Pozzolanic Reactivity of High-Alkali Supplementary Cementitious Materials and Its Impact on Mitigation of Alkali-Silica Reaction

Abstract

The growing scarcity of conventional supplementary cementitious materials (SCMs) such as Class F, Class C fly ashes, and slag has necessitated exploring alternative SCMs previously considered suboptimal. In particular, high-alkali SCMs are often avoided because of the potential concern that their alkali content could release into the concrete pore solution, thus exacerbating the potential for alkali-silica reaction (ASR). However, preliminary research indicates that not all high-alkali SCMs are deleterious, and some can effectively suppress the ASR expansive reaction when used in sufficient dosage levels. This study evaluates the feasibility of using high-alkali SCMs, such as high-alkali natural pozzolans and reclaimed fly ashes, focusing on their pozzolanic reactivity and the correlation between the reactivity and their ASR mitigation performance. The pozzolanic reactivity of the SCMs was evaluated by the R³ test per ASTM C1897 and strength activity index test per ASTM C311. Thermogravimetric analysis was used to determine the calcium hydroxide consumption by the SCMs. ASR mitigation performance of SCMs was evaluated in accordance with American Association of State Highway and Transportation Officials (AASHTO) T380 miniature concrete prism test. Additionally, pore solution expression and analysis of paste specimens were conducted to determine the correlation between the total alkali and the released alkali levels into the pore solution. Based on the results of this study, all SCMs indicated high pozzolanic reactivity; however, individual performance varied by test method. Ultimately, the high-alkali SCMs, particularly natural pozzolans, did not appear to release any significant levels of alkalis into the pore solution readily and, therefore, show potential for ASR mitigation when used in sufficient dosage levels.

Keywords

infrastructure materials properties of concrete and constituent materials cement pozzolons micro silica and slag durability

Supplementary cementitious materials (SCMs) have been widely used as Portland cement replacement materials throughout the concrete industry ( 1 ). ASTM C618 recognizes two primary categories of SCM: natural pozzolans and coal ashes ( 2 ). Natural pozzolans include volcanic ash, pumice, volcanic tuffs, and calcined clays such as metakaolin. Industrial by-products such as fly ashes are derived from coal combustion for power generation. Other SCMs, such as slag and silica fume, are derived from iron and silicon metal production, respectively, while ground glass pozzolans are derived from processing glass waste. Slag, silica fume, and ground glass pozzolans for use in concrete are specified through ASTM C989, ASTM C1240, and ASTM C1866, respectively.

Concrete is the second most widely used material after water and is responsible for at least 8% (36.3 billion tons) of global CO₂ emissions ( 3 – 5 ). Using SCMs as a partial replacement for cement in concrete is a sustainable approach that reduces the overall carbon footprint of concrete by not only aiding in the sustainable disposal of industrial residues that otherwise would need to be landfilled but also improving the durability of the Portland-cement-based binder matrix. Also, replacing Portland cement with SCMs will lead to using less clinker in the concrete, which reduces the overall carbon footprint of concrete ( 6 – 8 ). Typical SCMs are finely ground siliceous and alumino-siliceous materials that are amorphous in nature and are reactive with calcium hydroxide (CH) in the presence of water at ambient temperature ( 9 ). These characteristics allow SCMs to react with the hydration product of cement (CH), thus producing calcium silicate hydrate (CSH) and/or calcium alumino-silicate hydrate (CASH) ( 10 ). This process is also called the pozzolanic reaction, and the ability of SCMs to consume CH is the pozzolanic reactivity ( 1 ). The pozzolanic reaction improves concrete durability by decreasing the permeability ( 11 ). In addition, the pozzolanic reaction can produce CSH/CASH gel that can serve to sequester alkali ions from the pore solution. This process can help to reduce the hydroxyl ion concentration in pore solution and thus improve the mitigation of durability problems such as alkali-silica reaction (ASR) ( 12 ). The pozzolanic reactivity of SCMs depends on several factors, such as their amorphous content, particle size distribution, and chemical composition. High amorphous content, smaller particle size, and high amounts of silica and alumina can lead to higher pozzolanic reactivity of SCMs ( 13 – 15 ).

Among all the test methods to evaluate the pozzolanic reactivity of SCMs, ASTM C311 strength activity index (SAI) test is the most common method used, where the compressive strength of the test sample containing SCMs is measured and compared with that of a control sample at 7 and 28 days ( 16 ). ASTM C618 requires the SCM-containing mixtures to gain at least 75% of the compressive strength of control mixture at 7 days or 28 days to qualify as an effective pozzolan ( 2 ). The ASTM C1897 R³ test, developed by Avet et al., employs isothermal calorimetry to evaluate the pozzolanic reactivity of SCMs ( 17 , 18 ). This test method determines the pozzolanic reactivity of SCMs by measuring the cumulative heat generated in mixtures, where SCMs are mixed with a simulated pore solution made of CH, calcium carbonate, potassium sulfate, and potassium hydroxide.

Thermogravimetric analysis (TGA) is a method of thermal analysis that measures the amount and rate of change in the mass of a sample as a function of temperature and time in a controlled atmosphere. The mass loss that occurs at a specific temperature range can be attributed to a specific compound, and this parameter can be used to quantify the compound in the matrix. Several studies have shown the effective use of TGA in quantifying the CH content in the mixture, and this, in turn, can be used to assess the pozzolanic reactivity of an SCM ( 19 , 20 ). Typically, CH decomposes between 450°C and 500°C ( 21 ). With active pozzolanic reaction, CH in a cementitious matrix is consumed and converted to CSH gel, and the decrease in the CH content, as measured by the TGA method, can be used to gauge the pozzolanic reactivity of SCMs.

Fly ash is the primary SCM used in the concrete industry. However, because of global environmental policy changes, much of the coal-based power generation has transitioned to using clean energy sources to reduce carbon emissions, resulting in a shortage of availability of high-quality fly ash ( 22 , 23 ). Therefore, the need to find sustainable alternatives to fly ash as an SCM in concrete is urgent and necessary. High-alkali SCMs, the alkali content of which is generally over 4%, are generally avoided in concrete because of concerns arising from the potential leaching of alkali ions into the pore solution and an increase in the alkali loading in the concrete pore solution, with potential to exacerbate the deleterious ASR. ASR is a common concrete deterioration mechanism that results from the reaction of reactive amorphous silica found in some natural aggregates with alkali hydroxides (OH^-, Na⁺, and K⁺) present in the concrete pore solution. This reaction produces ASR gel, which is hygroscopic in nature and has a tendency to absorb moisture and expand. When ASR gel is restrained from expansion within the concrete matrix, tensile stresses are generated that can cause cracking in concrete. The presence of CH in the matrix plays a significant role in determining the expansive nature of the ASR gel ( 24 ). SCMs are the most common and effective method for controlling ASR, as the pozzolanic reactions consume CH and further develop the CSH binder ( 25 ). This mechanism will further increase the alkali-binding ability of CSH gel, lowering the pH of the pore solution, and refining the pore structure to decrease the permeability ( 11 , 26 , 27 ). Also, reducing the readily available CH content in the matrix will inhibit the formation of a more expansive ASR gel. Therefore, SCMs’ ASR mitigation performance can also be viewed as an indirect method to evaluate the pozzolanic reactivity of SCMs. The common test methods that are used to evaluate the effectiveness of SCMs in mitigating ASR are ASTM C1567, ASTM C1293, and American Association of State Highway and Transportation Officials (AASHTO) T380 ( 28 – 30 ).

Pore solution analysis is the typical method used to evaluate the impact of SCMs’ alkali release mechanism, which is also used to predict the ASR expansion process based on the concentration of alkali ions ( 31 ). Shehata and Thomas’s research evaluated the alkali release characteristics of blended cement with high-alkali SCMs, and their research indicated that some SCMs with high total alkali content released alkali ions into the pore solution, which increased alkali concentration in the pore solution ( 32 ). However, Mehta’s research indicated that alkali content in some natural pozzolans exists as crystal phases, which did not readily release into the concrete pore solution ( 33 ). The same finding was discovered by Rodrıíguez-Camacho and Uribe-Afif ( 34 ). In their study, one of the SCMs had a total alkali content of 6.89% Na₂O_eq, but the available alkali content was only 1.09% Na₂O_eq, thus only 15% of the total alkalis were available to be released into the pore solution. These findings provide the potential feasibility of using high-alkali SCMs in concrete without concerns for ASR.

This study investigates the pozzolanic reactivity and ASR mitigation performance of selected high-alkali SCMs. For this purpose, five natural pozzolans and two reclaimed fly ashes were studied. The SCMs were characterized for their mineralogy and particle size distribution, using X-ray diffraction (XRD) and laser diffraction, respectively. X-ray fluorescence (XRF) was conducted to study the chemical composition of the materials. The pozzolanic reactivity of the SCMs was assessed in accordance with ASTM C311 and ASTM C1897 ( 16 , 17 ). TGA was conducted on paste specimens to quantify the amount of CH consumed by SCMs’ pozzolanic reactions at various ages. Pore solution analysis was conducted on paste specimens to study alkali release and alkali binding in mixtures with high-alkali SCMs at different sample ages. ASR mitigation performance of mixtures with SCMs was investigated using AASHTO T380 miniature concrete prism test (MCPT) method ( 30 ). Finally, the results of pozzolanic reactivity experiments and ASR mitigation studies were compared and analyzed, and correlations between these performance measures were evaluated.

Materials and Methods

Materials

Two types of ordinary Portland cement meeting ASTM C150 ( 23 ) were used in this study: A low-alkali Type I/II Portland cement (Na₂O_e = 0.38%) from Argos, U.S., and a high-alkali Type I Portland cement (Na₂O_e = 1.00%) from Lehigh Hanson Inc. The chemical composition and physical properties of both Portland cements are presented in Table 1.

Table 1.

Cement and High-Alkali Supplementary Cementitious Materials Chemical Composition

		SiO₂	Al₂O₃	Fe₂O₃	S+Al+Fe	CaO	MgO	Na₂O	K₂O	Na₂O_e	LOI	SG	Amorphous level (%)
	Low-alkali Portland cement	19.93	4.77	3.13	27.83	62.27	2.70	0.06	0.48	0.37	2.6	3.15	NA
	High-alkali Portland cement	19.00	4.99	2.11	26.1	62.45	2.84	0.31	1.05	1.0	NA	3.15	NA
Pumice	NP 1	73.42	12.30	1.41	87.13	0.79	0.23	2.9	4.2	5.61	4.72	2.35	98.55
Pumice	NP 2	65.48	11.19	1.75	78.42	2.99	0.33	3.6	3.4	5.85	10.87	2.26	3.38
Volcanic rhyolitic tephra	NP 3	71.95	12.26	1.50	85.71	0.93	0.39	3.9	4.0	6.51	4.88	2.35	87.76
Volcanic glass	NP 4	71.21	12.99	0.90	85.1	0.56	0.13	3.9	4.1	6.57	5.95	2.40	100
Pumice	NP 5	71.91	11.68	2.18	85.77	0.32	0.09	5.5	4.2	8.28	3.94	2.34	91.17
Reclaimed fly ash	RFA 1	53.06	15.13	6.88	75.07	13.70	4.53	3.4	1.9	4.69	0.55	2.56	82.48
Reclaimed fly ash	RFA 2	56.81	14.20	2.69	73.7	10.13	1.41	2.8	2.7	4.56	8.42	2.42	88.88

Note: NP 1–5 = natural pozzolan samples; RFA 1–2 = reclaimed fly ash samples; NA = Not Available.

In this study, five natural pozzolans and two reclaimed fly ashes were investigated. The natural pozzolans are identified as NP 1 through NP 5, and reclaimed fly ashes as RFA 1 and RFA 2. The material chemical compositions and particle size distributions were measured by XRF and laser diffraction, respectively. These results are presented in Table 1 and Figure 1. XRD data of high-alkali SCMs were collected using the Rigaku X-ray diffractor. Measurements were made in flat-plate Bragg–Brentano θ–2θ geometry, and their angular range was from 10° to 80° 2θ values with a 0.02° 2θ step size. The scan rate for the test was 1° 2θ per minute. The amorphous level, that is, the amount of non-crystalline material, was determined by Rietveld analysis, which used the integrated surface area of the crystal compared with the total surface area ( 35 ). In this study, XRD results, shown in Figure 2 and Table 1, indicate that all the materials have a high amorphous content except NP 2. NP 2 did not exhibit a significant amorphous hump, rather it had many crystal peaks that often overlapped.

Figure 1.

Charts showing: (a) supplementary cementitious material particle size and (b) particle size distribution of natural pozzolans.

Figure 2.

X-ray diffraction pattern of supplementary cementitious materials.

The reactive aggregate used in this study was a siliceous argillite aggregate from Goldhill Quarry in North Carolina, which consists of reactive metatuff–argillite. The aggregate’s specific gravity and the percent water absorption were 2.60 and 1%, respectively.

ASTM C311 Strength Activity Index (SAI)

The SAI test was used to evaluate the pozzolanic activity of SCMs in mixtures blended with low-alkali cement, and the mixture proportions were followed in accordance with ASTM C311. In this test, eight mixtures—one control and seven mixtures with SCMs—were prepared for 7-day and 28-day strength measurements. After casting, the specimens were placed in the standard curing room for 24 h. After demolding, the samples were cured at ambient temperature in lime-saturated water. Compressive strength was measured using TEST MARK CM-3000 SD compression testing machine at a loading rate of 50 psi/s. The SAI was calculated using Equation 1:

SAI = \frac{Average compressive strength of test mixtures}{Average compressive strength of control} \times 100 %

(1)

ASTM C1897 R³

The pozzolanic reactivity of SCMs was evaluated using the R³ test per ASTM C1897-20 method A. Isothermal calorimetry was used in this test to quantitatively determine the heat signature. The mixtures consisted of SCMs, CH, calcium carbonate (CaCO3), potassium sulfate (K₂SO₄), and potassium hydroxide (KOH). The mass ratio of SCMs to CH and CaCO₃ was 1 to 3 and 2 to 1, respectively. The potassium solution was prepared by dissolving 4.00 g of KOH and 20.0 g of K₂SO₄ in 1.00 L of reagent water. The mass ratio of potassium to the solids, that is, the blend of SCMs, CH, and CaCO3, was 1.2. The cumulative heat was measured at 40°C for 7 days. All the materials were mixed at 1,600 ± 50 r/min for 2 min using the high-shear blender to achieve a homogeneous paste.

Thermogravimetric Analysis (TGA) for Determining Calcium Hydroxide (CH) Consumption

TGA was conducted to determine the amount of CH in the cement paste. For this testing, the AutoTGA Q5000 instrument was employed. The pastes were prepared by blending low-alkali cement with SCMs at a 20% mass replacement of cement, at a water-to-binder ratio of 0.42. The prepared paste samples were stored in a sealed container and were tested at the ages 12 h, 1 day, 3 day, 7 day, 28 day, and 56 day. After casting, the specimens were sealed in air-tight test tubes to avoid potential carbonation and stored in an air chamber maintained at 23°C and 50% relative humidity (RH). Before testing, the samples were de-molded from the tubes and ground using an agate mortar and pestle to pass the No. 100 sieve (150 μm). Then, the powder samples were immersed in 50 ml isopropanol for 15 min to remove moisture from the powder. The suspension was filtered by using a Büchner funnel to obtain the dehydrated powder, and 10 ml diethyl ether was added to the powder to remove extra isopropanol. After preparation, the sample was immediately stored in air-tight vials and tested. The weight loss observed in the samples between the temperatures of 400°C and 500°C was recorded and the amount of CH (Ca(OH)₂) per gram of cement in the mixture was calculated using Equation 2:

Ca {(OH)}_{2} = \frac{(Mas s_{400^{o} C} - Mas s_{500^{o} C}) \times (Ca {(OH)}_{2} Molar mass)}{H_{2} O Molar mass}

(2)

Pore Solution Analysis

Pore solution extraction and analysis were performed on binder paste specimens at different ages to determine the pore solution chemistry. The samples were mixed with binders consisting of high-alkali cement and SCMs, and deionized water. The water/binder ratio for this study was maintained at 0.60 for all samples in the experiment. The method used to extract pore solution in this study was using a pore solution expression die based on Barneyback and Diamond ( 36 ). Maximum stress of about 260 MPa was applied to extract the pore solution from the samples. The load rate was maintained between 1 and 1.8 kN/s. The pore solution was collected into centrifuge tubes, preventing potential contamination from carbonation, and they were stored at 4°C in a refrigerator before testing.

Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to analyze the pore solution to determine the concentration of alkalis ions (Na⁺ and K⁺). Before running ICP-OES, the pore solution was centrifuged for 10 min to separate any solids and the liquid. An amount of 1 ml pore solution was extracted from the storage tubes and diluted with 2% nitric acid (HNO₃) based on mass. The pore solution’s dilution factor was 100, meaning a 100 ml mixture solution contained 1 ml of pore solution. All water used in this study was deionized water.

AASHTO T380 Miniature Concrete Prism Test (MCPT)

AASHTO T380 (MCPT) was used to evaluate the ability of high-alkali SCMs to mitigate ASR in this study. In this method, the cementitious materials content of concrete mixtures was maintained at 420 kg/m³, with a water/binder ratio of 0.45. The ratio of bulk volume of dry-rodded coarse aggregate per unit volume of concrete was maintained at 0.65, and the coarse aggregate gradation followed the recommended gradation per AASHTO T380. The fineness modulus of fine aggregates was 2.60. Reagent-grade NaOH pellets were dissolved in the mixing water to boost the alkali content of the concrete to 1.25% by the mass of cement. SCMs were used at dosage levels of 20%, 30%, and 40% by mass replacement of cement.

The test specimens were cast and cured at ambient temperature and 100% RH for 24 h. After demolding, the specimens were placed in water at 60°C for another 24 h. The zero-day reading was taken at the end of 24 h of water bath curing. Then, the specimens were transferred into a sealed container with 1N NaOH maintained at 60°C. The prism length changes were recorded periodically at 0, 3, 7, 10, 14, 21, 28, 42, 56, 70, and 84 days. The criteria for evaluating the efficacy of SCMs in mitigating ASR in the MCPT method at 56 days are as per AASHTO T380:

Expansion <0.020% = effective ASR mitigation

0.020% <expansion <0.025% = uncertain ASR mitigation

Expansion > 0.025% = not effective ASR mitigation

If the samples exhibit expansion between 0.020% and 0.025% at 56 days, the average expansion between 56 days and 84 days (8 weeks and 12 weeks) should be less than 0.010% per 2 weeks for the mitigation measure to be considered effective, or, alternately, additional experiments should be conducted at higher dosage levels of SCM.

Results and Discussion

Strength Activity Index (SAI)

Figure 3 shows the SAI results, expressed as a percent of the Portland cement control. At 7 days, all the test materials, except NP 2 and RFA 1, had an SAI of approximately 85%. NP 2 indicated a greater value than the other materials and was close to that of the control mixture at 99%. However, the SAI of RFA 1 was only 70%, lower than the requirement of ASTM C618 of 75%. At 28 days, apart from NP1, other SCMs showed a significant increase in SAI compared with the corresponding value at 7 days, especially RFA 2, which increased from 85% to 112%. The pozzolanic reaction is the predominant reason for the relative increase in the SAI at this age ( 37 – 39 ). The 28-day SAI of NP 3, NP 4, and NP 5 increased by around 10% compared with 7-day values, reaching around 95%. However, NP 1’s 28-day SAI stayed similar to the value at 7 days, at 86%.

Figure 3.

High-alkali supplementary cementitious materials strength activity index results.

ASTM C1897: R³

Figure 4 indicates the 7-day cumulative heat release from the R³ results. All the paste specimens with SCMs investigated in this study showed a 7-day cumulative heat above 300 J/g. A previous study concluded that SCMs achieving a 7-day cumulative heat in excess of 200 J/g indicate highly pozzolanic reactivity; on the contrary, if the cumulative heat was below 100 J/g, it indicated an inert material ( 40 ). Therefore, all the high-alkali SCMs in this test are considered as highly pozzolanic materials. Two reclaimed fly ashes had the highest cumulative heat in this test, with RFA 1 and RFA 2 leading other natural pozzolans. The cumulative heat of NP 3 and NP 4 was around 350 J/g, and NP 1 and NP 2 were about 300 J/g.

Figure 4.

Heat release of high-alkali supplementary cementitious materials in R³ test.

The results from the R³ test did not correlate well with SAI results. For example, RFA 1 was a highly pozzolanic material with the highest cumulative heat release at 7 days in the R³ test. However, in the SAI test, it had the lowest SAI at both 7 days and 28 days. Additionally, NP 2 had a very high SAI, second only to RFA 2, but its cumulative heat release was the lowest among all materials in the R³ test.

Thermogravimetric Analysis (TGA)

Figure 5 indicates the relative CH contents in the paste. Considering the cement dilution that occurs when cement is replaced with 20% SCM, the CH content of the test mixtures was divided by 0.80 to correct for the dilution. The calibrated high-alkali SCMs CH consumption values were compared with the control, expressed as percent control, and the results are shown in Figure 5.

Figure 5.

Relative calcium hydroxide content in pastes containing 20% high-alkali supplementary cementitious material replacement.

At early ages, before 7 days, it is clear that all the high-alkali SCMs increased the CH content in the mixtures, which resulted from the filler effect of SCMs. The filler effect increased nucleation sites, further accelerating the cement hydration process ( 41 ). With the sample age increasing and pozzolanic reactivity, the CH content of mixtures with SCMs started to decrease, and some groups were lower than the control, indicating the occurrence of pozzolanic reactions caused by high-alkali SCMs. NP 1 and RFA 1 did not effectively lower the CH content in the mixtures compared with the control, and their CH content level was maintained at a constant level between 28 days and 56 days. This could have resulted from the larger particle size of NP 1 and RFA 1. These two materials have larger median and mean particle sizes compared with the rest of the SCMs. For the rest of the materials, they decreased the CH content by at least 5% compared with the control.

The CH consumption in the TGA did not correlate well with the results from the R³ test. The materials indicated as highly pozzolanic materials in R³ test did not effectively consume CH in the mixtures such as the RFA 1 and NP 1. The potential reason for the results was the different test environments. The R³ test’s environment was highly alkaline and high temperature (40°C), potentially increasing high-alkali SCMs’ reactivity and reducing the impact of particle size; this tendency was also observed in a previous study ( 42 ).

Pore Solution Analysis

Figures 6 and 7 show the potassium (K⁺) and sodium (Na⁺) concentrations of pore solutions expressed from various paste specimens containing SCMs at 28- and 84-day test durations. Figure 8 indicates the sum of alkalis ions concentration (K⁺ + Na⁺) in the same paste specimens. Since mixtures with SCMs contain only 80% cement, the red line in the figure is included to represent the 80% alkali ions concentration, resulting directly from the Portland cement. The pore solution analysis results indicate K⁺ concentration was much higher than Na⁺ in all the mixtures, with the exception of NP 2, and this is because K₂O is the principal alkali oxide in Portland cement. The alkali ions concentration in the control mixture remained constant from 28 days to 84 days, as the majority of Portland cement hydrations occur during this period and all alkalis are essentially released during this period.

Figure 6.

K⁺ concentration.

Figure 7.

Na⁺ concentration.

Figure 8.

Total alkali ions concentration.

In Figure 6, the K⁺ concentration of all test mixtures with SCMs was below the 80% line at 28 days and 84 days. NP 2 had the lowest K⁺ but increased significantly from the 28^th to the 84^th day, from 0.127 to 0.172 mmol/L. For the rest of the mixtures with other SCMs, the K⁺ concentration decreased with increasing sample age. RFA 1 indicated only a slight decrease from the 28^th to the 84^th day. Additionally, the K+ concentration of RFA 1 was much higher compared with mixtures with other SCMs. The change in K+ concentration strongly suggests the alkali binding by the pozzolanic reaction products.

Na⁺ concentration behaved differently from K⁺. Other mixtures, except for NP 1, indicated higher concentrations than the 80% control, especially NP 2. The Na⁺ ion concentration of NP 2 was highest at both 28 days and 84 days, with 0.242 and 0.235 mmol/L, respectively, while that of the control was only about 0.145 mmol/L. RFA 1 still did not perform well. At 28 days, RFA 1’s Na⁺ concentration was about 0.188 mmol/L, the second highest, and it increased by 0.04 mmol/L to reach 0.231 mmol/L at 84 days. The increase in Na⁺ concentration of all mixtures compared with the 80% of the control mixture indicated that the majority of the high-alkali SCMs released Na⁺ ions into the pore solution during the pozzolanic reaction, although these values were lower than that of K⁺ ion concentrations.

It was observed that the mixtures with RFA 1 had the highest total alkalis ions concentration among all the mixtures, which was also higher than the 80% of the control. NP 2 showed the lowest total alkali ions concentration, and NP 1 lowered the alkalis ions from 28 days to 84 days. The total alkali content of pore solution in mixtures with NP 3, NP 4, NP 5, and RFA 2 remained constant between the 28- and 84-day measurements. Both NP 2 and RFA 1 showed an increase in total alkali concentration in pore solution from 28 days to 84 days, and this is potentially because, when the pozzolanic reaction of high-alkali SCMs occurred, these high-alkali SCMs also potentially released alkali ions into the pore solution.

AASHTO T380 Miniature Concrete Prism Test (MCPT)

Figures 9 and 10 exhibited the length change of concrete prisms in AASHTO T380 (MCPT) with 20% SCMs. In MCPT, SCMs are considered to effectively mitigate ASR when expansion is below 0.020% at 56 days, and the expansion rate should not exceed 0.010% every two weeks from day 56 to day 84.

Figure 9.

American Association of State Highway and Transportation Officials T380 length change of concrete prisms with 20% supplementary cementitious material replacement.

Figure 10.

American Association of State Highway and Transportation Officials T380 56-D and 84-D expansion value with 20% supplementary cementitious material replacement.

Except for NP 1 and RFA 1, other materials effectively limited the ASR expansion to lower than 0.020% at 56 days. The Expansion of NP 1 and RFA 1 was 0.026% and 0.059%, respectively. RFA 1 showed the most inferior performance in the MCPT testing. Additionally, NP 1 and RFA 1 also failed to satisfy the expansion rate requirement of less than 0.010% per 2 weeks between 56 and 84 days. NP 3 had the lowest expansion at 56 days. However, at 84 days, NP 5’s performance exceeded NP 3. During the interval from 56 to 84 days, the rest of the mixtures, with the exception of NP 5, continued to expand, and their 2 week expansion rate exceeded 0.020% at 84 days.

NP 1, NP 3, NP 4, and two RFAs were selected to study the effect of SCMs replacement level on ASR mitigation. The replacement levels were increased from 20% to 30% and 40% by mass of cement in this test. The test results from this study are presented in Figure 11. The results indicated that, with an increase in SCM replacement level, the ASR expansion was significantly mitigated. Apart from RFA 1, the 30% replacement level was adequate for all the SCMs to control ASR expansion to less than 0.020% at 56 days, and the mitigation was effective even at 84 days. RFA 1 failed even at 30% replacement level, as the average test prism expansion was 0.035% at 56 days, which is much higher than the threshold of 0.020%. However, at 40% replacement level, RFA 1 successfully controlled the expansion below the 0.020% limit; even at 84 days, the expansion was limited to only 0.018%.

Figure 11.

American Association of State Highway and Transportation Officials T380 replacement level results comparison.

Correlation Between Various Pozzolanic Reactivity Experiments and MCPT

Table 2 shows the simple linear regression between MCPT expansion with pozzolanic reactivity experiments displaying raw data, regression equation, P-values, and R². To mitigate the influence of material types on the experimental results, the data is categorized into two groups: one includes all SCMs, and the other comprises only five types of natural pozzolan. Additionally, in contrast to the other experiments, the dataset for analysis includes the values of the control group of TGA and total alkali ions, as these two tests are quantitative analyses. Furthermore, in this analysis, the median size of SCMs was selected as a parameter to investigate the correlation between SCM’s particle size and MCPT expansion because, compared with the mean size, the median size was less affected by the extreme values. Also, in this analysis, the percentage of CH consumed as determined through TGA was used. Additionally, the linear regression plots are shown in Figure 12.

Table 2.

Correlation between Miniature Concrete Prism Test (MCPT) Expansion with Pozzolanic Reactivity Experiments

	MCPT-84D expansion (%)	Median size (µm)	Sum of Si+Al+Fe in XRF (%)	Amorphous (%)	SAI (%)	R3 cumulative heat release (J/g)	TGA weight loss (400°C–500°C) (%)	Total alkali ions (mmol/L)
Control	0.240						4.233	0.624
NP 1	0.044	21.90	87.13	8.55	86	307.8	3.428	0.413
NP 2	0.021	15.22	78.42	3.38	107	300.6	3.038	0.407
NP 3	0.022	8.38	85.71	87.76	97	351.2	3.082	0.419
NP 4	0.026	9.58	85.10	100.00	92	354.6	3.217	0.416
NP 5	0.018	7.29	85.77	91.17	95	315	3.021	0.435
RFA 1	0.080	18.86	75.07	82.48	75	389.7	3.358	0.542
RFA 2	0.022	9.95	73.70	88.88	112	387.2	3.207	0.441
Regression equation (all SCMs)		MCPT-84D expansion = −0.00312 + 0.0027949×median size	MCPT-84D expansion = 0.14069 − 0.0013169×sum (Si+Al+Fe)	MCPT-84D expansion = 0.0246617 + 0.0001093×amorphous	MCPT-84D expansion = 0.1735684 − 0.0014789×SAI	MCPT-84D expansion = −0.054294 + 0.0002548×R3 cumulative heat release	MCPT-84D expansion = −0.559003 + 0.1860149×TGA weight loss	MCPT-84D expansion = −0.356097 + 0.8985068×total alkali ions
Intercept P-value (all SCMs)		0.8645	0.3478	0.3636	0.0105	0.3742	<0.0001	0.0008
Predictor variable P-value (all SCMs)		0.0733	0.4641	0.7224	0.0223	0.5504	0.0001	0.002
R² (all SCMs)		0.5054	0.1115	0.0275	0.6781	0.1771	0.9388	0.8638
Regression equation (natural pozzolans)		MCPT-84D expansion = 0.0082299 + 0.0014406×median size	MCPT-84D expansion = −0.085934 + 0.0013282×sum(Si+Al+Fe)	MCPT-84D expansion = 0.0194006 + 8.9264e-5×amorphous	MCPT-84D expansion = 0.1188651 − 0.0009713×SAI	MCPT-84D expansion = 0.053445 − 8.3615e-5×R3 cumulative heat release	MCPT-84D expansion = −0.55163 + 0.1838643×TGA weight loss	MCPT-84D expansion = −0.399647 + 1.0202216×total alkali ions
Intercept P-value (natural pozzolans)		0.3298	0.5609	0.1910	0.1035	0.5311	0.0016	0.0007
Predictor variable P-value (natural pozzolans)		0.0698	0.4572	0.5594	0.1680	0.7422	0.0001	0.0004
R² (natural pozzolans)		0.7182	0.1946	0.1250	0.5219	0.0416	0.9476	0.9674

Note: NP 1–5 = natural pozzolan samples; RFA 1–2 = reclaimed fly ash samples; SAI = strength activity index; SCM = supplementary cementitious material; TGA = thermogravimetric analysis; XRF = X-ray fluorescence.

According to the results, an excellent correlation existed between MCPT expansion and TGA weight loss and total alkali ion results, with R² values of 0.939 and 0.864, respectively. However, the R² values significantly improved to 0.948 and 0.967, respectively, when considering only natural pozzolans were considered. These results revealed that the ASR mitigation of SCMs was proportional to the CH consumed. A similar finding was discovered in Oruji et al.’s research ( 43 ). Their results showed that ASR expansion and CH content indicated a linear proportional regression with 0.993 R². Additionally, the impact of pore solution alkalinity on ASR expansion, as observed in this study, can also confirm the findings from previous research, which showed that concrete exhibited a significant increase in expansion with an increase in pore solution alkali ion concentration ( 44 ).

However, MCPT 84-D expansion did not significantly correlate with the high-alkali SCMs’ amorphous level, the sum of Si, Al, and Fe oxides, and cumulative heat release in the R³ test, and their R² values were lower when considering all the SCMs or only natural pozzolans. Firstly, the high alkali content and temperature of the R³ reaction environment can explain its weak correlation with MCPT, which reduced the impact of relevant factors, such as particle size, on the pozzolanic reactivity ( 42 ). Secondly, the reactivity of SCMs and their ability to mitigate ASR is generally believed to be proportional to the sum of Si, Al, and Fe oxides, and the amorphous content of the pozzolan. However, the results of this study were not in agreement with the previous study, and this inconsistency can be attributed to the presence of high crystalline compounds in some SCMs ( 13 ). The existence of crystal content in SCMs, such as NP 2, indicated that not all the available silica and alumina were amorphous and reactive, making the sum of Si, Al, and Fe oxides unsuitable for directly determining the pozzolanic reactivity of SCMs. Thirdly, concerning the amorphous level, the low R² could be attributed to the activation of the crystal phase of SCMs during the reaction ( 45 ). This finding suggests that some high-crystal materials were not indeed “inert,” and the specific phase of the crystal also influences their reactivity.

The R² correlation between SAI and ASR expansion was only 0.678 when considering all SCMs but decreased to 0.522 when only considering natural pozzolans. Neither value was significant, suggesting that using SAI to predict SCMs’ ASR mitigation performance was ineffective. On the impact of SCMs on strength gain, apart from the densification caused by the pozzolanic reaction, the filler effect was another factor in accelerating the strength development, which enhanced the packing density of mixtures and allowed for further binder development ( 41 ). Furthermore, to mitigate the influence of material category differences, only natural pozzolan specimens were used for analyzing the correlation between MCPT expansion and the particle size of SCMs. In this case, the R² value reached 0.719, which indicates a better relationship between particle size and the suppression of ASR expansion. This relationship has also been confirmed in previous research: smaller particles possess a higher pozzolanic reactivity ( 14 ).

Figure 12.

Comparison between all SCMs and NPs: (a) median size versus ASR, (b) sum (Si+Al+Fe) versus ASR, (c) amorphous versus ASR, (d) SAI versus ASR, (e) R3 heat release versus ASR, (f) TGA CH content versus ASR, and (g) alkali ions concentration versus ASR.

Conclusion

From the data obtained in this study, the following conclusions can be drawn:

1) Compared with the control, the use of high-alkali SCMs can effectively reduce ASR expansion. RFA 1 did not perform as well as the rest; however, its use reduced ASR-induced expansion at the 20% replacement level compared with the control. Preliminary results indicate that, at higher replacement levels (30% and 40%), high-alkali SCMs can perform much more effectively in mitigating ASR.

2) The amorphous level of high-alkali SCMs was found to have less affect than other parameters. No apparent correlation was discovered between the amorphous level and other test results. NP 2 was a highly crystalline material but still indicated good pozzolanic reactivity and ASR mitigation performance.

3) The R³ test cannot be directly used to predict the ASR mitigation performance of high-alkali SCMs. Also, a “weak” correlation exists between R³ and SAI results. The primary reason for those results is that the R³ test evaluates the SCMs’ pozzolanic reactivity at a high-alkali and higher-temperature environment, which can increase the reactivity of SCMs.

4) High-alkali SCMs can effectively lower the alkali ions concentration in the pore solution, supporting the MCPT results. Also, there is a dynamic equilibrium of total alkali ions that exist in the pore solution when using high-alkali SCMs in the concrete. While high-alkali SCMs lowered the alkali ion content in the pore solution through alkali binding resulting from the pozzolanic reaction, these materials also potentially released some alkali ions into the pore solution. Additionally, the use of the total alkali content of high-alkali SCMs as criteria to evaluate their ASR-mitigating ability is not appropriate. Materials with low-alkali content may release more alkali ions into the pore solution compared with high-alkali SCMs and this can increase the challenge of controlling the ASR expansion. No correlation exists between the total alkali content of SCMs and the concentration of alkali ions in the pore solution, as this appears to be a function of the nature of the alkaline phase present in the SCMs.

5) Findings from TGA and pore solution analysis studies supported the expansion observed in the MCPT tests. The results of the two methods indicated “excellent” correlations with ASR expansion. Compared with other pozzolanic reactivity experiments, TGA and pore solution analysis are more effective in predicting high-alkali SCMs’ ASR mitigation performance.

6) Not all SCMs can be treated equally. Natural pozzolans and industrial by-products, such as fly ashes, must be analyzed independently for their ability to mitigate ASR.

Footnotes

Acknowledgements

We acknowledge the support of Boral CM Services, National Pozzolan Association (NPA), and its member companies for providing the materials for this study.

Author Contributions

The authors confirm contribution to the paper: study conception and design: W. Wang, P. Rangaraju; data collection: W. Wang, J. Roberts; analysis and interpretation of results: W. Wang; draft manuscript preparation: W. Wang, J. Roberts, P. Rangaraju. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This project was supported by the Natural Pozzolan Association through in-kind contribution of materials for the research. Also, the project was funded through internal grants at Clemson university (Incentive Grant – 1480355).

ORCID iDs

Weiqi Wang

James Roberts

Prasad Rangaraju

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Pozzolanic Reactivity of High-Alkali Supplementary Cementitious Materials and Its Impact on Mitigation of Alkali-Silica Reaction

Abstract

Keywords

Materials and Methods

Materials

ASTM C311 Strength Activity Index (SAI)

ASTM C1897 R3

Thermogravimetric Analysis (TGA) for Determining Calcium Hydroxide (CH) Consumption

Pore Solution Analysis

AASHTO T380 Miniature Concrete Prism Test (MCPT)

Results and Discussion

Strength Activity Index (SAI)

ASTM C1897: R3

Thermogravimetric Analysis (TGA)

Pore Solution Analysis

AASHTO T380 Miniature Concrete Prism Test (MCPT)

Correlation Between Various Pozzolanic Reactivity Experiments and MCPT

Conclusion

Footnotes

Acknowledgements

Author Contributions

Declaration of Conflicting Interests

Funding

ORCID iDs

References

ASTM C1897 R³

ASTM C1897: R³