Limestone filled Portland cements

Abstract: Limestone filled Portland cement wherein the fineness of limestone particles ranges from 5,000 to 12,000 Blaine and the fineness of clinker particles ranges from 1,500 to 3,000 Blaine.

Description: The present invention relates to limestone filled Portland cements characterized by an optimal particle size and to the process for the preparation of same. State of the art Cement is a hydraulic binder, i.e. a finely divided inorganic powder that, in admixture with water, forms a paste which cohagulates and hardens by hydration, and once hardened, it maintains its resistance also under water. As a hydraulic binder it must produce mortar or concrete capable of maintaining the necessary flow as long as required to reach, after given periods of time, predetermined mechanical resistance levels, and be endowed with a long-term stability in terms of volume. A major hydraulic constituent of cement is clinker, which consists of calcium silicates (at least two thirds by wt.), the remainder being aluminium oxide, iron oxide and other oxides. In compliance with UNI ENV 197.1 standard, limestone filled Portland cements (type II) are hydraulic binders containing clinker (65 to 94% by wt.), limestone (6 to 35% by wt.) and minor constituents (0 to 5% by wt.), said minor constituents being selected from blast furnace slag, microsilica, pozzolana, flying ashes, calcined shales or mixtures thereof. The above per cent amounts are exclusive of calcium sulphate, used to regulate the setting time, and optional additives. UNI ENV 197.1 standard covers types of cement, with requirements for compressive strengths that, depending on the various cement classes, vary from 32.5 N/mm 2 to 52 N/mm 2 , at 28 days. It follows that both cements consisting exclusively of clinker and gypsum and the so-called mixed cements, like those filled with cements, have to possess mechanical properties complying with standards. However, as shown from the data reported in the literature, the addition of a calcareous filler brings about a more or less marked reduction in the mechanical resistance properties of cement in standard mortar, if compared with those of Portland cement without filler (type I as per UNI ENV 197.1). P. Krustolovic et al . (CCR, vol. 24, No. 5, 1994) state that the addition of 20% non-hydraulic filler reduces the compressive strength of mortar and concrete by 27%. P. Livesey, in Performance of limestone filled cement , p. 6.1, confirms said results and states that the addition of 5% max. of calcareous filler does not affect the cement resistance, whereas the addition of 25% limestone causes a reduction in the mechanical resistance by 25%. As disclosed in Materiales de construcction , vol. 24, No. 233, 1944, the addition of 10% max. of calcareous filler would not affect the mechanical resistance properties of standard mortar. However, should the amount of calcareous filler be as high as 15%, the resistance will fall by 27% and only by decreasing the water/cement ratio, the resistance properties may be kept constant. S. Sprung and E. Siebel ( Zement Kalk Gips , No. 1, pp. 1-11, 1991) maintain that, with calcareous filler contents ranging from 5% to 10%, the increased compactness of the cementitious paste structure caused by intragranular holes filling may bring about a slight increase in the standard resistance properties. With higher calcareous filler contents, the diluting effect produced by the filler on the clinker causes a reduction in the resistance of cement in standard mortar. Said authors state that, should a Portland cement be added with a calcareous filler in amounts of 15%, the standard mortar resistance will be reduced by 14%. To obviate said inconvenience, Sprung and Siebel state that Portland cements filled with 20% to 30% limestone can have a mechanical resistance of 45 N/mm 2 only if prepared with clinker having fineness as high as 4,500 to 5,000 Blaine. The commercial-scale production of clinker with such a high degree of fineness requires long grinding times and high power consumption. The present invention The Applicant has surprisingly been found that Portland cement containing 20 to 25% limestone, having mechanical properties as high as and, in some cases, better than those of Portland cement without inorganic filler, may be obtained with clinker having fineness of 1600 to 2500 Blaine. It is a characteristic of the claimed limestone filled Portland cement that the limestone particles fineness ranges from 6,000 to 10,000 cm 2 /g (Blaine). In fact, it has been found that, when the clinker and limestone particles satisfy the aforementioned conditions, the mechanical properties of said mixed cement are substantially the same as and sometimes even better than those of Portland cement without filler. It is a further object of the present invention to provide a process for the preparation of the limestone filled Portland cement according to the present invention, comprising a separate grinding of clinker and limestone, followed by homogenization. In fact, it has been found that the process of the invention envisaging a separate grinding of clinker and limestone is highly energy saving in respect of the processes encompassing a co-grinding of said components. Description of the drawing Figure 1 shows the plant wherein grinding of the components of the present invention takes place. Detailed description of the invention As already pointed out, in the limestone filled Portland cement according to the present invention, the fineness of the clinker (ground with gypsum) ranges from 1,600 to 2,500 Blaine. The clinker particle size distribution preferably ranges from 0 to 80 µm or, even better, from 10 to 40 µm. In a particularly preferred embodiment of the invention, the clinker has 40 µm maximum nominal diameter, and does not contain any fraction of fine particles ranging from 0 to 10 µm. The fineness of the limestone of the invention ranges from 6,000 to 10,000 Blaine. At fineness values above 10,000 Blaine, the effect of the filler becomes less evident and at values above 12,000 Blaine the economic pros due to higher resistance properties reduce in respect of the economic cons due to higher energy consumption for grinding. The average diameter of limestone particles ranges from 10 to 30 µm and preferably is not higher than 15 µm. The limestone to be used according to the present invention has to comply with UNI ENV 197.1 standard, i.e. have a CaCO 3 content ≥75% by wt., a methylene blue adsorption ≤1.20 g/100 g and an organic matter content ≤0.50% by wt. The limestone content in the cement ranges from 20% to 25% by wt. in respect of the cement, excluding gypsum and additives. The best results are obtained with clinker fineness ranging from 1,600 to 1,700 Blaine, limestone fineness of 10,000 Blaine and with a distribution of the clinker particle size ranging from 10 to 40 µm. In the process for the preparation of Portland cement according to the present invention, clinker and limestone grinding takes place separately, preferably in a closed-circuit grinding mill essentially consisting of a hopper, a ball mill, an air classifier and a bag filter, as follows: i) the product to be ground is fed to a ball mill from a hopper; ii) the ground product goes through a vibrating pipeline to an air classifier, where the fraction of the desired fineness is separated from the coarser fraction, which is returned to the ball mill through a pipeline; iii) the product having the desired fineness and particle size is filtered through a bag filter and collected. The resulting components are homogenized by any mixer known in the art. With a view to optimizing energy consumption, the constituents, before being subjected to separate grinding, may be conveniently crushed, e.g. in a jaw-type crusher. Figure 1 shows a scheme of the grinding plant wherein grinding as per the above process is carried out. In particular in this figure: (1) is the hopper, (2) the recycle vibrating pipeline, (3) the air classifier, (4) the ball mill, (5) the pipeline wherethrough the particles from ball mill (4) enter air classifier (3), (6) the bag filter. The following examples illustrating the preparation of the claimed limestone filled cement and the relevant physico-mechanical properties are conveyed by way of indication, not of limitation, of the present invention. Example 1 Limestone and clinker, premixed with gypsum in a ratio of 5% by weight, were separately crushed in a jaw-type crusher. The particle size of the crushed materials is shown in Table 1. Clinker and limestone particle size after crushing Clinker/gypsum Limestone ⊘ [mm] Residue ⊘ [mm] Residue 10 - 4 12 4 30.5 2.8 23.7 2.8 58 1.5 34.6 1.5 80 1 38.3 1 86.2 0.5 43.9 0.5 91.2 0.3 54.4 0.15 94.9 0.15 82.5 0.088 95.8 0.1 99.1 0.06 97.5 The two components were then ground in the closed-circuit grinding mill shown in Figure 1. The mill was ball type. The ball size and distribution were as follows:  40 mm = 20%,  15 mm = 70%,  10 mm = 10%. The per cent volume of balls in the mill was 33% and the power rating was 4 kW. The material to be ground was taken from a hopper and fed to the ball mill by means of a vibrating pipeline. The product leaving the mill went to an air classifier, where the desired fraction was separated from the coarser fraction, which was recycled to the mill. During the several trials, the power rating adsorbed by the mill was measured by a wattmeter in order to calculate the specific power consumption with regard to the obtained product. In the aforesaid grinding mill, the mixture of clinker and gypsum was ground until obtaining two degrees of fineness, corresponding to (2,640 Blaine and, 2,320 Blaine. The relevant products are hereinafter denominated CLK2 and CLK1. The fine particle fraction ranging from 0 to 10 µm in diameter was partially removed by the air classifier from product CLK1 to give product CLK3 having fineness of 1,640 Blaine. Limestone was ground in said grinding mill until obtaining a fineness of ≈ 10,000 Blaine. The products obtained were analysed by a laser apparatus SYMPATEC HELOS. Table 2 shows the particle size distribution recorded. Per cent cumulative undersize materials produced in the closed-circuit grinding mill CLINKER/GYPSUM LIMESTONE  [µm] CLK 1 % undersize CLK 2 % undersize CLK 3 % undersize  [µm] % undersize 0.90 2.69 3.04 1.21 0.90 9.92 1.10 3.89 4.43 1.74 1.10 14.78 1.30 5.00 5.73 2.21 1.30 19.56 1.50 6.03 6.97 2.63 1.50 24.22 1.80 7.46 8.7 3.19 2.60 30.90 2.20 9.19 10.81 3.81 3.10 39.12 2.60 10.75 12.75 4.33 3.70 46.48 3.10 12.54 14.96 4.86 4.30 54.40 3.70 14.51 17.39 5.40 5.00 62.09 4.30 16.34 19.62 5.89 6.00 68.02 5.00 18.34 22.05 6.47 7.50 73.27 6.00 21.03 25.30 7.43 9.00 78.89 7.50 24.75 29.83 9.29 10.50 85.07 9.00 28.22 34.03 11.72 12.50 89.79 10.50 31.53 38.00 14.68 15.00 93.36 12.50 35.78 42.99 19.30 18.00 96.30 15.00 40.75 48.83 25.80 21.00 97.39 21.00 46.28 55.28 41.72 25.00 97.39 25.00 51.32 61.12 51.44 30.00 98.95 30.00 57.32 68.06 62.28 36.00 99.68 36.00 63.75 75.53 73.38 43.00 100.00 43.00 70.21 82.94 83.65 51.00 100.00 51.00 76.52 89.66 91.75 61.00 100.00 61.00 82.49 94.85 97.18 73.00 100.00 73.00 88.4 98.23 99.44 87.00 100.00 87.00 93.71 99.6 100 103.0 100.00 103.00 97.68 100 100 123.0 100.00 123.00 99.57 100 100 147.0 100.00 147.00 100.00 100 100 Example 2 Ground clinkers prepared as per Example 1 (CLK1, CLK2 and CLK3) were mixed and homogenized with 20% and 25% by wt. calcareous filler at 10,000 Blaine (prepared as described in Example 1). Standard mortars in compliance with UNI EN 196.1 were prepared from the cements produced by mixing, to determine the mixture compressive strength and rheological properties. The determination of the mortar consistency (workability) was carried out by means of a jerking plate in compliance with UNI 7044. The results obtained are shown in Table 3, where CLK 2 is a comparative example. Specific surface (Blaine), flow and compressive strength of the cements of the invention Blaine Flow % Standard mortar CS (N/mm2) 2 days 7 days 28 days CLK 1 2320 46.7 20.13 31.0 45.4 CLK 1 + 20%C 3856 53.9 24.32 35.3 45.2 CLK 1 + 25%C 4240 55.0 23.62 35.8 - CLK 2 2640 73.7 29.36 43.6 49.8 CLK 2 + 20%C 4112 64.5 27.78 42.0 49.6 CLK 2 + 25%C 4480 61.4 28.41 41.5 46.0 CLK 3 1640 61.5 18.01 35.9 51.2 CLK 3 + 20%C 3312 71.2 23.97 41.4 51.4 CLK 3 + 25%C 3730 70.3 24.29 40.04 46.1 As may be inferred from Table 3, limestone filled cement develops with time the same compressive strength as the cement obtained with the same clinker but without addition of inert materials. In particular, the addition of 20% limestone increases the final resistance of cement CLK3 even at 28 days. A really surprising feature concerns the increase in flow obtained with limestone filled cements CLK1 and CLK3 in respect of the corresponding Portland cements. The effect of the calcareous filler is the greater, the lower the content of clinker fine particles ranging from 0 to 10 µm in diameter. More specifically, a lower content of clinker fine particles can be made up by an addition of calcareous filler. Under said conditions, instead of exerting a diluting action, the filler appreciably increases short-term resistance without, however, worsening resistance properties at 28 days. Example 3 Three series of clinker and gypsum (CLK4, CLK5, CLK6) having average fineness of 2,000, 2,500 and 3,000 Blaine, respectively, were prepared substantially as described in Example 1. Said test cements were added with limestone ground as per Example 1, at 6,000, 8,000 and 10,000 Blaine, respectively. Standard mortars for the determination of compressive strengths were prepared as per Example 2. The results obtained are shown in Table 4, where CLK 6 is a reference example. Type of cement Resistance Properties N/mm2 2 days 7 days 28 days CLK4 16.0 27.4 36.0 CLK4 + C 6000 18.4 31.2 37.5 CLK4 + C 8000 20.3 32.8 37.9 CLK4 + C 10000 19.0 32.9 37.8 CLK5 20.9 37.1 45.7 CLK5 + C 6000 21.2 37.2 46.3 CLK5 + C 8000 23.2 37.8 46.7 CLK5 + C 10000 23.4 37.7 46.4 CLK6 23.7 39.8 50.2 CLK6 + C 6000 23.0 36.5 45.3 CLK6 + C 8000 24.2 36.3 42.8 CLK6 + C 10000 23.3 36.9 42.5 As may be inferred from Table 4, the effect of limestone is the greater, the more the fineness of the clinker is below 3,000 Blaine.

Claims: Limestone filled Portland cement wherein the fineness of limestone particles ranges from 6,000 to 10,000 Blaine and the fineness of clinker particles ranges from 1600 to 2500 Blaine, having a limestone content of 20% to 25% by wt. in respect of cement, excluding gypaum and additives. A limestone filled Portland cement as claimed in claim 1, wherein the limestone particles range from 10 to 30 µm in average diameter. A limestone filled Portland cement as claimed in claims 1 to 2, wherein the clinker particle size ranges from 0 to 80 µm. A limestone filled Portland cement as claimed in claim 3, wherein the clinker particle size ranges from 10 to 40 µm. Process for the preparation of limestone filled Portland cement as claimed in claims 1 to 4, wherein clinker and limestone are ground separately and then homogenized. A process for the preparation of limestone filled Portland cement as claimed in claim 5, wherein: (i) the product to be ground is fed to ball mill (4) from hopper (1); (ii) the ground product goes through a vibrating pipeline to an air classifier (3), where the fraction of the desired fineness is separated from the coarser fraction, which is returned to the ball mill through a pipeline (5); (iii) the product having the desired fineness and particle size is filtered through bag filter (6) and collected. A process for the preparation of limestone filled Portland cement as claimed in claim 6, wherein clinker and limestone are crushed in a jaw-type crusher, before being separately ground, and finally homogenized after said separate grinding step.

Inventors: CANGIANO STEFANO, CASSAR LUIGI, FRIGENI GIANMARIO, TOGNON GIANPIETRO, CANGIANO, STEFANO, CASSAR, LUIGI, FRIGENI, GIANMARIO, TOGNON, GIANPIETRO
Applicants: ITALCEMENTI SPA, ITALCEMENTI S.P.A.
Application year: 1997
Main classification: C04B14
IPC codes: C04B20, C04B28, C04B14