Comparison of Accelerated Mortar Bar Tests for Evaluating Alkali–Silica Reactivity of Reactive vs Non-Reactive Siltstone Aggregates: Case Study from the Qinghai–Tibet Plateau

Abstract

The accurate identification of the degree of alkali reactivity of aggregates is crucial for preventing alkali–silica reaction (ASR) damage in concrete. Two siltstones were collected from the main stream and a tributary of the Yarlung Tsangpo River. The alkali reactivity of these siltstones was examined through a combination of the Petrographic Method, the Accelerated Mortar Bar Test (AMBT), and the Chinese Universal Accelerated Mortar Bar Test (CAMBT). The influence of the petrographic characteristics of siltstones on the applicability of two accelerated expansion tests was also evaluated. Results show that both siltstones exhibit blocky textures and contain cryptocrystalline–microcrystalline quartz. However, the major alkali reactive components in the two siltstones show significant differences in morphology, distribution and content, with cryptocrystalline–microcrystalline quartz contents of approximately 20% in FS-1 and 10% in FS-2. This study reveals that the CAMBT identified two siltstones as alkali-reactive, whereas the standard AMBT classified them as non-reactive. The petrographic structures of the siltstones and the microstructural distribution of reactive quartz constituents are the key factors governing their expansion behavior and the applicability of different testing methods. By employing a single-graded aggregate that preserves the original rock fabric, the CAMBT improves the reliability of the detection of alkali reactivity in siltstones.

1. Introduction

The alkali–silica reaction (ASR) is one of the major factors causing the deterioration of concrete durability. Since its discovery in the 1940s, the ASR has led to the destruction of many dams, harbors, bridges, and highways, as well as industrial and civil buildings worldwide. Due to the diversity and complex composition of the fine and coarse aggregates, combined with the variation in mix proportions and service environments of concrete, the reaction becomes extremely complicated [1,2,3,4,5].

Siltstone is a fine-grained clastic sedimentary rock that is widely distributed in continental and shallow water depositional environments and is locally used as a source of construction aggregate. Siltstone is defined as a lithified sedimentary rock dominated by silt-sized particles, with particle sizes ranging from 0.004 to 0.0625 mm. From a geological classification perspective, siltstone and sandstone (particle sizes ranging from 0.0625 to 2.0000 mm) are both clastic sedimentary rocks derived from similar source materials and sedimentary processes and commonly occur as transitional or interbedded lithologies in natural sedimentary successions. Their fundamental distinction lies in a dominant grain size, rather than in their mineralogical origin or depositional mechanism. Siltstone consists mainly of fine-grained quartz with feldspar and lithic fragments and is lithified through cementation by siliceous, calcareous, or clayey materials [6]. Because of the high proportion of cryptocrystalline–microcrystalline quartz, clastic sedimentary rocks such as siltstone and sandstone can exhibit pronounced alkali–silica reactivity under highly alkaline conditions and therefore constitute potentially reactive aggregate sources in accelerated ASR evaluation tests. Different types of siltstone and sandstone are generally characterized by high quartz contents and have been widely reported as alkali-reactive rock types associated with ASR damage in concrete structures [7,8,9].

It should be noted that siltstone can present complex and diverse compositions and microstructural characteristics. These compositional and structural factors can affect the dissolution, reaction process, and expansion behavior of quartz in high-pH concrete pore solutions, often making the reliable determination of its ASR reactivity an engineering challenge and also a major obstacle in developing rapid and reliable testing methods.

Two hydroelectric stations in Canada that suffered the most severe ASR damage worldwide—the Beauharnois Dam and the Mactaquac Dam—were both affected by a misjudgment of the sandstone alkali reactivity [10]. The Cambrian Potsdam sandstone used at Beauharnois, located in Montreal, consists of well-crystallized quartz grains cemented by epitaxially grown siliceous components, with its alkali reactivity derived from the secondary siliceous components. The fine-grained graywacke used in the Mactaquac Dam in New Brunswick contains cryptocrystalline–microcrystalline quartz dispersed in the rock matrix as its reactive source. Both sandstones were judged as non-reactive aggregates by the mortar bar method (ASTM C227) used at the time, but ASR damage occurred after 7 and 10 years of dam operation, respectively [10]. Even the Accelerated Mortar Bar Test (ASTM C1260), later developed to replace ASTM C227 and widely used internationally, failed to correctly detect the alkali reactivity of the Potsdam sandstone [11]. This case reveals that ASTM C1260 carries a risk of false negatives when dealing with specific microcrystalline quartz aggregates. These two cases not only reflect the geological complexity of clastic sedimentary rocks such as siltstone and sandstone but more importantly highlight the limitations of early accelerated test methods in identifying the ASR susceptibility of heterogeneous siliceous aggregates.

Although documented cases of accelerated ASR evaluation tests failing to identify the alkali reactivity of siltstone aggregates remain limited, the close similarities between siltstone and sandstone in mineralogical composition and structural characteristics indicate that the ASR susceptibility of siltstone aggregates may also be underestimated by such accelerated test methods.

The Chinese Universal Accelerated Mortar Bar Test (CAMBT) was developed by combining the advantages of ASTM C1260 and the RILEM AAR-5 concrete microbar method for detecting alkali–carbonate reactions (ACRs) [12]. This method uses single aggregate gradation aggregates (2.5–5.0 mm) and can be applied to detect both ASRs and ACR reactivity. Compared with ASTM C1260, the CAMBT significantly improves the accuracy of judging aggregate ASR activity and its applicability to different rock types. For the two sandstones used in the Beauharnois and Mactaquac dams, the CAMBT not only correctly determined whether they were alkali-reactive, but its evaluation of the degree of reactivity was also highly consistent with the results of the concrete prism test (ASTM C1293), which is considered the most reliable internationally. Based on comparative studies of nearly 50 aggregates from the United States, Canada, Norway, South Korea, and Australia, using a 14 d expansion of 0.093% as the criterion, this method significantly improved reliability compared with the AMBT [12]. Based on the aforementioned testing methods, a new accelerated approach for ASR evaluation was established by adopting a single aggregate gradation in the CAMBT. The implementation of this method simplifies the preparation procedure of aggregate samples, enhances the reliability of testing results, and provides a new direction for the continuous optimization of key experimental parameters.

The effective prevention of alkali–silica reaction (ASR) damage in concrete fundamentally depends on the reliable identification of aggregate alkali reactivity. In engineering practice, Accelerated Mortar Bar Tests are widely employed as rapid screening tools; however, their applicability to aggregates with complex mineralogical and microstructural characteristics remains a critical concern. In particular, heterogeneous siliceous aggregates may exhibit expansion behavior that is not fully captured by certain accelerated test methods, leading to the potential underestimation of the ASR risk. This issue is of practical importance in regions such as the Yarlung Tsangpo River basin on the Qinghai–Tibet Plateau, where locally sourced siltstone is expected to be extensively used in large-scale concrete infrastructure owing to economic and logistical constraints.

Against this background, the present study evaluates the alkali–silica reactivity of representative siltstone aggregates from the Yarlung Tsangpo River basin using a combined approach involving a petrographic analysis, the Accelerated Mortar Bar Test (AMBT), and the Chinese Universal Accelerated Mortar Bar Test (CAMBT). Particular attention is paid to the influence of siltstone microstructural features on expansion behavior and to the discrepancies observed between different accelerated test methods. By systematically comparing the AMBT and CAMBT, this study aims to assess the reliability of Accelerated Mortar Bar Tests for ASR prevention and to clarify the potential advantages of the CAMBT in identifying the alkali reactivity of siltstone aggregates in practical engineering applications.

2. Materials and Methods

2.1. Cement

The cement used in the experiment (PC) was PI52.5R Portland cement produced by Jiangnan Onoda Cement Co., Ltd. (Nanjing, China). The chemical composition is shown in

Table 1. Chemical composition of cement (wt%).

SiO2	Fe2O3	Al2O3	CaO	MgO	K2O	Na2O	SO3	Loss
18.55	3.41	3.95	65.32	1.01	0.72	0.18	0.78	2.88

.

2.2. Siltstone

The sampling of the siltstone was carried out in accordance with representative geological characteristics of the region and future infrastructure construction planning. Referring to the Guide to the Field Geological Survey of the Tethys–Qinghai–Tibet Plateau Geological Evolution [13], the two siltstones were collected from the main stream and a tributary of the Yarlung Tsangpo. The basic information of the two siltstones is shown in

Table 2. Information on the location and type of the siltstones.

Sample	Aggregate	Latitude/°	Longitude/°	Elevation/m	Location
FS-1	Black siltstone	29.4577	84.4212	4728	Main stream
FS-2	Ferruginous siliceous lithic siltstone	29.0724	88.0061	4341	Tributary

FS represents the siltstone structure.

. Representative siltstones were selected using the quartering method for petrographic thin section preparation; some siltstones were crushed and sieved for expansion AMBTs and CAMBTs.

2.3. Petrographic Method

Following ASTM C295 [14], the two siltstones were cut and ground, bonded to glass slides with epoxy resin, and then further ground and polished to a thickness of 0.03 mm to prepare standard thin sections. A Nikon LV100POL polarizing microscope (Nikon Corporation, Tokyo, Japan) was used to identify the microstructure and mineral composition of the siltstone.

The Gazzi–Dickinson statistical method was used to quantitatively analyze the content of alkali-reactive components in siltstone. Quartz grains smaller than 62.5 μm were counted.

2.4. Accelerated Mortar Bar Test (AMBT)

The AMBT was carried out following ASTM C1260 [15]. PI 52.5R cement with an alkali content of 0.6% ± 0.1% Na₂O_e was used, with five graded aggregates and a water–cement ratio of 0.47. After curing in an environment of 20 ± 2 °C and a relative humidity > 90% for 1 d, the specimens were demolded then cured in 80 °C water for 1 d, and the initial length was measured. Subsequently, they were cured in 1 mol/L NaOH solution at 80 °C until 28 d. The length of specimens was measured periodically, and the corresponding expansion at each age was calculated. This method uses 14 d expansion ≥ 0.1% as the criterion for judging reactive aggregates.

2.5. Chinese Universal Accelerated Mortar Bar Test (CAMBT)

This method combines ASTM C1260 with the RILEM AAR-5 [16] micro-concrete prism test for detecting the aggregate alkali–carbonate reaction (ACR). It uses single-sized aggregates of 2.5–5.0 mm, with a cement–aggregate ratio of 1:1 and a water–cement ratio of 0.33. The mortar molding method, curing conditions, measurement schedule, and expansion calculation are the same as the AMBT. This method uses 14 d expansion ≥ 0.093% as the criterion for judging reactive aggregates.

3. Results

3.1. Petrographic Characteristics of Siltstones

Figure 1 shows thin section photomicrographs of the two siltstones (cross-polarized light), displaying typical alkali-reactive components.

Table 3. Petrographic characteristics of siltstones: textural and structural features and mineral composition.

Sample	Aggregate	Textural and Structural Characteristics and Mineral Composition of Siltstones
FS-1	Black siltstone	The siltstone exhibits sub-angular to sub-rounded rounding, is well sorted, and has a silt texture with a blocky structure. The main mineral constituents are quartz, feldspar, carbonaceous fragments, and limonite. The cryptocrystalline–microcrystalline quartz below 40 μm is uniformly and diffusely distributed, and its total content is approximately 20%.
FS-2	Ferruginous siliceous lithic siltstone	The siltstone has an argillaceous silt texture with a blocky structure. The main mineral constituents are quartz, accompanied by clay, opaque limonite, and organic matter. Cryptocrystalline–microcrystalline quartz below 40 μm is uniformly distributed with a clustered texture, and its total content is approximately 10%.

lists the structural and mineralogical features identified in thin sections.

Petrographic analyses from Figure 1 and Table 3 show that the two siltstones from different locations in the Yarlung Tsangpo River basin are blocky in structure according to the geological classification but display different structural characteristics. The two siltstones exhibit a silt-dominated structure, with minor variations in grain rounding and sorting observed between them. In addition, the quartz grains in sample FS-1 are characterized by a more diffuse distribution, whereas those in FS-2 are characterized predominantly by a clustered texture.

Typical alkali-reactive quartz, including cryptocrystalline and microcrystalline varieties, is identified as the dominant reactive component in the two siltstones. The corresponding alkali-reactive quartz contents are approximately 20% in FS-1 and 10% in FS-2, respectively.

3.2. Expansion of Siltstones in AMBT and CAMBT

The expansion curves of the two siltstone samples under the AMBT and CAMBT are shown in Figure 2 and Figure 3, with their 14 d and 28 d expansions listed in

Table 4. AMBT and CAMBT expansion results of siltstones.

Aggregate	Specimen/ Parameter	Expansion
		3 Days	7 Days	10 Days	14 Days	28 Days
FS-1 (AMBT)	1-1	0.035	0.049	0.068	0.101	0.132
	1-2	0.027	0.044	0.061	0.099	0.137
	1-3	0.032	0.048	0.063	0.084	0.132
	Average	0.031	0.047	0.064	0.095	0.134
FS-2 (AMBT)	2-1	0.019	0.036	0.051	0.077	0.111
	2-2	0.012	0.037	0.057	0.075	0.121
	2-2	0.016	0.037	0.051	0.069	0.103
	Average	0.016	0.037	0.053	0.074	0.112
FS-1 (CAMBT)	3-1	0.025	0.056	0.087	0.151	0.303
	3-2	0.021	0.041	0.080	0.163	0.313
	3-3	0.019	0.040	0.091	0.157	0.316
	Average	0.022	0.046	0.086	0.157	0.311
FS-2 (CAMBT)	4-1	0.015	0.041	0.080	0.105	0.273
	4-2	0.016	0.044	0.076	0.096	0.269
	4-3	0.016	0.037	0.064	0.093	0.255
	Average	0.016	0.041	0.073	0.098	0.266
FS-1	Ratio of CAMBT to AMBT	0.700	0.972	1.340	1.657	2.322
FS-2	Ratio of CAMBT to AMBT	1.000	1.108	1.387	1.331	2.382

.

According to ASTM C1260, a 14 d expansion of ≥0.1% is used to identify reactive aggregates. As shown in Figure 2, the two siltstone samples exhibited 14 d expansions below 0.1% in the AMBT and were classified as non-reactive. In contrast, based on the CAMBT criterion with a 14 d expansion threshold of ≥0.093% [12], Figure 3 indicates that the two siltstone samples exceeded this threshold and were identified as alkali-reactive.

In addition, the two siltstone samples showed different expansion characteristics with the extension of the curing time. In general, alkali-reactive aggregate samples may expand slowly at early ages and increase significantly after 14 d, typically showing an “S”-shaped curve. Some alkali-reactive aggregate samples expanded almost linearly over the entire curing period. However, the two siltstone samples showed significant differences in expansion behavior between the two methods. In almost the entire curing period, AMBT expansions were lower than those of the CAMBT, and after 14 d, the differences in the expansion increase became especially evident. At 28 d, FS-1 and FS-2 expansions in the CAMBT were 2.321 and 2.375 times that of their corresponding AMBT values. These differences in curve characteristics indicate that the structural features and particle size of siltstone significantly affect ASR expansion behavior and the reliability of the judgment of its alkali reactivity. Moreover, sample FS-1 showed a greater expansion than FS-2 in both tests, with a more pronounced difference observed in the CAMBT. Both siltstones exhibited a smaller expansion in the AMBT than in the CAMBT throughout the testing period. The divergence increased notably after 14 days, with CAMBT expansions at 28 days being approximately 2.3 times greater than those in the AMBT (Table 4).

4. Petrographic Structures and Applicability of Test Methods

A comprehensive evaluation using the Petrographic Method, AMBT, and CAMBT indicates that the two siltstones from the Yarlung Tsangpo River basin are characterized by blocky structures and contain typical alkali-reactive quartz components, with variations in their crystallinity, grain size, content, and distribution. These petrographic characteristics significantly influence their macroscopic expansion behavior in mortar bar tests and the resulting activity classifications. Notably, pronounced discrepancies in expansion behavior are observed for the siltstones between the AMBT and CAMBT. In particular, the siltstones exhibit substantially higher 14 d and 28 d expansions in the CAMBT than in the AMBT, leading to inconsistent activity/non-reactive classifications when the 14 d expansion criteria are applied [11,17].

Based on studies of aggregate particle size and ASR expansion [18,19,20], the petrographic structure of aggregates significantly affects their ASR expansion behavior and particle size effect. The relationship between the aggregate particle size and ASR expansion is not simple or direct but is closely related to the crystallinity of alkali-reactive components, the proportion of aggregates and the ratio of coarse particles to fine particles, curing conditions, and other factors [19]. The differences in the expansion results of the siltstone samples between the AMBT and CAMBT are mainly owing to the different aggregate gradations used in the two methods. Single-sized aggregates (2.5–5.0 mm) are used in the CAMBT, while five graded aggregates (2.50–5.00 mm, 1.25–2.50 mm, 0.63–1.25 mm, 0.30–0.63 mm, and 0.15–0.30 mm) are used in the AMBT. During the specimen preparation in the AMBT, crushing and grinding to obtain finer aggregate gradations may destroy the original structure and texture of the aggregate, affecting the space for the ASR gel accumulation and hardening [21]. In addition, for some rocks with special microstructural features, grinding and sieving may cause the loss of alkali-reactive components [11,22], thus underestimating the alkali reactivity of these aggregates.

In this study, the two siltstones, FS-1 and FS-2, exhibited significantly smaller expansions in the AMBT than in the CAMBT, and their judged results also differed. The alkali-reactive components in both siltstones were cryptocrystalline–microcrystalline quartz. However, the distribution pattern and content of the quartz are quite different, as shown in Figure 1 and Table 3. About 20% cryptocrystalline and microcrystalline quartzes are uniformly distributed in FS-1, while about 10% cryptocrystalline and microcrystalline quartz agglomerates are clustered in FS-2. As shown in Figure 4 and Figure 5, in FS-1, where alkali-reactive quartz is more uniformly distributed, cracks develop preferentially along the aggregate–paste interface, and a substantially higher number of internal cracks is observed compared to FS-2. This crack pattern in FS-1 is consistent with the classical ASR rim model, reflecting a relatively high internal homogeneity of the aggregate. In contrast, FS-2, characterized by a less uniform distribution of alkali-reactive quartz and a more clustered texture, exhibits internal cracks that are predominantly manifested as single through-going cracks, which may be attributed to localized contributions from a limited number of clustered alkali-reactive quartz domains. The micrographs of cracks shown in Figure 4 and Figure 5 indicate that the extent of the expansion-induced damage in sample FS-1 is greater than that in FS-2. In summary, FS-1, which contains a higher proportion of alkali-reactive quartz with a more uniform distribution, exhibits greater ASR expansion than FS-2, in which the alkali-reactive quartz content is lower and occurs in a more clustered distribution, suggesting that the content and distribution of alkali-reactive quartz are key factors controlling the magnitude of the ASR expansion in siltstone aggregates.

The crushing and grinding during the AMBT preparation should not significantly affect the microstructural homogeneity. The small expansion of the two siltstones in the AMBT may, on the one hand, be due to the loss of alkali-reactive components during crushing and sieving. On the other hand, since the sample preparation in the AMBT does not include washing and drying, some fine reactive particles (<0.15 mm) may show pozzolanic effects [20,23], which could suppress the ASR expansion and thus underestimate the alkali reactivity.

The CAMBT uses larger-sized aggregates, and specimen preparation requires almost no grinding, preserving the original microstructure, distribution state and content of alkali-reactive components, avoiding the enrichment/loss of components due to the specimen preparation and the possible pozzolanic effect of excessive fine particles. Therefore, the CAMBT can more accurately reflect the true alkali reactivity of aggregates and has better applicability to different rock types, including siltstone [11,12].

5. Conclusions

Through the petrographic analysis, AMBT, and CAMBT, the alkali reactivity of two siltstone samples from the Yarlung Tsangpo River basin of the Qinghai–Tibet Plateau was systematically evaluated. The influence of petrographic features on the ASR expansion behavior and the applicability to the AMBT and CAMBT were studied. The main conclusions are as follows:

• The siltstone samples are blocky in structure. Both contain typical alkali-reactive quartz components, with significant differences in crystallinity, grain size, content, and distribution. In the two siltstones, the alkali-reactive quartz components are mainly diffusely distributed cryptocrystalline–microcrystalline quartzes, accounting for approximately 10% and 20%.
• The alkali-reactive quartz with a different distribution pattern (uniform or clustered) showed significant differences in expansion development and activity/non-reactive results in the AMBT and CAMBT. The two siltstones were non-reactive in the AMBT but reactive in the CAMBT.
• The smaller expansions of siltstones in the AMBT compared with the CAMBT were due to the differences in the aggregate gradation used and the effects of crushing and sieving on the microstructure and reactive components. The CAMBT uses particle sizes that preserve rock structural features, reducing the damage to the original structure and the enrichment or loss of alkali-reactive components caused by crushing, grinding, and sieving, thus improving the reliability of the detection of the alkali reactivity of aggregates, including siltstone.
• The findings of this study are primarily based on macroscopic expansion and optical microscopy. Future research should employ advanced microanalytical techniques, such as SEM-EDS and XRD, to characterize the composition and morphology of ASR gels, thereby providing a deeper understanding of the underlying reaction mechanisms.
• To further validate the predictive capability of the CAMBT for siltstone aggregates, future studies will include a quantitative analysis and concrete prism tests (ASTM C1293). Furthermore, the applicability of the CAMBT will be explored for a wider range of sedimentary rocks, such as greywacke and shale, to establish its general reliability for complex lithologies.