 | 13.63 | 74.93 |
3 | 48.4 | 68.0 | 22.53 | 112.2 | 119.4 | 10.33 | 43.92 | |
5.5 | 54.5 | 75.7 | 37.60 | 115.1 | 121.0 | 6.02 | 25.57 | |
9.5 | 58.7 | 81.5 | 51.10 | 116.6 | 122.7 | 0.29 | 1.24 | |
13 | 62.7 | 87.6 | 63.13 | — | — | — | — | |
16.5 | 59.5 | 84.8 | 66.49 | — | — | — | — | |
24 | 58.5 | 84.5 | 71.74 | — | — | — | — | |
| ||||||||
OPC-CAH2 | 1.8 | 44.4 | 63.0 | 17.20 | 110.5 | 118.3 | 13.54 | 67.60 |
3 | 49.4 | 69.3 | 21.10 | 111.7 | 119.6 | 11.15 | 55.67 | |
5.5 | 55.2 | 76.9 | 37.32 | 114.7 | 120.8 | 4.74 | 23.68 | |
9.5 | 64.7 | 85.1 | 54.86 | 116 | 121.8 | 0.51 | 2.54 | |
13 | 61.9 | 85.8 | 64.14 | — | — | — | — | |
16.5 | 61.4 | 85.1 | 58.51 | — | — | — | — | |
24 | 60.2 | 86.6 | 66.50 | — | — | — | — |
In this report, the amounts of C3S that have reacted after heat evolution experiments at different time period under normal conditions were determined by the quantitative analysis (QXRD). The quantity of C3S was calculated from the intensity change of the basic reflection (
The quantity of unreactive C3S in pure OPC, OPC-CAH1, and OPC-CAH2 samples after different duration of hydration.
It was determined that after 5.5 h of hydration, only 9% of C3S reacts in pure OPC samples and the further reduction of its quantity depends on duration of hydration (Figure
Presumably, the synthetic CAH sample also induced the formation mechanism of ettringite.
In order to prove this fact, the intensity change of the basic reflection of ettringite (
The obtained results showed that after 3 hours of hydration, the area of the main diffraction peak typical to ettringite increased in two times in comparison with the pure OPC samples (Figure
The values of the main diffraction peak area of ettringite in OPC, OPC-CAH1, and OPC-CAH2 samples.
Composition | Hydration time (h) | |||||||
---|---|---|---|---|---|---|---|---|
0 | 1.8 | 3 | 5.5 | 9.5 | 13 | 16.5 | 24 | |
OPC | — | 0.14 | 0.15 | 0.26 | 0.40 | 0.60 | 1.00 | 1.45 |
OPC-CAH1 | — | 0.15 | 0.37 | 0.42 | 0.89 | 0.90 | 1.18 | 1.39 |
OPC-CAH2 | — | 0.21 | 0.22 | 0.54 | 0.51 | 1.03 | 1.18 | 1.30 |
The change of the main diffraction peak of ettringite in samples after 3 h of hydration: 1, OPC; 2, OPC-CAH1; 3, OPC-CAH2.
In addition, due to the accelerated dissolution of gypsum, a larger amount of formed ettringite was observed in OPC-CAH1 and OPC-CAH2 samples. Meanwhile, in an excess of gypsum in the pure OPC samples, the area of the main diffraction maximum of ettringite was slightly higher after 24 h of hydration (Table
It was examined that the additives accelerated the initial reaction and shortened the induction period. Also, the second and third exothermic reactions begun earlier, and, during the latter reaction, the higher values of the heat flow were obtained in comparison with pure OPC samples. At later stages of hydration, synthetic CAH affect the OPC hydration as the usual pozzolanic additives; moreover, the larger values of cumulative heat were reached.
It was noticed that additives stimulate the earliest C3S hydration (5.5–16.5 h): after 5.5 h of hydration, only 9% of C3S reacts in pure OPC samples and the further reduction of its quantity depends on duration of hydration. Meanwhile, in a case of CAH1 and CAH2 additives, 21% and 18% of this compound reacted.
It should be noted that the nature of synthetic CAH samples accelerated the dissolution of gypsum. In the pure system, the latter compound fully reacted only after 16.5 hours of hydration, whereas in the samples with additives it reacted already after 13 h. For this reason, a larger amount of formed ettringite was observed in OPC-CAH1 and OPC-CAH2 samples till 16.5 h of hydration.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was funded by a grant (no. MIP-025/2014) from the Research Council of Lithuania.