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Vietnam Journal of Science and Technology 56 (1A) (2018) 159-166 STUDY OF THE EFFECTS OF GYMPSUM, POZZOLANIC ADDITIVE MIXTURES AND THEIR FINENESS ON THE STRENGTH OF PCB50 FICO CEMENT Hoang Canh Nguyen, Nguyen Thi Tuyet Mai*, Tran Thi Xuan Lan, Huynh Dang Chinh, Ta Ngoc Dung School of Chemical Engineering, HUST, 1 Dai Co Viet, Hai Ba Trung District, Ha Noi * Email: maibk73@gmail.com Received: 15 August 2017; Accepted for publication: 5 March 2018 ABSTRACT In the study, raw materials including clinker, gypsum, pozzolan were separately milled on a ball mill (the ball mill size: φ500 mm × L500 mm, productivity: 5 kg / grinding time) to achieve the desired blaine fineness. A mixture is mixed at the calculated ratio includes of 90 % fineness clinker, 5 % fineness gypsum and 5 % fineness pozzolan (In it, pozzolanic additive is mixed at different fineness levels). The effects of the mixture of gypsum and fineness pozzolanic additive as well as the fineness of pozzolanic additive to strength of PCB50 Fico cement were investigated. The study shows that, pozzolanic additive have reduced the strength of cement. But, when pozzolanic additive is mixed with gypsum, this additive mixture has increased the strength of the cement at the late time (at the age of 28 days). The fineness of pozzolan has little effected on the early strength of cement. Keywords: gypsum, pozzolan, additive of cement, strength of cement. 1. INTRODUCTION According to statistics of Vicem, total cement production in Vietnam in the years 20152016 is about 75-76 million tons. Forecast production plans in the years 2016-2020 should reach 93-95 million tons to meet the demand for cement in the domestic and export [1, 2]. Therefore, the cement industry is the spearhead of building materials industry. It plays an important role in the national economy. There have been many studies in the world that have studied the effects of additive on improving the properties of cement, as well as improving the strength of cement. Research by T. Vuk et al. studied the effect of limestone additive on grinding process of the cement [3]. Research by E. Sakai et al. studied the effect of fineness of limestone and gypsum additive as well as clinker’s fineness on early strength of cement [4]. Research by Wang P. Z. et al. studied the effect of fineness and particle size distribution of activated blast furnace slag on the strength of cement [5]. These studies are aimed to increase productivity, improve product quality, reduce the cost of energy consumption and raw materials contributing to lower product cost. Besides, Hoang Canh Nguyen, et al. the cement industry is constantly innovating in both technology and manufacturing equipment to increase competitiveness in the domestic cement consumption market and export. The cement industry needed to ensure the properties of cement as activity, curing speed, etc. It is decided not only by the mineral and chemical composition of clinker, shape and size of the crystals as tricalcium silicate (C3S), dicalcium silicate (C2S), etc. but also depends a lot on the fineness of the product, composition and shape of cement and additive particles. Compressive strength of cement depends on hydration level of cement, the amount and essence of hydrated crystals and especially the arrangement of hydrate substances (structure of hardened cement paste). Therefore, the smoother the cement grains, the higher the surface area of the reaction. This leads to increase hydration speed, the amount of crystals created is also greater. It is the cause that affects much to the early strength of cement. If there is more coarse cement, it will be difficult to completely hydrate. Because of the fact that the reaction only occurs at a certain depth [3-11]. With the above analysis, this study investigated the effect of combination of gypsum and pozzolanic additive and fineness of mixed additive on the compressive strength of PCB 50 Fico cement. 2. EXPERIMENTAL 2.1. Materials Materials used in the study include: Tay Ninh Fico clinker with standard of C PC50 (TCVN 7024:2013); Thai Lan gypsum additive with gypsum standard of G90 (TCVN 9807:2013); Binh Phuoc pozzolanic additive- the active mineral additive has a strong activity (according to standard of TCVN 6882:2001). Chemical and mineral composition of Tay Ninh Fico clinker were shown in Table 1. Table 1. Chemical and mineral composition of Tay Ninh Fico clinker. Chemical composition, % Mineral composition, % Insoluble matter MKN SO3 SiO2 Al2O3 Fe2O3 CaO MgO 0.24 0.89 0.40 21.04 5.62 3.60 64.31 3.76 C3S C2S C3A C4AF 58.92 16.19 8.81 10.94 2.2. Preparation of cement samples The clinker, gypsum raw materials were preliminarly crushed separately to a size < 5 mm. Then each was crushed in the ball mill for 65 minutes (the ball mill size: φ500 mm × L500 mm, productivity: 5 kg/ground time). Puzolanic additive was ground separately in the ball mill with different grinding times: 20, 35, 65 minutes. A mixture of 95 % clinker and 5 % pozzolan (at three ground times of pozzolanic additive: 20, 35, 65 minutes) were mixed evenly to form the cement - pozzolan samples. The formed cement samples were denoted P20, P21, P22, respectively. Another mixture of 90 % clinker, 5 % gypsum and 5 % pozzolan were mixed evenly to form the cement - (pozzolan, gypsum) samples (pozzolanic additive used on three fineness with ground time at 20, 35, 65 minutes). The cement - (pozzolan, gypsum) samples 160 Study of the effects of gympsum, pozzolanic additive mixtures and their fineness … were denoted TP20, TP21, TP22, respectively. Two types of cement samples were formed by the same process. But, one type of sample was formed by a mixture of 95 % clinker and 5 % gypsum, was denoted T22. Another type of sample was formed by 100 % clinker (without gypsum or pozzolanic additive), was denoted M22. The formed cement samples P20, P21, P22, TP20, TP21, TP22, T22, M22 were studied of the effect of composition, fineness of gypsum and pozzolanic additive on the properties of cement. 2.3. Study methods The methods used in the study are the following. Water demand and setting time of cement were determined by Vicat and Le Chatelier methods (TCVN 6017-1995 ISO 9597-1989), soundness of cement was determined by observing the volume expansion of cement paste that has the normal consistency, through the displacement of two pincer of the Le Chatelier mold; the blaine fineness of cement was determined following the standard TCVN 4030-2003; the compressive strength of cement was determined following the standard TCVN 6016-1995 ISO 679-1989; the scanning Electron Microscope (SEM) method was used to study the microstructure of cement paste. 3. RESULTS AND DISCUSSION 3.1. Effect of puzolanic additive on the compressive strength of cement The results of the effect of pozzolanic additive on the blaine fineness and on the compresive strength of cement - pozzolan samples P20, P21, P22 and M22 (comparative sample) were shown in Table 2 and Figure 1. Table 2. The results of fineness and compressive strength of cement - pozzolan samples P20, P21, P22 and M22. The percentage of weight (%) CLK Pozzolan Ground times of pozzolan (minutes) P20 95 5 20 4169 4241 10.6 23.5 32.3 38.2 P21 95 5 35 5226 4261 10.9 23.7 33.3 38.6 P22 95 5 65 6700 4341 11.6 25.3 33.8 38.8 M22 100 0 65 - 4286 11.7 23.4 31.6 45.3 Sample The blaine fineness of pozzolan (cm2/g) The blaine fineness of the mix (cm2/g) R1 R3 R7 R28 Compressive strength (MPa) The results in Table 2 and Figure 1 show that within the studied fineness range are 4169÷6700 cm2/g, when increasing the fineness of pozzolanic additive, the compressive strength of cement samples tends to increase but not much (increased level of the compressive strength of cement samples < 5 %). The studied fineness of pozzolanic additive has little effect on the compressive strength of cement. Referred to the studied results on the effects of gypsum and limestone additive (replacement rate equivalent), pozzolanic additive tends to reduce the strength of cement. Meanwhile, gypsum and limestone additives tend to increase the early strength of cement [12, 13]. This can be explained as limestone additive can participate in 161 Hoang Canh Nguyen, et al. 50 P20 P21 P22 M22 TP20 Compressive strength, MPa Compressive strength, MPa reactions that produce products such as C3A.CaCO3.11H2O and C3A.3CaCO3.32H2O. These products have small volume and easy to swell up. So, it is easy to go into sealing porous holes, cracks in cement. Thereby, limestone additive can improve the early strength of cement. Gypsum additive also has similar effects, it can participate in reactions that produce products such as C3A.4CaSO4.31H2O (ettringite). This compound has needle-shaped crystals and can interwoven to create original skeleton structure of cement. Thus its volume increased 2 times compared with the original. This contributes to increasing compaction of cement and leads to increasing intensity of cement. 40 30 20 10 0 1 day 3 days 7 days Figure 1. The graph showing the effect of puzolan additive on compressive strength of cement samples P20, P21, P22 and M22. TP22 T22 50 40 30 20 10 0 1 day 28 days TP21 60 3 days 7 days 28 days Figure 2. The graph showing the effect of mixture of pozzolan and gypsum additives on compressive strength of cement samples TP20, TP21, TP22, T22. 3.2. Effect of the combination of gypsum and puzzolanic additives on the compresive strength of cement The results about the effect of combination of gypsum and pozzolanic additives on the compresive strength of cement - (pozzolan, gypsum) samples TP20, TP21, TP22 and T22, P22 (comparative samples) were shown in Table 3 and Figure 2. Table 3. The results of fineness and compressive strength of cement - (pozzolan, gypsum) samples: TP20, TP21, TP22 and two comparative samples: T22, P22. CLK Gypsum Puzolan Ground times of pozzolan (minutes) TP20 90 5 5 20 4410 20.8 31.3 40.1 53.1 TP21 90 5 5 35 4450 21.3 33.3 41.3 54.5 TP22 90 5 5 65 4530 21.7 33.6 41.6 55.4 T22 95 5 - - 4141 21.8 35.2 48.9 54.1 P22 95 - 5 65 4341 11.6 25.3 33.8 38.8 Sample 162 The percentage of weight (%) The blaine fineness of the mix (cm2/g) R1 R3 R7 R28 Compressive strength (MPa) Study of the effects of gympsum, pozzolanic additive mixtures and their fineness … The results in Table 3 and Figure 2 show that the combination of gypsum and pozzolanic additives has increased the fineness of cement compared to single pozzolanic additive (at the same ground time 65 minutes, sample P22 achieve the fineness of 4341 cm2/g, the fineness of sample TP22 is 4530 cm2/g). This shows the cost of grinding energy of cement - (pozzolan, gypsum) samples is lower than that of cement - pozzolan samples. On the other hand, the cement samples with the combination of gypsum and pozzolanic additives tend to increase the strength at the late time compared to the cement sample with single gypsum additive [13] or single pozzolanic additive in section 3.1 (at the age of 28 days, the compressive strength of sample TP22 was 55.4 MPa; sample P22 : 38.8 MPa; sample T22 : 54.1 MPa). + The results of some physical index of cement samples TP22, P22, T22 are shown in Table 4. The results indicate that the water demand of cement samples ranged from 23 to 26 %. The initial setting time and the final setting time are within the allowed standard (except for cement – pozzolanic sample P22, the initial setting time and the final setting time are slightly lower than the standard). The soundness of cement samples TP22, T22 và P22 is very high compared to standard (≤ 0.12 mm). Table 4. Evaluation of some physical indicators of cement samples: TP22, P22 and T22. Sample Water demand (%) The initial setting time (minute) The final setting time (minute) The blaine fineness (cm2/g) Soundness (mm) TP22 25.2 85 115 4530 0.1 T22 23.2 80 120 4141 0.1 P22 23.6 30 50 4341 0.12 Standard indicator - ≥ 45 ≤ 375 ≥ 2800 ≤ 10 3.3. Scanning electron microscopy results of cement samples TP22, T22, P22 and M22 Scanning electron microscopy results (SEM) of the cement samples TP22, T22, P22 and M22 were shown in Figure 3. SEM images in Figure 3 showed cement sample M22 has a surface structure with many pores. There was an appearance of complete hexagonal crystals of Ca(OH)2. At this magnification there was no appearance of needle shape structures or fibrous structures. This makes the compresive strength of cement is not high and this matches with the results of studying the strength of cement (3.1). Cement sample T22 has needle-shaped crystals of Ettringite focused mainly in porous holes. They contribute to enhancing the ability to link among other components that help improve strength more than sample M22. From the SEM image of cement sample P22 it can clearly be seen that it has many hexagonal crystals of Ca(OH)2. However, these crystals are quite small mostly less than 2 μm. There is a small amount of needle-shaped crystals of Ettringite formed and focused in porous holes. The structure of cement sample P22 is still many porous holes and this leads to the strength of the cement sample P22 is not high. The cement sample TP22 has many needleshaped crystals of Ettringite formed and focused mainly in porous holes. Hexagonal crystals of Ca(OH)2 of sample TP22 are quite large compared to other samples. The pozzolanic reaction in 3-day-old samples is not clear. The amount of Ca(OH)2 in the cement sample TP22 is still high. The porous holes are still yet to be filled with the products of pozzolan and Ca(OH) 2 because 163 Hoang Canh Nguyen, et al. they are not created in this age of 3 days. This leads to the early strength (3, 7 days) of the cement sample TP22 is not high, but it helps to increase the strength at the late time (28 days). This matches with results were studied about the strength of cement samples in section 3.1 and 3.2. Figure 3. SEM images of cement samples TP22, T22, P22 and M22. 4. CONCLUSION Pozzolanic additive has reduced the compresive strength of cement when mixed separately. However, when pozzolanic additive is mixed with gypsum additive, this additive mixture has increased the compresive strength of the cement at the late time (at the age of 28 days, the compresive strength of sample TP22 was 55.4 MPa; sample P22 : 38.8 MPa; sample T22 : 54.1 MPa). The fineness of pozzolanic additive has little effect on the compresive strength of cement (In the range of fineness studied (4169 ÷ 6700 cm2/g), the pozzolanic additive gave an insignificant increase of the compresive strength of cement (< 5 %)). On the other hand, when pozzolanic additive is combined with gypsum additive it increases the cement fineness more than the cement sample with single pozzolanic additive (at the same ground time of 65 minutes, the cement sample P22 has a fineness of 4341 cm2/g; the cement sample TP22 has a fineness of 4530 cm2/g). This could help reducing the cost of grinding energy in the cement industry. 164 Study of the effects of gympsum, pozzolanic additive mixtures and their fineness … Acknowledgement. The authors would like to thank the Hanoi University of Science and Technology for the financial support to complete the article under the grant number T2016-DN-002 and T2017-HTDN-03. REFERENCES 1. 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