High-performance low-carbon GGBS concrete for sustainable development: a case study in Hong Kong - Facing forthcoming OPC & PFA shortage and carbon neutral challenges
By the Civil Engineering and Development Department, AECOM Asia Co Ltd and Excel Concrete Limited with the coordination of the Civil Division
It is well known that the addition of a supplementary cementitious material (SCM) to replace part of ordinary Portland cement (OPC) can improve concrete performance, reduce cement consumption to lower carbon footprint, and reduce waste disposal if the SCM is a waste.
Among the various SCMs, pulverised fuel ash (PFA) is the one most commonly used in Hong Kong, but its production is diminishing due to reduced use of coal for power generation. Facing the anticipated PFA shortage, Ground Granulated Blast-furnace Slag (GGBS) may be considered as an alternative. However, most construction professionals in Hong Kong lack experience in the use of GGBS. In this Cover Story, the first major application of high-performance low-carbon GGBS concrete in Hong Kong is reported. Apart from lower carbon footprint, the use of GGBS also has other advantages, such as improved durability, dimensional stability and aesthetic appearance. In theory, it can also reduce alkali-aggregate reaction (AAR) to allow the use of volcanic aggregate.
Both of the SCMs, PFA and GGBS, are types of waste that may be added into concrete to replace part of the OPC. PFA is the ash produced by burning coal for power generation. It is generally processed by air classification to reduce the loss on ignition (mainly unburnt carbon), which could adversely affect performance. It is round in shape and has similar fineness as OPC. On the other hand, GGBS is obtained from the blast-furnace of steel mills by quenching the molten slag in water to form granulated slag, which is then dried and ground to become GGBS. It is angular in shape and may have higher fineness than OPC, depending on the grinding process. Although the use of PFA and GGBS in concrete is partly to reutilise the waste for reducing waste disposal, their use would also improve the overall performance of the concrete produced. For this reason, both PFA and GGBS have become essential ingredients for the production of highperformance concrete.
In Hong Kong, the use of PFA is more popular because of local availability. However, the amount of PFA produced in Hong Kong is diminishing due to the change from coal to natural gas for power generation and import of electricity from Mainland. Eventually, there will be an acute shortage of PFA amounting to more than 300,000 tons in the years to come, if we do not look for alternative sources of SCM. There is therefore need to import PFA to Hong Kong.
Unlike PFA, which may be added to replace up to 35% of the cement, GGBS may be added to replace up to 75% of the cement. Hence, its effectiveness in reducing cement consumption is much higher. As cement production generates about one ton of CO2 per ton of cement produced1 and contributes to about 8% of the total world CO2 emission2, the use of GGBS as SCM in concrete can better help to mitigate global warming, which is causing sea level rise and higher frequency of extreme weather. There is now an urgent need to lower carbon footprint for sustainable development, and one practical and effective way is to add GGBS to replace at least 50% of cement to lower cement consumption.
However, GGBS concretes do not have the same properties as PFA concretes, and most construction professionals in Hong Kong are not familiar with them. Basically, if the GGBS are ground to have only a similar fineness as the cement, and a substantial portion of the cement is replaced by them, the GGBS concretes so produced would have a lower early strength and thus would require a longer period of curing and temporary support. This shortcoming can be resolved by grinding the GGBS to a higher fineness than the cement so as to increase its reactivity. In fact, different sources of GGBS with different finenesses and/or different chemical compositions may have different properties albeit all complying with the relevant standards. For this reason, each source of GGBS should be evaluated before use.
Although the standard3 only requires the GGBS to have a similar fineness as the cement, finer GGBS is generally preferred because of its higher reactivity. Actually, the smaller particle size of the finer GGBS would enable the GGBS particles to fill into the voids between the cement particles to increase the packing density of the blended OPC + GGBS mixture so as to enhance the overall performance of the GGBS concrete produced4. In other words, with the use of a finer GGBS, there is the opportunity to produce a high-performance GGBS concrete, with all-round high performance in terms of workability, early strength, durability and dimensional stability, as exemplified in the first major application of high performance GGBS concrete in Hong Kong presented herein.
The Kong Nga Po project
For sustainable development and to pioneer the use of highperformance concrete with low carbon footprint, it has been decided that a high-performance low-carbon GGBS concrete be developed for use in the Kong Nga Po project. Some details of the Kong Nga Po project are given below:
Project title: Site Formation and Infrastructure Works for Police Facilities in Kong Nga Po
Project owner: The Civil Engineering and Development Department, The HKSAR Government
Consultant: AECOM Asia Co Ltd
Contractor: Build King Construction Ltd
GGBS may be used to replace OPC and PFA to decrease cement consumption and overcome PFA shortage in Hong Kong.
The project works comprise mainly: site formation of about 19 hectares of land at Kong Nga Po, upgrading of an existing road, construction of a vehicle bridge, some retaining walls and a water tank, and associated infrastructure works including road works, drainage and sewerage works, geotechnical works and landscaping etc. An aerial photo of the project is shown in Figure 1.
Figure 1: Aerial view of Kong Nga Po project site
The concrete mixes used are of grade C40 or C45, and all exposed concrete surfaces are designed to have fair-face finish with consistent and preferably light colour and no efflorescence. The total volume of concrete is about 50,000 m3.
High-performance GGBS concrete for Kong Nga Po project
The concrete supplier engaged is Excel Concrete Ltd (Excel), which has been producing ordinary GGBS concrete for some other projects in Hong Kong. To develop a high-performance low-carbon GGBS concrete, Excel has chosen to use a new source of GGBS with a relatively high fineness. The GGBS is white in colour and has a fineness of 415 m2/kg, which is higher than that of OPC. As measured by a laser particle size analyser, its median particle size is 11.5 μm, which is finer than that of OPC particles (14.4 μm). It has a density of 2,830 kg/m3, which is about 10% lighter than OPC. Compared to OPC, its seven-day activity index is about 60%, whereas its 28-day activity index is at least 75%. Its chemical compositions are as depicted in Table 1, from which it can be seen that the CaO content is 43.4%, the SiO2 content is 34.5%. The Al2O3 content is 13.8% and the MgO content is 5.8%.
Table 1: Chemical analysis of GGBS used
The GGBS concrete was designed to be used as both C40 and C45 concretes with a high workability of 200 mm slump. To achieve low carbon footprint, the GGBS content was set at 50% of the total cementitious materials. In other words, the GGBS was added to replace 50% of the cement so as to reduce the cement content by one-half. Moreover, to achieve high dimensional stability, the total cementitious materials content was set at only 410 kg/m3. The other concrete mix parameters were: water-cementitious materials ratio = 0.40 and aggregate-cementitious materials ratio = 4.44. From the total cementitious materials content of 410 kg/m3 and a water content of 164 kg/m3, the cementitious materials paste volume may be calculated as 30.2%. The adoption of such a small paste volume should help to improve the dimensional stability of the concrete for reducing both its temperature rise during curing and its drying shrinkage in the longer term. Hence, this GGBS concrete may be regarded as a high-performance low-carbon concrete.
For comparison with PFA concrete, a small quantity of PFA concrete with the same water-cementitious materials ratio and aggregate proportions was also produced for use in certain parts of the construction works.
Concrete performance improvement
The variations of the seven-day and 28-day cube strengths with the sequence of production are shown in Figure 2. From these results, it is evident that even in wintertime, during which the relatively low ambient temperature should have caused the early strength to be on the low side, the sevenday strengths were most of the time higher than 40 MPa. And, the moving-4 average of 28-day strength varied within the range of 64.7 to 84.8 MPa, indicating that the GGBS concrete produced was actually good enough to be regarded as a grade C50 concrete. Regarding quality performance, the moving-40 standard deviation of 28-day strength varied from as low as 3.86 MPa to at most 6.82 MPa, revealing that the GGBS concrete production was under good quality control. According to the concrete supplier, there was no particular difficulty in the quality control of the GGBS concrete production.
Figure 2: Variations of seven-day and 28-day cube strengths during production
The strength development with age of the GGBS (50%) concrete is compared, in Figure 3, to those of OPC concrete (100% OPC) and PFA (25%) concrete with the same watercementitious materials ratio of 0.40. It is noted that the sevenday to 28-day strength ratios of OPC concrete, PFA concrete and GGBS concrete were 0.829, 0.748 and 0.734 respectively. And the 28-day strength of GGBS concrete was higher than OPC concrete and PFA concrete by 5% and 31.6% respectively. Hence, the use of 50% GGBS would yield higher early-age concrete strength compared to PFA concrete.
Figure 3: Strength development of OPC, PFA and GGBS concretes
In order to evaluate the dimensional stability of the GGBS concrete produced, the temperature rise during curing has been measured by the Temperature Rise Evaluation Test (TRET) of a 1.0 m × 1.0 m × 1.0 m heat-insulated concrete block, and the drying shrinkage strain has been measured by the shrinkage test of 75 mm × 75 mm × 250 mm prismatic specimens at 27±2°C and 55±5% RH. The temperature-time curves obtained by the TRET are plotted in Figure 4, whereas the shrinkage strain-time curves are plotted in Figure 5. The temperature-time curves reveal that the maximum temperature rise was 48.5°C reached at a curing time of 73 hours. This temperature rise is about 15% lower than that of OPC concrete (OPC concretes with similar grade strength and workability usually have a temperature rise of around 57°C). Likewise, the shrinkage strain-time curves reveal that the shrinkage strain after 120 days of drying was 380 microstrain, which is about 25% lower than that of ordinary OPC concrete and 21% lower than that of ordinary PFA concrete with the same water-cementitious materials ratio.
Figure 4: TRET results of the GGBS concrete
Figure 5: Shrinkage test results of the OPC, PFA and GGBS concretes
To evaluate the durability, Rapid Chloride Penetration Test (RCPT) of the GGBS concrete has been carried out after 35 days of curing. Two core specimens were obtained from the concrete block used for TRET and two additional core specimens were obtained from 150 mm concrete cubes. The two core specimens from the TRET block yielded a mean value of total charge passed of 1,271 Coulomb, while the two core specimens from the concrete cubes yielded a mean value of total charge passed of 1,207 Coulomb. Both of these results are “low” according to ASTM C 1202 or Hong Kong Construction Standard CS1. These total charge passed values are lower than that of OPC concrete by more than 50% (OPC concretes with similar grade strength usually have total charge passed of about 3,000 Coulomb).
Regarding the colour and surface finish of the GGBS concrete, the surface appearance of the GGBS concrete is presented in Figure 6.
Figure 6a: External surface of U-trough
Figure 6b: External surface of retaining wall
Figure 6c: General view of surface appearance of GGBS concrete
Obviously, the GGBS concrete has a more consistent and whiter colour than the PFA concrete. However, to achieve the best appearance, the mortar used to fill the tie-bolt holes in the hardened concrete should also be made of GGBS in order to avoid colour difference between the concrete and the mortar in the tie-bolt holes. More importantly, there is no efflorescence on the surfaces of the GGBS concrete to cause contamination and discolouring of concrete surfaces. Hence, GGBS concrete should be particularly suitable for fair-face concrete.
Use of high fineness GGBS can produce high-performance concrete with high early strength, dimensional stability and durability. Such high-performance GGBS concrete is particularly good for fair face finish with consistent light colour and no efflorescence.
Carbon footprint reduction
Regarding the carbon footprint, with 50% GGBS added, the OPC content of the concrete is reduced to only 205 kg/m3, which is very low indeed. Comparing with ordinary OPC concrete with similar grade strength and workability, which usually has an OPC content of about 430 kg/m3, the cement content has been reduced from 430 to 205 kg/m3, that is, by about 52%. As a result, the CO2 emission from the cement consumption would be reduced by the same percentage. In other words, the total CO2 emission from cement consumption would be reduced by 0.52 × 1 × 0.430 × 50,000 = 11,180 ton. In larger scale projects, such a 52% reduction in carbon footprint would be a much bigger contribution to the mitigation of global warming.
However, the total carbon footprint of concrete production is comprised of not just the carbon footprint from cement consumption but also the carbon footprint from the other raw materials, the transportation of all materials to Hong Kong, and the electricity consumption of the concrete plant. Excel has conducted a carbon footprint assessment of its Lam Tei Plant using the CIC Carbon Labelling Scheme for Construction Products – Ready-mixed Concrete for the period from 1 July 2020 to 30 June 2021. This carbon footprint assessment by Excel has been verified by Hong Kong Quality Assurance Agency (HKQAA), who confirmed that the data input in the Carbon Footprint of Product (CFP) quantification tool are accurate and reliable. The carbon footprint assessment results for the various concrete mixes produced in the Lam Tei Plant are presented in Table 2.
Table 2: CO2e of various concrete mixes produced in Excel’s Lam Tei Plant
From the results presented in Table 2, it can be seen that for Grade 40 concretes, the addition of PFA to replace OPC would reduce the carbon footprint by 18.3%, whereas the addition of GGBS to replace OPC would reduce the carbon footprint by 47.5%. For Grade 45 concrete, the addition of PFA to replace OPC would reduce the carbon footprint by 24.7%, whereas the addition of GGBS to replace OPC would reduce the carbon footprint by 47.4%. It is therefore evidenced that GGBS is much more effective than PFA in reducing the carbon footprint of concrete production. In this particular project, in which the total concrete consumption is about 50,000 m3, the total reduction in CO2 emission may be calculated as (410.57 – 215.96) × 50,000 = 9,730,500 kg = 9,731 ton, which is close to the previously estimated reduction in CO2 emission from the cement consumption. If only OPC concrete containing no SCM is used for the construction and if the concrete mix contains 430 kg/m3 cement, the CO2 emission from the cement consumption would be about 1× 0.430 × 50,000 = 21,500 ton.
By the use of such 50% GGBS concrete, the carbon footprint of concrete construction can be reduced by about one half.
Summary and remarks
With the anticipated shortage of PFA in Hong Kong and the urgent need to reduce the carbon footprint of our concrete construction for sustainable development, it is now time to consider using alternatives, which may be added in place of PFA and at higher dosages to further reduce the cement consumption. For this purpose, GGBS is a good substitute for both OPC and PFA. The use of GGBS could lead to lower early age strength than ordinary concrete but this problem could be alleviated by employing a GGBS that is ground to a higher fineness than OPC.
With the use of a relatively fine GGBS, a high-performance low-carbon GGBS concrete has been developed for the Kong Nga Po project. This GGBS concrete did not pose any particular difficulty to the quality control and has been proven to have better early strength than PFA concrete. More importantly, this GGBS concrete has been tested to have much higher dimensional stability to the extent that the temperature rise during curing is 15% lower than OPC concrete and the drying shrinkage is 30% lower than OPC and PFA concretes. The durability is also higher to the extent that the RCPT total charge passed is below 50% of that of OPC concrete. In terms of carbon footprint, compared to OPC concrete, this GGBS concrete with an OPC content of only 205 kg/m3 would have a carbon footprint from cement consumption of about 52% lower and a total carbon footprint from concrete production of about 47% lower.
The GGBS concrete has been applied in this particular case to a civil project. Actually, it can also be applied to a building project, provided the GGBS concrete complies with the requirements stipulated in the Code of Practice for Structural Use of Concrete 2013 (CoP 2013). According to Clause 22.214.171.124 of CoP 2013, the usual range of PFA or GGBS content by mass of the total cementitious content should be (a) 25% to 35% for PFA; and (b) 35% to 75% for GGBS. Moreover, according to Clause 10.3.6.2 of CoP 2013, the minimum period of curing and protection should be (a) Portland cement: three days for average ambient condition and four days for poor ambient condition, and (b) all other cements (that is, with PFA or GGBS added): four days for average ambient condition and five days for poor ambient condition.
Since it is the early strength that signifies, the minimum period of curing and protection may be reduced by increasing the early strength of the GGBS concrete. In this regard, further study is recommended.
In fact, according to the literature, the use of GGBS may also mitigate AAR to the extent that even volcanic aggregate, which is prone to AAR, may be utilised to make good use of volcanic rock derived from site formation and rock cavern projects. In this regard, further study is recommended.
The provision of material data by Greentex Construction Materials Ltd and the advices given by Ir Prof Albert K H Kwan are gratefully acknowledged.
About the authors: Ir John W H Chung is from the Civil Engineering and Development Department. Ir Robert Y K Chan and Ir Gloria Y Y Tang are from AECOM Asia Co Ltd. Mr Raymond W M Au and Mr C M Tsang are from Excel Concrete Ltd.
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