Acid Activation in Low-Carbon Binders: A Systematic Literature Review

Abstract

Geopolymers have emerged as an alternative binding material to Ordinary Portland Cement (OPC). Recently, there has been an increase in studies exploring the synthesis of these materials using acid activation rather than traditional alkaline activation. This approach offers benefits such as good strength at an early age, better thermal properties, and a chemical activator that emits less carbon to be produced. In addition, it provides resistance to efflorescence and leaching, which are common challenges associated with alkali-activated products. This work analyzed the scientific advances in acid activation in synthesizing an alternative binder to OPC. To this end, a systematic review of the last five years of scientific literature was carried out using the Systematic Review for Engineering and Experiments (SREE) method. The results show a notable increase in research focused on acid activation over the last few years. The acid activators were always phosphate solutions, mainly phosphoric acid. Metakaolin was the most tested precursor, followed by fly ash, and volcanic ash. The research requires improvements in the methodological quality, providing data on molar ratios (Al/P, Si/Al, and Si/P), Liquid/Solid mass ratio, activator solution molarity, and curing process, in addition to statistical treatment and comparison of results. There exists a paucity of diversity in the examined precursors, activators, and additives. Future research developments need to clarify the behavior of mechanical resistance over time, better curing process, water resistance, durability, and the role of iron, magnesium, and calcium silicates and/or oxides. The paper identifies the main research gaps in the area and functions as a database, guiding researchers in selecting raw materials, dosing methodology, and curing processes.

1. Introduction

The manufacture of Ordinary Portland Cement (OCP) emits a significant amount of greenhouse gases, resulting in adverse environmental consequences, such as increased global temperatures and climate change. Due to this, interest in using various waste products from industry to create new types of cement has increased to reduce dependence on OPC in construction and material production. [1]. Among current research endeavors is the development of activated materials. These materials rely on chemical activation to attain properties similar to OPC, including mechanical strength and durability. However, they have the advantage of emitting fewer greenhouse gases during production [2]. They are produced by combining precursor sources of aluminosilicates (such as fly ash, blast furnace slag, metakaolin, etc.) with an alkaline solution (such as sodium silicate, sodium hydroxide, etc.), capable of initiating chemical reactions that result in the hardening of the mixture.

In recent years, there has been an increase in studies exploring acid activation rather than traditional alkaline activation. This shift in focus is primarily due to some advantages associated with acid activation, such as enhanced early-age properties, increased long-term mechanical strength, and improved thermal characteristics [3,4]. Additionally, acid activators are produced with lower CO₂ emission and energy consumption rates [5], present efflorescence resistance, and do not induce leaching problems [6]. However, acid activation faces some challenges, including the complexity of the chemical process, the curing process, and the cost of the acid. The microstructure and performance of acid-based geopolymers are also highly dependent on the precursors and activators’ physicochemical properties. These properties include particle size [7], curing temperature [8,9], molar concentration of the acid [5,10,11,12], activator dilution [13], and chemical composition [14].

Phosphoric acid (PA) and metakaolin (MK) are the main acid activators and precursors used in synthesizing acid-based geopolymers. However, environmental and economic concerns are driving efforts to replace MK partially due to the high temperatures required for its production and the exploitation of natural resources to supply kaolinite [6]. Fly ash (FA) [5,15,16,17,18,19,20,21,22], volcanic ash (VA) [9,23,24,25], calcined clay [26], burnt brick residue [27], bauxite [28], laterite [28,29], and mine tailings (MT) [30] were tested, alone or in blends, to reduce the required amount of MK and provide a recycling opportunity for these residues.

Acid activation with PA is favored by its high reactivity with aluminosilicate sources and by generating products with good thermal properties. In an acidic environment, the active Si-O-Al layer ruptures to produce the [SiO₄]⁴⁻ and [AlO₄]⁵⁻ units, which combine with the [PO₄]³⁻ tetrahedron to form amorphous gels [31]. Despite the extensive research on acid activation, there remain unanswered questions due to the lack of systematic studies on acid-activated systems. Furthermore, the wide variety of aluminosilicate sources, in terms of chemical compositions and particle sizes, makes it difficult to compare studies and determine ideal compositions [31].

This work focuses on the acid activation for synthesizing alternative binders to OPC through a systematic literature review (SLR) based on the Systematic Review for Engineering and Experiments (SREE) method. The selection of studies only includes paste analyses, excluding investigations involving mortars or concretes due to the influence of aggregates used. Furthermore, studies related to Supplementary Cementitious Materials (SCM) to OCP were not considered. Four review articles related to phosphate-based geopolymers or phosphate cement [6,31,32,33] were identified, providing information on the structure, reaction kinetics, some properties, and potential applications, notably concerning geopolymers produced from MK and PA. To date, this is the first systematic review article focusing on the production of cementitious material through acid activation, which encompasses different precursors and activators. The objective is to analyze the methodological quality of the research, discern conflicting data, establish a database grouping information on raw materials, activators, pre-treatments, additions, molar ratios, curing processes, mechanical performance, and durability, as well as identify deficiencies and research gaps related to the topic.

2. Methodology

2.1. Systematic Literature Review (SLR)

The SLR was conducted following the Systematic Review for Engineering and Experiments method [34], which is an adaptation of the ProKnow-C method (Knowledge Development Process-Constructivist) [35], incorporating a methodology for analyzing the methodological quality of scientific articles.

Initially, search parameters and databases for the research were defined. Keywords were combined using the boolean operators ‘AND’ and ‘OR’ in the title, abstract, and keywords of the articles (Figure 1). The search was carried out on 7 July 2023, considering all scientific articles in the Science Direct and Web of Science databases, excluding categories unrelated to the theme, such as Medicine, Dentistry, Biology, Energy, and Social Sciences. The search reported 750 articles from Science Direct and 1010 from Web of Science, totaling 1760 manuscripts.

These articles were extracted from the databases and inserted into a reference manager, in which duplicate articles were removed, resulting in 1334 remaining articles. Figure 2 shows the distribution of these articles by year of publication, revealing the increasing interest in acid activation for synthesizing alternative binders.

Subsequently, articles from the last 5 years (2018–2023) aligned with the research were selected by analyzing titles and abstracts. Following this phase, the search terms were validated by checking the articles related to the topic that were cited by the selected articles. If a referenced article was not found, the article was examined to identify terms similar to the search terms. This was the case for terms such as “phosphate-based” and derivations of “silicoaluminophosphate”, which were initially not included in the search terms. After reviewing the articles, it was noted that many studies are using phosphoric acid in acid activation and, for this reason, using specific terminology for geopolymers synthesized by this acid, sometimes called ”phosphate-based geopolymer” or “silicoaluminophosphate geopolymer”. Figure 3 illustrates the outcome of all stages in the selection process of the 69 articles in this SLR, comprising four review articles and 65 research articles.

Table 1. Distribution of papers from bibliographic portfolio according to the precursor.

Precursor	Units	Percentage
MK	28	43%
MK + additive	9	14%
MK + second precursor	7	10%
FA	5	8%
FA + second precursor	2	3%
FA + additive	1	2%
VA	5	8%
others	8	12%

displays the distribution of research articles based on the investigated precursor. Note the predominance of studies with MK (67%), followed by FA (13%) and VA (8%).

Using the VOSviewer program (version 1.6.20 was released on 31 October 2023), a software tool for constructing and visualizing bibliometric networks, Figure 4 was produced with the keywords of the papers from the bibliographic portfolio. There is a notable occurrence of terms such as “metakaolin”, “phosphoric acid”, and “geopolymer”, in addition to their combination with the terms “activation”, “based”, “phosphate”, and “silicoaluminophosphate”.

2.2. Methodological Quality Analysis

The SREE method [34] was employed to assess the methodological quality of the selected studies. This approach considers the analysis criteria outlined in

Table 2. Criteria for analyzing the methodological quality according to the SREE method.

Item			Description
I	Randomization		Evidence of randomness in the production and testing of the samples involved
II	Analysis	Basic	Use of mean and standard deviation to characterize the samples elements
		Statistic	Use of inferential statistics to characterize the method
III	Comparison	Basic	Comparison with reference elements
		Medium	Comparison with similar studies without indication of origin
		Advanced	Comparison with similar studies from a SLR
		Statistic	Use of inferential statistics for comparison with SLR studies

, and Figure 5 shows the number of research articles for each methodological quality analysis criterion.

The evaluated studies lacked evidence of implementing randomization during the sample preparation processes or experimental programs. The randomization of samples is a fundamental aspect of guaranteeing the statistical reliability of the work, as it helps to distribute statistical errors that may be present in laboratory tests, minimizing their impact on the results. As a consequence, there is a possibility that some human or material factor could influence the production or testing of samples. Additionally, Figure 5 indicates that 62 articles (95%) used only the mean and standard deviation and descriptive statistics measures to analyze the strength results in different treatments. Therefore, the results are restricted to these samples and do not explore more advanced analyses.

Only three articles presented inferential statistics to characterize the method, enabling the transfer of results to the population (statistical analysis). Regarding the comparison and discussion of results, 32 articles (49%) utilized only the comparison of the samples regarding treatments with the reference sample (basic comparison), while 33 manuscripts (51%) compared with data from similar studies (median comparison). Neither “advanced comparison” with studies from systematic reviews nor “statistical comparison” with standardized statistical comparisons were identified.

Although acid activation in alternative binders has recently become a highly relevant topic, scientific studies still need to improve methodological quality. The extrapolation of these studies for comparison with similar research cannot be understated, as it is a fundamental requirement for effective and meaningful progress in this area. This comparison is crucial for advancing regulatory standards for field applications of the products studied. Therefore, developing the methodology is hugely important, strengthening the knowledge base and allowing for consistent progress.

3. Results and Discussions

Table A1. MKSAPG—Materials and methods.

Ref.	Precursors	Additive	Activator	Curing Process	Theoretical Molar Ratio			L/S	Molarity Acid Solution	Compressive Strength (MPa)
					Al/P	Si/Al	Si/P			<3 d	7 d	14 d	28 d	>28 d
[13]	MK	-	PA	70 °C	1.0	1.0	1.0	0.5	6.5	80	-	-	-	-
[52]	MK	-	PA	RT	1.6	0.8	1.2	1.0	6.2	-	29	-	-	-
[37]	MK	-	PA	40 °C for 24 h → 60 °C for 24 h	1.2	1.0	1.1	1.0	12.0	123	-	-	-	-
		-		40 °C for 24 h → 80 °C for 24 h	1.2	1.0	1.1	1.0	12.0	97	-	-	-	-
[45]	MK	-	PA+ADP	20 ± 2 °C with 90% of RH	2.0	0.6	1.1	1.0	-	16	21	25	33	-
[48]	MK	-	PA	60 °C for 24 h → RT	1.0	0.6	0.6	0.8	10.0	-	-	-	58	-
[8]	MK	-	PA	60 °C	1.0	1.1	1.1	1.0	5.8	-	-	30	-	-
		-		RT	1.0	1.1	1.1	1.0	5.8	-	-	21	-	-
[46]	MK	-	PA+MAP	RT with 95–100% of RH	1.5	1.9	2.8	0.8	-	-	37	39	52	-
[27]	MK	-	PA	RT	-	3.3	-	-	10.0	-	-	-	52	-
		-		60 °C for 24 h→RT	-	2.4	-	-		-	-	-	28	-
	WB	-		RT	-	4.6	-	-		-	-	-	56	-
		-		60 °C for 24 h→RT	-	4.6	-	-		-	-	-	33	-
[62]	MK + PG	-	PA	RT for 24 h → 60 °C for 24 h→RT	-	3.3	-	1.4	10.0	-	-	-	49	-
[51]	MK	-	MAP	20 °C	2.2	1.3	2.7	0.8	7.4		20	-	-	43
		Ca(OH)2			2.1	1.3	2.7				20	-	-	54
		CaSiO3			2.0	-	-				15	-	-	56
[40]	MK	-	PA	60 °C for 24 h → RT	-	-	-	-	10.0	31	49	58	74	-
		rGO			-	-	-	-		34	54	61	79	-
[50]	MK	PEG	PA	RT	1.5	1.1	1.4	-	10.0	-	-	26	37	-
		-		RT	1.3	1.1	1.1	-		-	-	14	20
		-		80 °C	8.7	1.0	8.5	-		-	-	40	46	-
	Kaolin	-	PA	60 °C for 24 h → RT	1.0	2.1	2.1	0.7	9.8		3	4	6
	MK	-			1.0	2.1	2.1	0.7	3.7	-	3	3	5	-
[63]	Kaolin	-	PA	40 °C for 48 h → 80 °C for 48 h → RT	0.5	2.2	1.1	0.9	14.0	-	32	-	45	-
[30]	MK + MT	-	PA	RT	0.8	4.6	6.6	0.5	6.0	-	-	-	46	-
[28]	MK + Bauxite	-	PA	RT for 2 h → 60 °C for 24 h→RT with RH 68%	1.1	1.5	1.7	0.7	10.0	-	-	-	22	-
	Laterite + Bauxite	-			0.7	1.3	0.9			-	-	-	48	-
[85]	MK + FA	-	PA	40 °C and RH of 90%	1.0	1.1	1.1	-	-	-	-	-	-	-
				80 °C for 24 h → RT	1.0	1.1	1.1	-	-	-	115	-	-	60
[16]	MK + MSWI-FA	-	PA	RT for 24 h → 60 °C for 24 h → RT	1.1	2.1	2.3	0.9	12.0	-	32	-	-	34
[17]	MK + FA	-	MAP	RT for 24 h → under water at 25 °C	2.1	1.9	3.6	0.8	4.7	29	-	-	49	-
[15]	MK + FA	-	PA	40 °C with RH 90% for 6 d → 80 °C for 24 h → RT	1.0	4.1	4.1	0.9	3.6	-	30	-	-	44
[47]	MK	DBM	MAP	25 °C with RH 95%	1.8	1.1	2.0	1.1	11.6	14		-	-	-
[61]	MK	Mullite Fibers	PA	60 °C for 24 h → RT with RH 80%	0.8	1.3	1.9	0.9	6.0	-	26	-	27	-
[56]	MK	-	PA	RT for 2 h → 40 °C for 24 h → RT	0.4	2.3	0.8	0.8	10				88

,

Table A2. FASAPG—Materials and methods.

Reference	Activator	Curing Process	Theoretical Molar Ratio			L/S	Molarity Acid Solution	Compressive Strength (MPa)
			Al/P	Si/Al	Si/P			7 d	14 d	28 d	>28 d
[20]	PA	RT	5.6	1.5	8.4	0.3	5.4	6	7	12	21
[5]	PA	25 °C	5.6	1.4	7.7	0.4	4.0	11	-	-	-
		60 °C						13	-	-	-
		90 °C						11	-	-	-
[18]	ADP	RT	6.0	2.3	13.8	0.4	2.2	7	9	14	-
[19]	ADP	RT	7.8	1.5	11.4	0.3	3.4	10	21	28	53
[65]	PA	RT	1.9	2.6	4.8	0.3	8.0			12

,

Table A3. VASAPG—Materials and methods.

Reference	Activator	Curing Process	Theoretical Molar Ratio				L/S	Molarity Acid Solution	Compressive Strength (MPa)
			Al/P	Si/Al	Si/P	Fe/P			1 d	3 d	7 d	14 d	28 d	>28 d
[24]	PA	RT	0.6	4.2	2.4	0.3	0.5	10.0	21	-	-	-	31	-
[9]	PA	20 °C	0.7	4.5	3.0	0.4	0.5	7.5	0	9	13	-	19	-
		40 °C							11	18	20	-	42	-
		60 °C							20	16	19	-	26	-
	MAP	20 °C	1.0	4.5	3.0	0.4	0.5	7.4	17	15	24	-	32	-
		40 °C							27	22	20	-	25	-
		60 °C							19	21	18	-	25	-
[23]	PA	RT dry	1.1	3.7	6.5	0.7	0.4	9.1	-	-	-	-	83	-
		RT wet-dry							-	-	-	-	39	-
[25]	PA	RT	0.9	4.1	4.4	0.5	0.7	7.2	-	-	9	-	32	53

and

Table A4. SAPG based on others precursors—Materials and methods.

Ref.	Precursor	Acid	Cured Process	Theoretical Molar Ratio				L/S	Molarity Acid Solution	Compressive Strength (MPA)
				Al/P	Si/Al	Si/P	Fe/P			3 d	7 d	14 d	28 d	>28 d
[67]	Laterite	PA	RT for 2 h → 50 °C for 24 h → RT for 14 d	1.8	1.0	1.7	1.8	0.7	6.0	-	-	105	-	-
[29]	Laterite	PA	RT for 24 h → RT with RH 53%	1.0	1.0	0.9	1.0	0.8	10.0	-	-	-	83	-
			40 for 24 h → RT with RH 53%							-	-	-	48	-
			50 for 24 h → RT with RH 53%							-	-	-	65	-
			60 for 24 h → RT with RH 53%							-	-	-	46	-
			70 for 24 h → RT with RH 53%							-	-	-	42	-
			80 for 24 h → RT with RH 53%							-	-	-	32	-
			90 for 24 h → RT with RH 53%							-	-	-	24	-
[69]	Calcium Silicate Powder	PA	RT with RH 60%	0.1	13.1	1.4	0.0	1.3	13.0	34	46	58	56	55
[11]	Nanometric Tubular Halloysite	PA	50 °C for 48 h → 80 °C for 48 h	0.3	2.1	0.7	0.0	1.3	14.0	-	65	-	63	-
[70]	Aluminosilicate Powder	PA	60 °C for 6 h	-	2.5	-	-	-	-	5	-	-	-	-
[71]	Natural Pozzolan	PA	60 °C for 24 h → RT	0.3	4.2	1.3	0.2	0.6	11.7	-	21	-	23	-
[72]	SFCC	PA	60 °C for 6 h → RT	0.6	1.2	0.7	0.0	0.5	12.0	-	30	-	-	-

(Appendix A) summarizes the precursors, curing processes, molar ratios, L/S ratio, and compressive strength of this RSL.

Table A5. Experiments conducted in the studies of this RSL.

XRD	FTIR	SEM/MI	TGA	DTA	IC	DSC	MIP	NMR	EDS	EDX	WA	EC	F	ST	CS	LT	Others	Ref.
x	x					x		x							x		Atomic Absorption	[8]
x	x	x	x		x	x	x								x			[9]
x	x		x	x							x			x	x		Viscosity	[13]
x		x																[14]
x	x	x	x			x								x	x			[15]
x	x	x					x	x						x				[16]
x		x	x	x				x	x		x		x	x	x			[17]
x	x	x					x		x			x			x	x	XPS	[18]
x	x	x	x			x	x	x				x	x	x	x			[19]
x	x	x	x			x	x	x					x	x	x			[20]
x	x	x								x					x			[5]
x	x	x							x						x			[19]
x	x													x	x			[23]
x	x	x	x		x				x						x			[24]
x	x	x	x		x	x				x	x			x	x			[25]
x																	Atomic Absorption	[26]
x		x	x			x									x		Infrared spectra	[27]
x		x	x			x									x		Infrared spectra	[28]
x	x	x									x				x			[29]
x	x										x				x			[30]
x	x				x			x						x	x		ICP-OES	[37]
x	x	x						x			x				x		Brazilian Test	[12]
x	x	x				x		x		x							pH-metry	[38]
x	x	x													x			[40]
x	x	x	x	x	x		x						x	x	x		Shrinkage	[45]
x		x	x				x	x	x				x	x	x			[46]
x		x	x	x						x				x	x			[47]
x		x													x			[48]
x								x							x		Thermal Resistance	[49]
x	x	x	x			x									x		Flexural Strength	[50]
x	x	x	x			x				x	x				x		Water Resistance, Shrinkage	[51]
x	x	x	x			x									x		Finit Element Analysis	[52]
x			x	x			x					x						[53]
			x	x			x										Dynamic Vapour Sorption	[54]
x	x																	[57]
x					x													[10]
x	x	x	x			x				x					x	x	Shrinkage, Flexural Strength	[61]
x	x	x												x	x	x	Flexural Strength	[62]
x	x	x								x					x			[63]
x	x	x	x					x							x		Specific Surface Area (BET)	[64]
x	x	x													x		Semi-adiabatic calorimetry	[67]
x		x					x				x				x	x	Flexural Strength	[69]
x	x	x	x			x		x							x			[11]
x	x	x	x											x	x	x	Flexural Strength	[70]
x	x	x									x	x			x		Bulk Density, P-wave Velocity	[71]
x	x	x						x							x			[72]
x	x	x	x		x	x									x			[85]
x	x	x						x							x			[86]
x	x					x					x				x		Dry Density, Porosity, TC	[79]
x		x													x			[78]
x									x								Porosity, NGAIA, BET, XPS	[80]
x	x					x					x				x		Porosity, NGAIA, Batch Adsorption Test	[82]
x		x													x		NGAIA, Porosity, TC	[83]
x									x								Bulk density	[77]
x	x	x							x								Bulk density, Splitting Strength, TC	[84]
x		x													x		BET, NGAIA, XPS, Raman analysis, CDAA	[81]
x		x	x						x		x				x		Water Resistence	[56]
x		x	x						x						x		Thermal Resistance	[55]
																	Ph test, CBR	[66]
x	x	x	x			x									x			[65]
x	x		x	x				x										[73]
x												x			x	x	Flexural Strength	[60]
x	x	x					x		x						x			[59]
x		x							x						x	x	Thermal Water Resistence	[58]
x	x	x													x		Bulk Density, Infrared spectra, Linear Shrinkage	[68]

XRD = X-ray Diffraction; FTIR = Fourier Transform Infrared; SEM/MI = Scanning Electron Microscope/Micrography Images; TGA = Thermogravimetric analysis; DTA = Differential Thermal Analysis; IC = Isothermal Calorimetry; DSC = Differential Scanning Calorimeter; MIP = Mercury Intrusion Porosimetry; NMR = Nuclear Magnetic Resonance; EDS = Energy Dispersive X-ray Spectroscopy; EDX = Energy Dispersive X-ray Analysis; WA = Water Absorption; EC = Electrical Conductivity; F = Fluidity; ST = Setting Time; CS = Compressive Strength; LT = Leaching Test; ICP-OES = inductively coupled plasma-optical emission spectrometer; XPS = X-ray Photoelectron Spectroscopy; BET = Brunauer, Emmett and Taller; TC = Thermal Conductivity; NGAIA = Nitrogen Gas Adsorption Isotherm Analysis; CDAA = Carbon Dioxide Adsorption Analysis.

(Appendix B) presents the laboratory tests employed in each work.

3.1. Nomenclature

The diversity of nomenclatures often leads to conflicts and misunderstandings throughout the scientific community. For this reason, some terms must be presented and clarified. Alkali-activated material (AAM) is a binder formed by the relationship between an aluminosilicate precursor and an alkaline activator. Geopolymer can be considered a type of AAM containing little or no calcium, often derived from MK or FA precursor [1].

Cement or chemically phosphate-bonded binder, inorganic polymers with [PO₄]³⁻ instead of [SiO₄]⁴⁻, should be considered as a new class of materials in the geopolymer family [36]. Different names are given by the research community, with the common association of the term “geopolymer” with a term related to phosphate activation, such as “phosphoric acid-based geopolymers”, or a complete nomenclature that associates the aluminosilicate precursor, such as “silicoaluminophosphate geopolymers”.

As it is a concise and easily disseminated term, the term geopolymer is widely used by engineers and researchers today. Following this trend, this article chose to use the term silicoaluminophosphate geopolymer (SAPG) for materials chemically activated by phosphate, considering that the commonly used precursors have a low calcium content and, therefore, are synthesized through a geopolymerization process. Furthermore, it was expected that the literature would encompass acid activation using other types of acids. However, all studies in the bibliographic portfolio employed PA or a phosphate solution as an acid activator. SAPG with MK as its primary precursor will be denoted as MKSAPG, in the case of FA it will be called FASAPG, and in the case of VA, it will be referred to as VASAPG. Materials chemically activated by alkali will be utilized in the term alkali aluminosilicate (AAS).

3.2. Chemical Structure

The chemical structure of SAP gels is not yet a consensus in the literature. Generally, there is a consensus on the predominance of aluminum with six coordinates in SAPG, whereas aluminum in NASH gels formed in AAS geopolymers is characterized by four coordinates [11]. However, literature data mention the existence of structures similar to berlinite in SAP gels, implying the presence of four-coordinate aluminum as well [31].

The P/Al molar ratio significantly influences the SAPG formation process. At a low P/Al ratio, dissolved aluminum reacts with [PO₄]³⁻ to form the Al-O-P structure directly. At relatively high P/Al ratios, the metastable intermediate P-O-P structure appears during the reaction and is finally transformed into the Al-O-P structure with the consumption of [PO₄]³⁻. Furthermore, with the increase in the P/Al ratio, the contents of the 4-coordinate and 5-coordinate aluminum structures in MK are significantly reduced, and the newly formed Al_IV-OP structure transforms into an Al_VI-OP structure. Therefore, aluminum atoms preferentially exist in the Al_VI structure in SAPG with higher P/Al ratios [37].

SAPG with an optimal acid activator content presents both amorphous phases of aluminum phosphate (Al–O–P) and amorphous phase of silica (Si–O–Si), dispersed in an amorphous geopolymeric structure of aluminum phosphate silicate (Si-O-Al-O-P), with some terminal of silicate phosphate units (Si-O-P). For excessive P/Al ratio, the geopolymer presents a porous structure based mainly on a crystalline phase of aluminum phosphate silicate hydrate and some amorphous geopolymeric networks [12,38].

3.3. Acidic versus Alkaline Activators

As mentioned previously, PA is the most commonly used activator for acid activation of geopolymers. In contrast to conventional alkaline solutions, which use sodium hydroxide (NaOH) or sodium silicate (Na₂SiO₃) and require manufacturing temperatures above 1000 °C, PA is produced at temperatures below 300 °C. This offers economic benefits and contributes to environmental sustainability by leading to decreased energy consumption and lower CO₂ emissions [33]. Comparing FASAPG, AAS, and cement pastes, the CO₂ emission intensity was reduced by 70.9% and 35.6%, respectively. The reduction in energy consumption was 90.6% in both cases [5].

Regarding the mechanical resistance of SAPG and AAS, the literature is controversial, although the possibility of mechanical properties in the same order of magnitude can be stated. Higher compressive strength has been reported for high-calcium AAS (70–110 MPa) compared to low-calcium AAS (15–65 MPa) and SAPG (10–50 MPa) [6]. However, there were no investigations on SAPG with high calcium content, with a clearly smaller number of studies and more excellent dispersion of results for SAPG. Other studies present mechanical properties up to twice as high for SAPG compared to AAS [4,39,40].

Efflorescence in AAS results from incomplete consumption of alkaline and/or soluble silicates, forming sodium carbonate in the pores or on the surface of the geopolymeric matrix. This may occur because water weakens the sodium bond in the geopolymer. In the case of SAPG, the acidic synthesis environment causes SAPG to exhibit high resistance to efflorescence [6].

In addition to satisfactory mechanical properties, high efflorescence resistance, reduced carbon emissions, and moderate energy consumption, the phosphorus available in SAPG can be absorbed or extracted to form phosphorus-containing fertilizers, thus creating a virtuous ecosystem favorable to agricultural crops [6,41,42]. This is another considerable advantage of acidic activators since sodium leaching leads to soil salinization and water contamination in alkaline activators.

Despite the apparent advantages of acidic activators, it is important to note two limitations of phosphate-based activators. First, phosphate rock sources are limited, which makes large-scale production of PA-based geopolymers unsustainable. The search for an alternative source of PA is of great importance. Some studies have been interested in alternating the activator with other phosphates rich in aluminum elements [9,18,19,33,43,44,45,46,47]. Secondly, the reaction between the PA solution and aluminosilicates is highly exothermic [5]. In large-volume works, this means little control over the reaction rate, released heat, and the final structure formed [31]. This problem can be managed and avoided by using phosphates, such as NaH₂PO₄ or KH₂PO₄, alone or in combination with the PA solution. However, these activators seem to be less efficient when compared to PA [31]. Finally, it is noteworthy that the acid activator PA incurs higher costs. Comparing SAPG with AAS and cement pastes, there was an increased cost of 87.4% and 30.7%, respectively, due to the higher price of the PA solution [5].

3.4. L/S Ratio and Al/P Molar Ratio

Handling SAPGs in complex forms can be challenging compared to AAS geopolymers due to the high viscosity of their reactive mixture. However, this can be addressed by adding water to the formulation. Nonetheless, adding water changes the concentration of [P] and influences the material’s structure, forming new amorphous networks [13]. Interestingly, authors often mention only the dilution of the solution without providing information on the percentage of water in the mixture or the Liquid (activator solution + water)/Solid (L/S) ratio [29]. It is important to note that the parameter nP/nAl + nH₂O governs not only the chemical structure but also the mechanical resistance and porosity of the material [13].

The optimum Al/P molar ratio and L/S ratio are affected by temperature and curing time [14,26]. Variations in the Al/P molar ratio strongly affect the dealumination process only at low curing temperatures (between 22 and 50 °C) with short curing times (between 0.2 and 3 h). The same percentage of released aluminum can be achieved by increasing the curing temperature in the early stages of geopolymer formation or extending the curing time, even at low curing temperatures [26].

At room temperature (RT), SAPG consolidation occurs in compositions rich in [Al] and [Si] and poor in [P] [14]. More specifically, for Al/P = 1 and variable amounts of water, the networks are mainly based on the coexistence of Al–O–P with a hydrated silica network, conditions necessary for a more resistant sample [13,48,49]. The consolidation time is significantly reduced with increasing curing temperature, with a shorter time being observed for compositions rich in [P] (instead of poor, as in the case of geopolymers formed in RT) [14]. In general, studies suggest the absence of unreacted precursor, satisfactory stability, and mechanical resistance with an Al/P ratio close to 1.0 for ambient curing over a long period [13,48,49] and an Al/P ratio close to 0.5 for thermal curing with short time [26].

3.5. MKSAPG

MK is derived from the calcination of natural kaolinite at temperatures ranging from 700 to 800 °C. MK has been selected as a source of aluminosilicate due to its reactivity related to a significant amorphous fraction and low number of “impurities” (chemical components other than aluminosilicates). Owing to the need for calcination and limited availability, MK is confined to the research environment, aiming to investigate the mechanism of geopolymer formation, with recommendations for low-volume and high-value practical applications [47].

3.5.1. Compressive Strength

It is possible to see that the rate of increase in compressive strength is higher at early ages and decreases with increasing curing time [40,45,50,51]. The timeframe for gaining strength in these materials remains unknown, and it is common to evaluate the standard period of 28 days used for OCP, although the material apparently continues to gain significant strength beyond this period [45].

Figure 6 compiles the optimal compressive strength results from studies involving MK as a single precursor, distinguishing between curing at RT and thermal curing. A noteworthy increase in compressive strength is observed for thermally cured geopolymers, notably in the early stages. Thermal curing at 80 °C increased the compressive strength from 14 to 40 MPa at 14 days and from 20 to 46 MPa at 28 days [50]. Following the same trend, curing at 60 °C increased compressive strength from 21 to 30 MPa at 14 days [8]. Rising temperature produces a geopolymer structure richer in aluminum phosphate phases that increase the material’s compressive strength [8].

3.5.2. Curing Process

Thermal cures have been carried out at temperatures of up to 80 °C, although some researchers favor lower temperatures of 40 and 60 °C, to avoid cracks and fissures resulting from expansive stress due to the rapid exothermic reaction. A two-stage curing method has been found to prevent thermocracking, which involves a pre-cure at 40 °C for 24 h and a secondary cure at an elevated temperature of either 60 °C or 80 °C for another 24 h. In the second curing stage, a compressive strength of 123 MPa was achieved for SAPG with P/Al = 0.84 cured at 60 °C. The higher curing temperature of 80 °C in the second curing process did not benefit strength [37].

3.5.3. Chemical Activator

Most studies focused on geopolymer synthesis using PA, while only two studies investigated the impact of adding aluminum species to the activating solution. Adding aluminum dihydrogen phosphate (ADP) to the PA solution has several benefits such as reducing the heat of hydration of the geopolymer, improving thermal stability, initial compressive strength, and reducing drying shrinkage. However, this addition also has some drawbacks such as increased total porosity, harmful pores, and lower compressive strength with increasing curing age [45]. The addition of monoaluminum phosphate (MAP) leads to similar results. Aluminum species allow a rapid sol/gel transition, which improves rheological properties, setting time, and initial strength. However, soluble aluminum hinders the continuous reaction of MK in the subsequent period, exerting a detrimental influence on the continued development of resistance beyond seven days of curing [46].

3.5.4. Thermal and Water Resistance

Thermal stability of MKSAPG up to 1400 °C has been reported [14,52,53,54]. The initial temperature of the zeolite crystallization phase occurs at 180 °C, with conversion of this phase into phosphocristobalite from 300 °C to 1400 °C. The formation of tridymite is observed from 700 °C onwards. The amount of phosphocristobalite and tridymite crystals reached a maximum of 1100 °C, with partial dissolution of the detridymite and cristobalite phases at 1400 °C [53]. Hence, the utilization of this material in high-temperature applications is viable, given the development of stable and refractory phases at elevated temperatures [55].

MKSAPG samples soaked in water lose about 54% of compressive strength, attributed to the hydrolytic deterioration of Si-O-P bonds in the presence of water [56]. However, heat treatment improves water resistance, with limited release of acidic species into water even after low-temperature calcination [57]. It was also possible to achieve water resistance without heat treatment by changing the molar ratios to Al/P = 1 or 4 and Si/Al ≤ 1 [58].

3.5.5. Reaction Kinetics

The reaction kinetics were described in three steps [26]. In the first stage, dealumination occurs in about 30 min, attacking all of the Al-O-Al bonds and a small part of the Si-O-Al bonds. The second stage consists of the condensation process that lasts around 12 h and is divided into four possible phases. The third phase is the polycondensation step [24]. It was also found that increasing the PA concentration increases the activation energy of MK dissolution, with a maximum reaction rate achieved at 80 °C with a PA concentration of 8 mol/L [10]. A relatively higher reaction temperature decreases the maximum heat release of the geopolymer [10].

3.5.6. Additives

The additives Graphene oxide (rGO), Polyethylene Glycol (PEG), calcium hydroxide (Ca(OH)₂), calcium silicate (CaSiO₃), Dead Burnt Magnesia (DBM), nano silica (NS), nano alumina (NA), wood fibers, and mullite fibers were tested in MKSAPGs. Adding an optimal dose of 2% rGO does not affect the product’s existing gel network but densifies the microstructure and brings resistance gains of between 10 and 12% at all curing ages [40]. Intending to minimize the harmful effects of thermal curing and develop a shrinkage-free geopolymer, the addition of PEG prevented thermocracking and formed more amorphous aluminum silica phosphates with fewer micropores [50]. Replacement of up to 6% by weight with highly soluble Ca(OH)₂ improved compressive strength at any age, while poorly soluble CaSiO₃ decreased compressive strength for any percentage of replacement [51]. The incorporation of 20% DBM induces an acid-base reaction during the formation of SAPG and allows its preparation with an initial setting time of 8 min and compressive strength of 8.3 MPa in 1 day. This improvement in properties at an early age was attributed to phosphorösslerite detected in the geopolymer paste after 1 day of curing and absent on the third day of curing [47]. The addition of NS and NA at a low dose of 1% aided in the structural development of the composite and provided an increase in compressive strength of 25% and 45%, respectively [59]. The incorporation of wood fiber can reduce brittleness and increase fracture toughness, but it is unfavorable to the setting and hardening behavior, and compressive strength of MKSAPG [60]. Adding 10% mullite fibers in a SAPG produced a desirable bond at the fiber/matrix interface with inhibition of cracking in samples subjected to temperatures up to 1350 °C [61].

3.5.7. MK with Other Precursors

The effect of adding FA to MKSAPG was investigated. Incorporation of up to 10% high-calcium FA proved to be beneficial for compressive strength and setting time reduction due to the formation of calcium phosphate compounds (e.g., brushite and monetite) as secondary bonding phases [15,17]. The FA cenospheres prevented the plate-shaped MK from being bound by aluminum phosphate, thereby increasing the amount of MK to depolymerize [15]. However, additions above 10% decrease the amount of available aluminum, and the competition of the ash metal ions for phosphates hinders the development of the aluminophosphate geopolymeric matrix [16]. Furthermore, after long-term aging, calcium sulfate transformed into needle- and slice-like nanoparticles, reducing the compressive strength of the resulting geopolymers [15]. Overall, the results suggest the possibility of adjusting the proportions of FA and MK to achieve a desirable setting time without significant compromise to long-term compressive strength.

The partial replacement of MK with phosphogypsum (PG) and mine waste (MT) was studied to explore the possibility of reusing industrial waste. Adding 70% of PG increased the compressive strength by around 20%, from 40.14 MPa to 48.43 MPa, thanks to the limitation of crack propagation obtained by the good adhesion of the PG with the geopolymeric matrix [62]. Furthermore, the PG addition reduced the setting time and inhibited the hydrolysis process of geopolymers, limiting the release of metals contained in PG. The leaching values were found to be lower than the international standards [62]. Incorporating 90% by weight of MT resulted in optimal compressive strength (45.5 MPa) and water absorption (3.0%). Therefore, the binder obtained by acid activation could be used in structural materials manufacture [30].

3.5.8. Use of Kaolin

Replacing kaolin (non-dehydrated kaolinite) with MK to synthesize SAPGs avoids energy consumption and CO₂ emissions from the calcination stage. By increasing the molar concentration of the acid activator to 14 M, the kaolin-based SAPG reached compressive strengths of 32 MPa at 7 days and 45 MPa at 28 days [63]. Furthermore, mechanically activated kaolin by a grinding process appeared more reactive than thermally activated kaolin [64]. However, the environmental and economic impact of increasing acid activator content and the energy expended in the grinding process need to be further investigated to clarify the feasibility of non-dehydrated kaolinite.

3.6. FASAPG

FA is an industrial byproduct produced in large volumes resulting from the combustion of pulverized mineral coal in electricity generation plants. It consists of aluminosilicates, calcium, and other oxides, and is classified according to its composition into Class C (high calcium content) and Class F (low calcium content). The advantages of FA are its positive influence on the workability of fresh cement paste and the reduction in hydration heat [31]. Furthermore, due to its fine particle size, typically 10 μm, it does not require the grinding process. Disadvantages are the possible presence of dioxins that can be removed by heat treatment or leaching [31], and instability in mechanical resistance, with a reduction in compressive strength as the curing time increases [19].

3.6.1. Compressive Strength

The compressive strengths achieved by FASAPG (Figure 7) are lower than MKSAPG. The highest resistances were attained in studies that employed ADP as a chemical activator. In contrast to MKSAPG, FASAPG activated with ADP did not lose resistance over time [18,19].

There was a slight increase in compressive strength in thermally cured FASAPG. This increase is barely visible in Figure 7 because of the much higher strength of ADP-activated geopolymers at RT. Similar to MKSAPG, raising the curing temperature beyond 60 °C was not beneficial for compressive strength due to microcracks resulting from the rapid shrinkage process. Increasing the temperature from 25 °C to 60 °C enhanced the compressive strength from 11 to 13.2 MPa, but further elevation to 90 °C decreased it to 11.4 MPa [18]. Likewise, raising the temperature from 30 °C to 60 °C increased compressive strength from 7.2 to 11 MPa, but an increase to 80 °C reduced it to 4.1 MPa [21].

3.6.2. Curing Process

Four different curing processes were tested for the mixture of FA and high magnesium nickel (HMNS): (1) Water curing at 20 °C; (2) Air curing at RT (3) Standard curing at 20 °C with relative humidity (RH) ≥ 95%; (4) Curing with fresh film at RT. The results showed lower strengths for water curing and standard curing (with RH ≥ 95%). Furthermore, when the curing age was increased, only the sample covered with fresh film had no reduction in strength. In high humidity conditions, numerous pores form on the geopolymer surface, and unreacted PA is slowly dissolved, reducing hydration products in the subsequent curing period. Therefore, to escape a decrease in strength at late curing, high-humidity environments should be avoided [37].

3.6.3. Chemical Activator

As previously mentioned, FASAPG exhibited superior mechanical resistance when activated with ADP compared to PA. As curing time and ADP concentration increased, the pore diameter of the ADP-activated geopolymer decreased, and the matrix structure became denser due to the formation of more hydrates. Additionally, the setting time increased with increasing L/S and ADP concentration, a trend opposite to that observed in the PA-activated geopolymer. [19].

3.6.4. Thermal Resistance

Four stages of mass loss were observed in ADP-activated FASAPG. In the first, loss appeared in the temperature range of 0–180 °C due to the physical removal of water. In the second, a loss occurred in the temperature range of 180–780 °C due to the chemically bound water removal. In the third, a loss occurred in the temperature range of 780–1080 °C due to the phase transition of the geopolymer matrix. In the latter, mass loss occurred in the temperature range of 1080–1200 °C, associated with the evaporation of P₂O₅ [19].

3.6.5. Water Resistance

The water resistance decreased with the decrease in PA concentration, increase in L/S ratio, and extended immersion time in water. Leaching of certain crystalline compounds containing calcium and hydrolysis of Si-O-P units in the amorphous gel occurred after water immersion, also contributing to the reduction in the strength of FASAPG [65]. Enhancing the water stability of SAPG is crucial to meet the requirements of engineering applications [65].

3.6.6. Chemical Structure

The main hydration products of FASAPG are the same as those of MKSAPG, an amorphous matrix composed of –Si–O–P–, –Si–O–Al–O–P– and –Al–O–P [16,17,18]. The presence of calcium brings new crystalline products such as Brushite (CaPO₃(OH)·2H₂O), Monetite (CaHPO₄), and Berlinite (AlPO₄) [20].

3.6.7. FA with Other Precursors

FA was tested with HMNS, a mining waste that is recycled in small quantities. Due to the high MgO content in HMNS, the use of alkaline activators can potentially form Brucite (Mg(OH)₂) through the hydration of MgO, an expansive compound [21]. Although acid activation provides favorable reaction conditions for geopolymerization, the compressive strength of FASAPG and HMNS was much lower than MKSAPG geopolymers and AAS geopolymer based on FA and ground ferronickel slag (which also contains MgO). The main reasons were the low pozzolanic activity of HMNS and the porous and fissured structure due to the violent reaction between [Mg] and the PA activator [21].

3.6.8. Solidification/Stabilization (S/S)

A better S/S capacity of [Pb]²⁺ was obtained in FASAPG than in cement and AAS geopolymer. The presence of [Pb]²⁺ in FASAPG was beneficial for compressive strength for contents up to 0.6% and had little impact on the porosity of the geopolymer, although the proportion of macropores increased. The binding mechanism for [Pb]²⁺ stabilization included chemical precipitation, physical adsorption, and encapsulation. [Pb]²⁺ was stabilized mainly in the form of the stable compounds Pb₃(PO₄)₂ and PbHPO₄. [18]. FASAPG also performed better than cement and AAS binder in treating soils contaminated with Pb2+, particularly in acidic environments [66].

3.7. VASAPG

VAs are generated when solid rock breaks and magma separates into tiny particles during explosive volcanic activity. They can have a very variable composition but are generally a source of aluminosilicates and contain metallic oxides (calcium, magnesium, and iron), making them a potential natural material for synthesizing binders.

3.7.1. Compressive Strength

VASAPGs reach moderate compressive strengths at an early age and can reach high strengths at a later age (Figure 8), with no loss of strength over time. The highest strength was 80 MPa at 28 days with curing at RT [23].

3.7.2. Curing Process and Chemical Activator

The MAP as activator increased the compressive strength from 19 to 32 MPa at 28 days with curing at RT, while thermal curing at 40 °C increased it to 42 MPa [9]. Furthermore, there was a decrease in strength for thermal curing at 60 °C and for the combination of thermal curing at 40 °C and using MAP simultaneously [9]. To some extent, heat improves the dissolution of aluminum from VA and stimulates the solubility of other reactive components without overly accelerating the reaction kinetics. Although MAP at RT promotes strength development at an early age, thermal curing at a mild temperature (40 °C) seems to be the most relevant parameter for strength development at the late age of VASAPG [9].

3.7.3. Water Resistance

Water resistance is a crucial requirement for geopolymers in civil construction, and little attention has been given to this parameter in the studies found in this RSL. The water resistance of a VASAPG was evaluated by comparing the compressive strength of specimens before and after immersion in water for 24 h. A 50% reduction in strength was attributed to the unstable bonding phase in the amorphous structure, which was easily leached when the samples were immersed in water [23].

3.7.4. Chemical Composition

The amorphous phase of volcanic ash contains oxides of SiO₂, Al₂O₃, Fe₂O₃, MgO, and CaO [24]. Calcium and magnesium are responsible for the rapid setting of the VASAPG at RT, while aluminum and iron are responsible for compressive strength development [25].

The VA chemical composition and amorphous phase content directly interfere with the chemical activator dosage [23]. Therefore, the ideal mixture design should consider the molar ratios Ca/P, Mg/P, Al/P, and Fe/P [25]. The optimal molar ratio R = (CaO + MgO)/P₂O₅ = 4.2, considered relatively high, was achieved by reducing the PA concentration [23].

A high dosage of PA, obtained with Al/P ≤ 0.5 and Fe/P ≤ 0.27, harms the formation of a stable binder, which disintegrates when immersed in water and forms a white precipitate [25]. Compressive strength increased considerably with time, ranging from 9 MPa (7 days) to 53 MPa (90 days) for Fe/P = 0.50 and Al/P = 0.93 molar ratios. The main binder was a porous phase of ferro-silico-aluminophosphate. Secondary phases were also identified in some mixtures, including ferro-aluminophosphate and magnesium phosphate [25].

3.8. SAPG Based on Other Precursors

Studies investigating the acid activation of the following precursors were found: calcined laterite, calcium silicate powder, nanometric tubular halloysite, aluminosilicate powder, natural pozzolan, spend fluid catalytic-cracking (SFCC), and copper mine tailings (MT).

3.8.1. Calcined Laterite

Laterite is a natural precursor rich in iron. The calcined laterite showed the presence of kaolinite, hematite, anatase, and quartz. At the same time, the products indicated the formation of poly(phospho-ferro-siloxo) networks in the appearance of a broad protuberance structure between 17° and 40° (2θ). The intensity of this diffuse halo structure is lower for samples cured at 90 °C. Compressive strength at 28 days with PA concentration at 10 M was higher for samples cured at RT (83 MPa), median for curing at 50 °C (65 MPa), and lower for curing at 90 °C (24 MPa) [29]. For two different laterites, maximum compressive strengths of 98 to 105 MPa at 14 days were obtained for curing at 50 °C and 6 M and 8 M concentrations. At low acid concentrations, there is a higher concentration of Fe in the cement matrix based on laterite-phosphate, which may indicate a fine dispersion of small grains of hematite [67]. The acidic activation using calcined iron-rich laterite gave better mechanical performance than calcined clay-rich laterite at the same condition, suggesting the alteration of iron minerals strengthened and densified the geopolymer matrix. In comparison with Portland cement at the same conditions, laterite calcined-based SAPG ensured better fire resistance [68].

3.8.2. Calcium Silicate Powder

Calcium silicate-based SAPG is a material that undergoes an acid-base reaction to gain strength under RT conditions [69]. Produced with an L/S ratio of 1.0 and molded in non-hermetic molds, it presented an improved mechanical response of 37.9 MPa on the first day and 55.1 MPa at 14 days. SEM images confirmed the influence of molds. Samples from PVC molds are shaped like prisms in random orientation, while polystyrene molds have structures similar to needles packed in segments [69].

3.8.3. Nanometric Tubular Halloysite

Halloysite is a hydrated polymorph of kaolinite but has a distinct nanosized tubular structure and surface reactivity. Halloysite calcined at 450 °C showed low reactivity, while calcination at 750 °C improved its reactivity due to dehydroxylation. However, halloysite calcined at 1000 °C almost did not react to PA, as it contained γ-Al₂O₃ and had a lower Si-OH content [11]. During acid activation of calcined halloysite at 750 °C, PA reacted with amorphous Al₂O₃ and SiO₂ to form a geopolymer with Si–O–P–O–Al network. By increasing the PA concentration to 14 M, the extent of geopolymerization is improved due to the incorporation of more [Si] and [P] into the geopolymer network, which leads to the formation of a denser structure with greater compressive strength (63 MPa at 28 days) [11].

3.8.4. Aluminosilicate Powder

Aluminosilicate powder can be compared to a very pure MK, which allows us to evaluate the influence of Si/Al molar ratios on an impurity-free SAPG. With increasing Si/Al molar ratios, the flexural strength increased, reached a maximum value at the Si/Al ratio of 2.5, and decreased. However, the reaction speed and curing rate decreased with increasing Si/Al molar ratios. When the Si/Al ratio was equal to 1.5, the geopolymer showed high expandability, and the reaction speed increased. Furthermore, its tensile strength reaches 2 MPa after 5 min. The material could have applications in emergency rescue projects, such as water jets in tunnel engineering and pipeline jets in hydraulic engineering [70].

3.8.5. Natural Pozzolan

Natural pozzolan is a pyroclastic volcanic rock abundant in nature. Similar to VA, its composition is very variable but not found in fine powder like the former. SAPG based on ground natural pozzolan and PA were synthesized and cured at 60 °C for 24 h, followed by curing at RT. The results showed maximum compressive strength of 23 MPa at 28 days for a Si/P molar ratio of 1.25 and a H₂O/H₃PO₄ molar ratio of 1.0 [71].

3.8.6. SFCC

SFCC catalyst is an industrial solid waste used to prepare SAPG for the first time [72]. When the PA concentration was 12 M, the geopolymer reached a maximum compressive strength of 30.2 MPa. SFCC-based SAPG has a similar structure to MKSAPG. However, for geopolymer synthesized with SFCC catalyst with increasing acid concentration, the ratio of Al₆-OP to Al₆-OSi is 3:1 to 5:1, but for geopolymer prepared with MK, it’s close to 2:1. The former has more Al-O-P bonds and Si-O-P bonds, while the latter has more Si-O-Al bonds [72].

3.8.7. Copper MT

PA can be used to activate low-reactivity copper MT to produce SAPG. The optimal Al/P ratio of 2.21 is much higher than the ideal Al/P ratio of about 1.0 for the MKSAPGs but is within the range of optimal Al/P ratios for the FASAPGs and SAPG based on FA with high magnesium nickel slag. The MT-based SAPG shows good durability in both neutral and acidic environments. The concentrations of leached heavy metals from the MT-based geopolymer in both neutral and acidic environments are within the permissible ranges provided by different environmental organizations [73].

3.9. Porous SAPG

Porous materials (sometimes referred to as geopolymeric foams) can find diverse high-value-added applications in civil construction, serving as thermal and acoustic insulators, adsorbents, pH buffering agents, and catalysts [74,75,76]. In this RSL’s bibliographic portfolio, seven studies have been dedicated to the investigation of porous SAPGs. H₂O₂, aluminum and iron powders, and carbonate lime residue (a by-product of the sugar beet-making industry) have been successfully evaluated as foaming agents [77,78,79]. Porous SAPG using activated carbon as a foaming agent showed high NO_x conversion (greater than 85%) over a wide temperature range (from 250–350 °C) [80], and the possibility of CO₂/N₂ capture with high IAST selectivity [81]. Porous SAPGs using hydrogen peroxide as a foaming agent had a higher heavy metal adsorption capacity than AAS geopolymers [82], and significant fire resistance [83]. Porous SAPGs using limestone powder as a foaming agent showed good thermal insulation with acceptable yield strength [84].

4. Development of Future Research

Over the past five years, there has been a remarkable rise in research focused on SAPG. However, the methodological quality has compromised the reliability of existing research, and the absence of data elements in these investigations has amplified the challenges associated with result comparisons. Thus, the advancement of SAPG research demands methodological standardization. In the case of SAPGs, this means providing data on molar ratios (Al/P, Si/Al, and Si/P), L/S ratio, activator solution molarity, curing process and type of mold, in addition to statistical treatment and comparison of results.

Currently, most of the research is focused on SAPG based on MK as a precursor. MK has a high content of aluminosilicates and a lower content of “impurities”. This focus is significant for understanding the reaction kinetics and recognizing the chemical structures formed. However, MK’s low availability and high cost make it necessary to expand research into other types of precursors, notably industrial waste with low market value. There has also been no research on other types of acid activators besides PA, ADP, and MAP.

Regarding research with MKSAPG, most research indicates that the reaction is prolonged at RT. This aspect has been addressed with thermal curing, even so, this may not be the only way. Furthermore, there has been no research on long-term dimensional stability (shrinkage and expansion). Additionally, scant attention has been directed towards porous SAPG, despite their potential for serving as a high-value application in civil construction and environmental remediation.

In addition to MK, the other two most investigated precursors were FA and VA. The former have market value and are today considered a co-product. The latter are natural resources of limited availability. Research with other types of precursors is necessary. Although important, the few studies found with FA and VA still do not clarify mechanical resistance over time, better curing process, water resistance, and the role of iron, magnesium, and calcium silicates and/or oxides. Some studies caution that determining the exact nature of reaction products (morphology, chemical composition, and structure) from these precursors with PA is challenging, owing to the material’s amorphous nature and the chemistry’s complexity involved.

Beyond the limited amount of research on other precursors and possible additives to improve the quality of SAPGs, there has been no research on the durability of these materials and only a few studies have evaluated the leaching aspect. Although there is evidence that PA may have lower environmental impacts, the analyses carried out to date have been very preliminary, and the environmental impacts of these materials have not yet been wholly predicted.

5. Conclusions

The RSL of the last five years on acid activation in the synthesis of alternative binders to OCP allowed the creation of a database on SAPGs (see Appendix A and Appendix B) and reaching the following conclusions:

• Phosphorus-based acid activators offer advantages such as satisfactory mechanical properties, high efflorescence resistance, reduced carbon emissions, moderate energy consumption, and do not induce leaching problems. However, there are notable limitations, including the complexity of an exothermic chemical process, the curing process, and the cost of the acid.
• The principal activator is the PA. Some research tested MAP and ADP to evaluate the influence of adding an aluminum source. Adding ADP and MAP to MKSAPG reduced hydration heat, increased thermal stability, improved rheological properties, setting time, initial compressive strength, and decreased drying shrinkage. However, these activators hindered the continuous reaction of MK in the later period, leading to an increase in total porosity and harmful pores and lower compressive strength with increasing curing age. Adding ADP to FASAPG obtained better mechanical resistance results than PA activation. In contrast to MKSAPG, setting time increased and there was no loss of strength over time, with the matrix structure becoming denser due to the formation of more hydrates.
• The dominant precursor was MK, followed by FA and VA. MKSAPG and VASAPG achieved better mechanical strength, exceeding 20 MPa at 28 days. For FASAPG, the strength ranged around 12 MPa.
• The most commonly used pre-treatment is calcination. In the case of MK, kaolinite is calcined at temperatures of around 700 °C. In the case of FA and VA, these precursors are already calcined by the circumstances these materials are obtained. It was possible to obtain SAPG based on non-calcined kaolinite with good resistance by increasing the molar concentration of the acid activator to 14 M. Furthermore, kaolin mechanically activated by a grinding process appeared to be more reactive than kaolin thermally activated by the calcination process. However, the environmental and economic impact of the increase in acid activator and the energy expended in the grinding process need further investigation to clarify the feasibility of using non-dehydrated kaolinite.
• Some additives were tested for reducing retraction problems (PEG) or improving mechanical properties (rGO, Ca(OH)₂, mullite fibers) or setting time (DBM). However, the fact that we only have one study of each addition precludes the possibility of comparisons and conclusions about these additives’ effectiveness.
• Mechanical performance comparisons between SAPGs and AAS materials are controversial. Variables such as activator concentration, curing process, L/S ratio, and curing age can strongly influence the results. Under optimal conditions, SAPGs can obtain strengths good enough for structural applications in civil construction. There are indications in the literature that they may have thermal and water resistance and do not present leaching and efflorescence problems. However, further investigations are needed to confirm these indications and studies on the durability and environmental impacts of SAPGs.